www.UsHumans.net: Chapter 4
How and when the Earth began, and the affects of its moving continents on life
In this chapter we look at the initial formation of the Earth and then describe some evidence for its moving, tectonic plates and for its fluctuating climate. The main points of this chapter are that the global position of each continent slowly changes through time, that the climate of each continent changes as they move about in latitude and longitude, and that this in turn causes changes in the plant and animal species of each region of the Earth. You might to visit http://svs.gsfc.nasa.gov/search/Keywords/Rotating.html to see NASA’s animation of the rotating Earth. Visit www.solarviews.com/cap/earth/vearth4.htm for an Earth topography animation. For additional Earth-science information, visit http://textbookrevolution.org/Textbooks/EarthSciences.html.
The primary source for this chapter is A Short History of Planet Earth, Mountains, Mammals, Fire, and Ice by J. D. Macdougall. I have included the portions of his book that present the measured facts concerning the age of the Earth, the interior of the Earth, continental drift, changing sea levels through time, glacial advances and retreats, and the temperatures of past ages. These items are directly involved in changes in climate and in plant and animal types. You should read his entire book because it contains many other interesting topics.
Initial formation of the Earth
The previous chapter contained a description of the gravitational process by which a new solar system is formed out of the debris of older stars. In our Solar System, debris gravitated into one large central Sun and also into several smaller planets; near these planets, even smaller clumps formed into moons. The Earth accumulated out of a portion of this clump of dust, gas, and debris in a process that required about ten thousand years to be completed. When the Earth's size became large enough, the weight of its outer material crushed and heated its inner material, causing that material to melt. There is no video tape of the formation of our Earth (unless a family from Andromeda happened to take a few photos while on a long vacation); this gravitational-accumulation scenario is instead observed as a computer experiment where trajectories are calculated for tens of thousands of dust- and rock-sized particles that initially comprise an interstellar dust cloud. One on-line video is available at http://jrscience.wcp.muohio.edu/movies/solarsystemformation.mov and another at http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/33/video/b. You might like to visit http://yso.mtk.nao.ac.jp/~kokubo/moon/kit/movie.html to view a movie showing the formation of the moon through the collision of the Earth and a Mars-sized mass.
Within the molten portion of the Earth, the heaviest materials sank to the center to form the core while the lightest materials floated to the top to form the continents. For example, iron is heavier than many of the Earth's minerals so it sank toward the Earth's center. Iron also has a higher melting point so that it remains in solid form for a longer amount of time compared with many of those other chemicals. It has been calculated that a half-mile (one km) sphere of iron would take about one million years to sink through the molten Earth to its center. Such calculations are possible because scientists have measured the time it takes lumps of various materials to fall through various liquids–for example, the time for a pearl to fall through a bottle of hair remover or the time for a bucket of oil to leak through a hole, which is the way the viscosity of car oil is measured. This fall-time depends on the densities of the lump and of the liquid and on the viscosity of the liquid. These properties have been measured for thousands of chemicals. Remember that the Earth's heaviest elements came from stellar interiors and supernova explosions, and that the Earth's iron core is a remnant of the last fusion-stage of a star that later exploded. (This means there might be city-sized chunks of iron floating around the galaxy.) In just tens of millions of years, the materials of the early Earth had separated into layers of differing densities. In general, the heavier chemicals had moved toward the Earth's center while the lighter chemicals moved toward its surface to form a crust. The material of the continents consists of the lightest chemicals. The other rocky planets and moons have crusts also but no continents. The lack of continents on other planets indicates a lack of such easily accessible minerals because these minerals will not have already risen to the surface.
The Earth's crust first solidified about 4.5 billion years ago (for comparison, 4.5 billion seconds are about one hundred years). This fact has been determined in a several ways. For example, when a piece of liquid emits radiation there is no permanent effect on the surrounding liquid but a measurable track remains visible when this happens inside a solid material. In the same way that running your finger through cake batter does not leave any permanent effect while running your finger through a baked cake does leave a visible result. When a liquid material cools and solidifies it can then begin to accumulate radiation tracks. A higher number of tracks indicate that it has been in solid form for a longer period of time. The 4.5 billion-year-old age of the Earth can be determined by counting either uranium or thorium decay tracks in lead rocks.
Still today, the center of the Earth's core is hot enough that its iron would normally be a molten liquid. The huge pressure caused by the weight of the material above the core causes it to instead remain in solid form. We have laboratory machines that can generate these great pressures, and this allows us to make measurements of the properties of materials that are being subjected to such high pressures. This solid core has a radius of 750 miles or 1,200 kilometers, which is about the width of Texas, and is mostly iron. It is actually spinning relative to the Earth’s surface. It is surrounded by a molten layer of iron that is 1,400 miles (2,200 km) thick, which is about the same as the distance across the Atlantic from Reykjavik, Iceland to Bonn, Germany. In turn, this is surrounded by yet another layer, called the mantle, that is 1,800 miles (2,900 km) thick. Surrounding the mantle is the crust that is three or four miles (5.5 km) thick underneath the oceans but it is 18-25 miles (35 km) thick underneath the continents. The mantle contains more of the heavier chemicals, like iron and magnesium, and less of the lighter chemicals than does the crust. The continents form the Earth's outermost layer. You can see a picture of these layers at http://pubs.usgs.gov/publications/text/inside.html, which is part of the U.S. Geological Survey website.
These layer sizes and their densities are deduced by measuring the time it takes earthquake waves to pass through the Earth along various paths. To understand the details of just how a wave passes through matter, consider a solid block of a certain chemical, like iron. Each atom within the block is held in place by the electrical forces of its neighboring atoms, making each adjacent pair of atoms behave as if they are held together with a spring. (The positions of these atoms can be measured in many ways–for example, with a scanning-tunneling microscope, for example see www.nrel.gov/measurements/tunnel.html on the National Renewable Energy Laboratory website.) When one side of the block is pressed inwards, its "springs" will be compressed. These springs will rebound outward when that external pressure is removed, and this results in a wave motion that spreads throughout the iron. It turns out that a forced disturbance causes one wave to move inwards and outwards along the direction of the applied force, and a second, more slowly moving wave that wiggles perpendicularly to that first wave. (See www.kettering.edu/~drussell/Demos/waves/wavemotion.html for animations of these so-called longitudinal and transverse waves.) These speeds of these two types of waves have been measured for many materials and are determined by the density of the material and the strengths of the interatomic forces, which determine the "stiffness” of the springs. Waves move faster along strong springs then weak ones, just as waves move faster along a string held tightly–rather than loosely–between two hands. If you compress and release a liquid you will not get the perpendicular waves because you can't really wiggle a piece of liquid sideways, you can only compress it. To finally relate this discussion of waves to our description of the Earth's interior, it occurs that when an earthquake wave passes though a liquid, only the parallel waves continue onward while the perpendicular waves disappear. The USGS website http://pubs.usgs.gov/gip/interior has additional information about this. For the video Earthquake animation: Northridge showing the complicated earthquake process, visit http://solidearth.jpl.nasa.gov/rp.html.
An earthquake occurs when a piece of the solid Earth suddenly shifts. This sudden movement generates both perpendicular and parallel waves that travel outward from the point of origin. Geologists use seismometers to measure the magnitude of the earthquake waves and also their arrival times at many points around the globe. The distance between each receiving seismometer and the wave's point of origin tells geologists about the speed of the wave as it traveled through the Earth to arrive at those seismometers. Since wave speeds are affected by the density and the type of material that they pass through, geologists are able to deduce what sorts of materials occur within the Earth. Visit www.nchc.org.tw/english/vr_result_franctal1.php for an earthquake wave movie and accompanying music that is also art.
Geologists know that the Earth's middle layer is liquid because perpendicular waves do not pass through this region. Imagine an earthquake occurring at a point on the surface of the Earth. Waves move outward from the site of the earthquake in all directions. Some waves head straight to the opposite side of the Earth, others move toward the nearby horizon, and yet others travel at a forty-five-degree angle into the Earth to emerge at the more-distant surface. A perpendicularly wiggling wave which tries to move straight through the Earth toward its opposite side will be blocked by the liquid region described above. Other perpendicularly wiggling waves, which travel along on a line just below the Earth's surface, can emerge at the nearby surface and be detected by a seismometer. Those waves moving downward along a forty-five-degree angle might pass all the way through the Earth while those moving downward at sixty-degree-angles do not. The angles of those which do pass through the Earth serve to locate the Earth's molten region.
The interior of the Earth includes radioactive elements. As mentioned above, when a radioactive atom decays, its emits energetic particles in the form of a helium nucleus, electron, or light. Inside the Earth, these can travel but a short distance before being absorbed by surrounding material. The surrounding material is heated when it absorbs those energetic particles. Through the last four billion years, such radiation has continually heated the Earth’s interior. If there were no radiational heating, the Earth would cool off in just fifty million years. The interior of the Earth is the source of much of the background radiation that our bodies absorb everyday.
Moving tectonic plates and the factors that affect climate
We see that the interior of the Earth consists of several concentric, spherical shells. We live on the outermost shell. But the surface of the Earth is not formed of one solid piece of material that moves as a whole; instead, it is broken up into ten large and many small pieces in the manner of adjoining jigsaw pieces, which geologists call "tectonic plates." Below the Earth's crust, some hotter sections of the mantle are moving upwards while adjacent sections are cooler and moving downwards. This convective movement of mantle pushes the plates around the surface of the Earth.
We are all familiar with the Earth’s continents, including Africa and Australia and such. Due to their lower weight, the continents ride along on top of the heavier, crustal material–in the same way that bread would float on oatmeal. The molten material of the mantle slowly churns around with hotter sections moving upward and adjacent, cooler sections moving downward. The motions of the mantel push the continents around the surface of the Earth. To see an animation of the continents moving around the planet through the last 225 million years, and to also see a picture of where the continents might be 250 million years from now, you might like to visit the NASA website at http://liftoff.msfc.nasa.gov/news/2000/news-collision.asp. The arrangement of oceans and mountains as they appeared in the U.S. some 80 and 300 million years ago can be seen by visiting the United States Geological Survey (USGS) website http://pubs.usgs.gov/gip/continents. Animated globes showing continental positions and features can be seen at www.scotese.com.You can view on-line books describing the Earth’s interior, geologic time, moving continents, ice ages, glaciers, dinosaurs, volcanoes, and fossils by visiting http://pubs.usgs.gov/products/books/gip.html. The BBC website has an animation called Welcome to Britain’s Rocky Past, From the Big Bang to the Present that includes the continental movements through the last 500 million years, see www.bbc.co.uk/education/rocks/flash/indexfull.html.
Each continent rides on top of one or more of the moving plates. Sometimes a continent is sitting on top of two adjacent plates that begin to move apart due to an upwelling of mantel material, as seen in the animation Plate Tectonics II at www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp20/20_keypoints.mhtml. This causes that continent to become torn or split into two pieces which then begin to move away from each other. This is called a continental rift. About a billion years ago, a rift began to develop around the Great Lakes region of the U.S., but then stopped. The Rio Grande, Red Sea, and East African rifts are more recent developments.
In your kitchen you can see little "continents" moving, splitting, and colliding by placing two small separate drops of liquid oil on top of a pan of water and then slowly heating the pan on a stove. You'll see that some regions of the water will be rising while others are sinking. You might also place a line of pepper on top of the oil and then watch the pepper-line become split and dragged around the surface.
When a continent is split and its pieces begin to diverge, an ocean may develop between those former pieces. For example, about two hundred million years ago, the South American and African continents were not separated by an ocean but were adjacent to each other; still today, the shapes of their coastlines are similar. The plates move so slowly that it has taken about two hundred million years for the Atlantic ocean to widen by 2,500 miles (4,000 km). These two continents will eventually be pushed far enough apart that they will collide with other continents. The positions and widths of the oceans have continually changed through time.
Along the entire centerline of many oceans, the original rifts continue to grow into humongous, underwater mountain chains larger than the Himalayas and longer than the Andean-Rocky-Mountain system. The mid-oceanic ridge averages 4,500 meters (13,500 feet) above the sea floor and 2,000 kilometers (3,200 miles) in width. The entire ridge system is 50,000 kilometers (30,000 miles) in length and is the largest feature on the Earth; yet it is all underwater. You can see map of the mid-oceanic ridge system at the USGS website at http://pubs.usgs.gov/publications/text/developing.html and an animation of sea-floor spreading at the University of University of Ohio website at http://jrscience.wcp.muohio.edu/movies/sea-floor-spreading.mov. See also the animation at www.visionlearning.com/library/flash_viewer.php?oid=1683.
As one plate moves, it might scrape past an adjacent plate with a stick-slip motion or it might instead directly collide with its neighboring plate. During a collision, one of the plates is driven underneath (it is said to be subducting) while the other is pushed up and over. Through several million years, such a collision often pushes up a section of Earth into a new mountain range. It then takes another fifty million years for the erosive effects of the wind, ice, and rain to wear down that mountain range. There have been many such cycles of mountain formation and erosion throughout the Earth’s 4.5-billion-year history. New mountains are rugged, tall, and steep, like the Sierra Nevada, Andean, European Alps, and Himalayan ranges, while old mountains have been worn down in size, like the Australian Alps and the Appalachians. You might like to visit the University of Leeds website at http://earth.leeds.ac.uk/dynamicearth/himalayas/india/animation.htm to view an animation of the Himalayan-producing collision of India with Asia. For more information about the Appalachians, visit http://pubs.usgs.gov/gip/birth. The Appalachians were pushed up a few hundred million years ago when Europe, America, and Africa collided.
The moving, rising, and subducting plates cause earthquakes in several ways. For example, cool brittle subducting oceanic plates can fracture. Sometimes solid minerals from the crust can be carried down to a great enough depth that they suddenly melt and cause a movement due to their decreased rigidity. Subducted minerals can also suddenly become mixed with water that lowers their melting point, resulting in an earthquake. Laboratory measurements of rocks subjected to large pressures have shown that the decrease in melting point occurs at the pressures that are found at a 150-kilometer depth within the Earth. For more information, visit http://pubs.usgs.gov/gip/earthq1.
The tectonic plates move at the speed of about one inch (2.5 cm) per year, which is about the same speed at which your fingernails grow. This slow speed is hard to see with your eyes but is easy to measure with ground-based devices that bounce laser light off satellites orbiting the Earth. At this speed, every fifty million years or so, a continent can move to a very different location on the planet. Sometimes a particular continent may get carried from an equatorial position to a more polar position, causing its climate to change.
Through the last 4.5 billion years, most every continental region has spent time submerged under oceans and uplifted into mountains and has experienced climate ranging from glacial to desert to rain forest. Plate tectonics slowly changes a regions location, with the result that every few million years, the regions climate will have changed. In turn, these slow changes in climate result in slow changes in the successive generations of plants and animals living on each continent. Tectonic plate movements have a slow effect on climate and species, but the weather changes by the day and the season.
Energy from the Sun drives the daily weather and the seasonal climate of the Earth (You might like to view NASA’s Pulse of the Planet at either http://svs.gsfc.nasa.gov/vis/a000000/a002300/a002395/index.html or http://svs.gsfc.nasa.gov/search/Keywords/SIGGRAPH.html for music-accompanied plots of the dynamic Earth.) Energy from the Sun drives also feeds all of the life on Earth as plants use its energy to make organic molecules. The Earth's outer atmosphere, above the clouds and weather, receives about fourteen hundred watts per square meter from this sunlight for a total of twenty million-billion watts. This is equivalent to 200 million-million, 100-watt light-bulbs and is 20,000 times as much power as the ten trillion watts used by our entire civilization of six billion persons–or 100,000 times the power used by the people of the United States. (For further comparison, a single hurricane generates ten times the power used by the United States while a one-megaton nuclear bomb would supply only one-ten-thousandth of the U.S. needs.) How much power is one hundred watts? It is the power needed to lift fifty kilograms up a distance of two meters every second–or in English units, one hundred pounds being lifted six feet every second. The operation of your body requires about one hundred watts of power. This power is also obtained from the sun, no matter if you eat plants directly or if you eat animals that have eaten plants.
To see the cause of the seasons, imagine the Sun and the Earth as two spheres placed on a table, each with a line painted around its center or equator. The Earth is placed to the right of the Sun. The center line of the Sun is parallel to the table top but the Earth must be tilted so that its center line points upward by twenty-three degrees from the horizontal table top. The Earth's axis remains tilted this way as it orbits the Sun. Light coming from the Sun always shines more directly onto the equatorial region but glances along the polar regions. One pole is tilted toward the Sun but the other is tilted away from it. This makes the regions near the equator warmer than the polar regions. Six months later, the Earth has traveled halfway around the Sun. In terms of our table-top model, we would slide the Earth around the Sun, being careful not to change the orientation of the Earth's center line, until it is on the left side of the Sun. The pole which had been tilted toward the Sun is now tilted away from it. This means that each pole drastically cools for a few months of each year while it is receiving no sunlight. The north pole receives no sunlight at all around January, while the south pole is in the dark around June. Visit www.astro.uiuc.edu/projects/data/Seasons/seasons.html to view an animation of the orbital-caused seasons.
As air circulates around the planet, it cools whenever it moves into a sunless, polar region and heats while in daylight, especially whenever it travels near the equator. The same thing happens to ocean water as it travels around the planet, moving between equatorial and polar regions. Water circulates around the world’s oceans following paths largely determined by the current positions of the continents. Circulation patterns change as the positions of the continents change. When ocean currents flow from the equator toward the poles, they carry heat that warms the poles (air movements have a smaller role in moving heat from the equator toward the poles). The ability of ocean currents to move equatorial heat pole-ward changes as the location and shape of the continents change through time. A plot of global ocean currents is shown at the NASA website http://vathena.arc.nasa.gov/curric/oceans/drifters/topo_arr.gif, which can also be reached by clicking on the map shown at http://vathena.arc.nasa.gov/curric/oceans/drifters/ocecur.html. A 3-D movie clip showing warm water moving away from the equator, cooling at the north pole, and returning to the equator as cold water, can be seen at http://svs.gsfc.nasa.gov/search/Keywords/OceanCirculation.html. For other satellite images of the oceans, you might visit http://topex-www.jpl.nasa.gov/science/jason1-quick-look.
Climate changes as temperatures and the distribution of rainfall changes. The annual amount of rainfall in a region is sometimes altered because the incoming air currents shift from passing over land or mountains to instead passing over oceanic areas. As mountains are built through a several-million-year time-span, the surrounding region’s climate changes. One such climate change is of special importance in our own biological history: A continental rift developed in Africa about fifteen million years ago, resulting in the formation of a new mountain range that changed the area's rainfall patterns. As Eastern Africa become drier than Western Africa, the ensuing changes in its plant and animal life led to a type of ape that walked upright. It also occurs that as mountains are worn away through a fifty-million-year time-span, the regional climate again changes.
The Earth’s climate is also affected by the number of volcanoes around the world that are active at the same time. In the year 1815, a large volcano threw a lot of dust into the atmosphere that in turn blocked enough sunlight to cancel that year's summer. Larger volcanoes can cause several summers to be skipped and trigger a glacial advance, as has occurred in the past. In the Philippines in 1991, the volcanic eruption of Mt. Pinatuba released dust that blocked enough sunlight to cool the Earth's air by 0.5 centigrade degree (one degree Fahrenheit), see http://svs.gsfc.nasa.gov/search/Keywords/Mt.Pinatubo.html. For an explanatory animation of a volcano, visit www.sci.sdsu.edu/volcano/volcano.mov. A very large volcanic eruption that occurred 71,000 years ago may have caused a six-year-long winter and triggered a 1,000-year-long ice age, as described at www.ngdc.noaa.gov/paleo/ctl/clihis100k.html..
The Earth’s climate is first of all driven by the Sun. Variations in the energy output of the Sun have been directly measurable for only the last twenty years. Satellites have found a 0.1% change. The Sun's magnetic activity varies with an eleven-year cycle and also a 100,000-year cycle. Mukul Sharma found the longer cycle when studying changes in the amount of radioactive beryllium-10, which has a 1.5-million-year half-life, produced on the Earth by cosmic rays. This amount depends both on the magnitude of the Earth’s magnetic field and on the magnetic activity of the Sun.
Several astronomical factors affect the Earth's climate. The 23-degree tilt of the Earth relative to its orbital plane results in the yearly seasonal changes with which we are all familiar. But the tilt of the Earth varies between 21.5 and 24.5 degrees on a 41,000-year cycle. The elliptical orbit of the Earth around the Sun slowly alternates between being more circular and less circular, and this 100,000-year cycle changes the distance of Earth's closest approach to the Sun and so changes the maximum amount of received sunlight. All these cycles are simultaneously occurring. Sometimes their effects cancel, sometimes they combine into a heating trend, and sometimes the combine into a cooling trend. The Earth’s temperature necessarily follows the net effect, but these astronomical variations alter only slightly the amount of received sunlight. Asteroid and comet collisions can instantly and drastically alter the Earth's climate.
When asteroids having a diameter of one kilometer (0.6 mile) or more collide with the Earth, the collision might throw enough dust into the air to cause darkness for over a year. In addition, several years would elapse before the dust would settle back to the ground and stop blocking sunlight. The darkness and cold would cause the death of many plants and in turn, result in the death of the animals that eat the plants–and in the animals that eat these animals. Such a sequence of events might lead to the extinction of a large portion of the species of life on the Earth. In fact, there have been five such extinctions where 50% to 90% of the Earth's species have suddenly disappeared. (Visit http://www.arkive.org for lists and video of today’s endangered species. Visit www.unesco.org/culture/worldreport/html_eng/stat2/table30.pdf for an international comparison of numbers of endangered species.) These extinctions may be due to asteroid or comet collisions, or they may be due to rapid changes in climate. A large crater off the Mayan peninsula is a record of a collision that might have caused the extinction of the dinosaurs. Visit http://sherpa.sandia.gov/planet-impact/asteroid/movies/aster_vr2.qt for an animation of a meteor hitting the ocean near New York City. For an animation of the propogating tsunami from a meteor impact, visit http://es.ucsc.edu/%7Eward/eltanin_small.mov. For an animation of the kt-impact, visit www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_mov.html. Throughout the surface of the Earth, a buried layer of iridium (a component of comets) has been found that dates to the time of the disappearance of the dinosaurs. Scientists estimate that the energy from this collision was about 10,000 times larger than the energy from all of the world's nuclear weapons. About once per year, a much smaller but still massive 20,000-ton asteroid burns up in the atmosphere. In the year 1908 an extensive forest area of Siberia was damaged by an explosion of unknown origin, speculations concerning its cause range from antimatter to asteroid impacts.
Individual species also come and go through time, typically lasting a few million years. A single species will become extinct whenever the climate changes too rapidly or too drastically or if its food sources, predators, or competitors change too rapidly. There might be competition for food by other species that are better suited to the current environment. Changes in plants and animals–that is, evolution–is driven by these changes. The climate changes less in the oceans than on the land so that there has been less change in ocean life. Ocean life changes slowly as food finds new ways of avoiding predators and as predators find new ways of finding food.
Scientists have gathered millions of facts enabling them to piece together the picture of past continental positions and the past location and period-of-existence of the plant and animal species on those continents. Working region by region, this has been a large task. Evidence for the past positions of the Earth's plates has been gathered in many ways and by many scientists. The first clue that the continents were moving around the planet's surface came from the simple observation that the coasts of Africa and South America have similar shapes. If one could push them back toward each other, they would fit together like two jigsaw-puzzle pieces. We have seen that geological layers and their fossils can be dated using relative and radiological means. In another approach, geologists and paleontologists have run around the planet noting each region's geologic formations and its species of fossilized plants and animals. As a continental tear or rift develops, it begins to spread apart the area's previously unbroken geological formations and to separate the area's plant and animal species. The rift continues to separate the two areas for millions of years so that scientists today see them separated by a large distance. The similarity of the geological formations and the fossilized plant and animal species on both sides of the rift indicates that the two regions used to be connected. As two continents move away from each other, an ocean often grows in between them.
A mid-oceanic ridge is built as molten rock emerges from the mantle. The recently emerged rock is continually pushed away from the ridge by yet-newer material emerging behind it. This creates new ocean floor and pushes originally-adjacent continents apart, widening the ocean, while the ridge stays in place and grows. The pushing goes on for one or two hundred million years until another tectonic collision occurs.
The histories of the ridges are deduced from measurements of their magnetism. To see this, consider that the atoms of the emerging, molten rock act like little magnets. As this rock cools, its atomic magnets will line up with the current direction of the Earth's magnetic north pole. But it has been found that the Earth's magnetic field occasionally flips its orientation; In fact, the dates have already been determined in which 170 of these reversals took place through the last 75 million years. The result is that the atomic magnets within newly-emerged rock will point in the opposite direction compared to rock that had emerged when the Earth’s magnetic field pointed in the opposite direction. The atomic-magnet orientation of the emerged rock alternates many times as one moves outward from the rift's location. Paleomagnetic data is available from the National Geophysical Data Center at www.ngdc.noaa.gov/seg/geomag/paleo.shtml, and a figure showing a series of reversals as the plates diverge can be seen at http://pubs.usgs.gov/publications/text/developing.html.
The histories of mid-oceanic ridges are also deduced from measurements of their density because heavier basaltic rocks erupt where crust is pulled apart. Mantle basalt has a higher iron content and a higher density than crustal material (we have seen that the proportion of iron increases toward the Earth’s center). Measurements of a region's density detect this difference in materials. The heavier basaltic ocean floor upwells along mid-oceanic ridges and then moves slowly toward the continents. After a 100-million-year journey, this heavier basaltic rock subducts underneath the lighter continents that keep floating above this material. Deep sea trenches may form along the line where oceanic crust is forced back down into the mantle. For example, the Mariana trench in the Pacific Ocean is more than 11,000 meters (33,000 feet) deep. The continents are 4.5 billion years old, but the ocean floors are never more than about 200 million years old because they are continually opened and submerged. By the way, ocean floors are more often rocky, not sandy, except near the shore.
The past positions of continental shorelines can be deduced from the horizontal movement of beaches, whose sand forms as a strip of quartz ground up by crashing waves. Near a shoreline there is coarse-grained sand, while a little way offshore there is sand and sandstone. Mud and shell is found farther out. Farthest from shore is a line of limestone, which consists of calcium carbonate accumulated from seashell skeletons. As sea levels rise and fall, this series moves horizontally as it follows the juncture of land and sea. Offshore sediment accumulates at the rate of one centimeter (one-half inch) every five to fifty thousand years. Some information about the coastal geology of the Southeastern U.S. seashore can be found at the NOAA website at http://www3.csc.noaa.gov/beachnourishment/html/geo/geo.htm.
The total amount of water on the Earth has changed little through time. It is mainly found in the oceans or locked up in glacial ice, only 3% is held within rivers. But scientists have found that the sea level rises and falls through time as the volume of the Earth’s glaciers change due to global temperature changes. (The height of the sea is also affected by the volume of the oceanic ridges.) As glaciers appear, grow in time, and then melt and disappear, the Earth's ocean level rises and falls. Whenever there are large glaciers then there will be less water in the oceans. As glaciers have advanced and retreated, the ocean levels have been found to raise and lower by a 300-yard (300 meter) vertical distance. It has risen by 120 meters (120 yards) in the last 20,000 years. When the glaciers melt, a lot of water is temporarily held in many lakes. These lakes occasionally burst free, releasing huge torrents that create instant canyons. About 8,000 years ago, a single Canadian lake-burst released enough water to raise the level of the ocean by 20 to 40 cm (4 to 8 inches).
In the last two million years there have been about twenty cycles of glacial advance and retreat. These cycles have roughly followed a 100,000-year period and may be related to the 100,000-year cycle in the Sun’s magnetic activity. Within this more-lengthy period occurs shorter cycles of glaciation. In Northern Europe, some of these have been dated to 75, 65, 59, 40-29, 19-13, and 11-7.5 thousand years ago. Some relatively recent, large temperature changes occurred about 14,000, 11,500 and 7,600 years ago. Smaller and more recent changes include a warmer period during the years 900 to 1200 ad and a cooler period from 1450 to 1850, during which time Holland's canals and London's Thames river would freeze–the last time was in 1814. In the sixteenth century, Spanish Conquistadores encountered snow in Mississippi.
During a glacial maximum or “ice age,” summer temperatures are ten to twenty degrees Fahrenheit (five to ten degrees Celsius) colder than they are now. Four of the ice ages that occurred in the last one million years–the Nebraskan, Kansan, Illinoian, and Wisconsinan–were named for the most southerly advance of year-round ice. Sometimes the northern, year-round glacial region extends down to Britain and Kansas, while at other times there is no region of year-round ice anywhere on the planet. Since there is less year-round ice today, we are living in a relatively mild phase of an ice age. You can see the glacial retreat during from 18,000 to 8,000 years ago in the Midwestern U.S. by visiting www.museum.state.il.us/exhibits/larson/content.html or www.museum.state.il.us/exhibits/ice_age/laurentide_deglaciation.html for the North America retreat. For an overview and movies, see http://jesse.usra.edu/articles/iceagemodule/iceagemodule-paper.html.
The last glacial advance began 130,000 years ago, peaked 20,000 years ago when one-third of the Earth's land was covered by ice, and has been in a steady decline since that time. This glacial extreme created a land bridge between North America and Asia and allowed many groups of us humans to migrate from Asia into the Americas. As the glacier retreated, the land bridge became submerged under the ocean. This process can be seen in the video Postglacial Flooding of the Bering Land Bridge: A Geospatial Animation made by the Institute of Arctic and Alpine Research at the University of Colorado. This video is available at their website
http://instaar.colorado.edu/QGISL/bering_land_bridge. The institute also shows a plot of sea level changes through the last 20,000 years at http://instaar.colorado.edu.QGISL/bering_land_bridge/blb_overview.html. Rising ocean levels submerge flat islands and cause higher, coastal land to be stranded out at sea, as was the case for Britain. Visit www.pastperfect.info/sites/lowhauxley/climate/coastalclip_c.html to see the ocean rise from ten to five thousand years ago, stranding Britain out to sea. In the distant future, the ocean level will again fall and Britain will rejoin the continent. The sea level changes with the Earth’s temperature.
The age of the glacial advances and retreats are determined in many ways. For one, as a glacier retreats it exposes underlying rock to cosmic radiation. The radiation builds up through time and can be measured to deduce how much time has passed since the glacier retreated. Coral reefs always remain near the surface of the ocean, growing upwards and downwards as the level of the ocean changes; a history of their height reveals ocean levels and hence glacial volumes through time. Ocean levels and glacial volumes also give information about past temperatures, as do past longitudinal distributions of warm- and cold-water plankton.
Geologists and paleontologists have pieced together a history of the Earth's past temperatures in many ways. Information is obtained from the types of plants and animals that are found in geologic layers because the temperature range of each species is known. Insects are especially useful in this way because each species lives in a particularly limited temperature range. If a ten-million-year-old geological layer contains seeds from palm trees rather than pine trees, then we know something of that region's past climate. It is known that in tropical regions, with high temperatures and high rainfall amounts, leaves are broad instead of narrow and have smooth instead of jagged edges. The ratio of broad-to-narrow and smooth-to-jagged-edged leaves indicates past temperatures.
Many websites contain information about the flora and fauna of past geological epochs. Jonathan Adams has constructed maps of the vegetation found in each of the Earth’s continents at the time of the last glacial maximum 18,000 years ago, see www.esd.ornl.gov/projects/qen/adams1.html. He also shows plots of vegetation cover ranging from 150,000 to 500 years ago. The National Climate Data Center (NCDC) of the National Oceanic and Atmospheric Administration (NOAA) maintains the World Data Center for Paleoclimatology at www.ncdc.noaa.gov/paleo/data.html. This website has information about climate and plants, mammals, and insects of past ages obtained from corals, glacial ice cores, and pollen samples. The current land cover of the U.S. can be seen at http://geography.usgs.gov/www/products/geoface.html.
The annual layering of snow, dust, and pollen tells us much about past temperatures and climate. Pollen is released each spring; some of it forms annual layers on lake bottoms, giving yearly records of the species present at the time each annual layer was formed. The ratio of fern spores to pollen grains indicates the mix of species that had been present. Annual pollen layers are also found just offshore along seacoasts. (We'll see in Chapter 12 that this fact was used to determine the changes in rainfall occurring at the time that a group of us humans first began to become full-time farmers in ancient Mesopotamia.) Through time, the accumulated weight of the dust and organic material that has settled onto lake bottoms begins squeezing the earlier, lower layers into so-called sedimentary rocks. We know that the Earth's oceans are at least 3.8 billion years-old because the oldest known sedimentary rocks were formed that long ago. Snowfalls are similarly layered annually on top of glaciers, giving information about winter-to-summer temperature and precipitation changes. The chemicals found in trapped glacial-air also gives a record of past atmospheric contents. For an example of what can be learned about fauna and climate from lake-bottom layering, see the USGS website at http://pubs.usgs.gov/fs/fs-0059-99. It. contains a report of the mineral contents of the annual layers of Elk Lake in Minnesota. This lake bed provides a 10,400-year record of terrestrial vegetation, windiness, and moisture levels that shows how this now-forested region experienced dry, warm, windy, dusty, sagebrush covered prairie conditions during the time span from 8,000 to 4,000 years ago. You can see a number of ways in which past temperatures are deduced by visiting www.ncdc.noaa.gov/paleo/proxies.html.
Limestone is composed of dead sea-shells that in turn consist of dissolved seawater components, including oxygen. The percentages of oxygen isotopes in limestone indicate past ocean temperatures. Oxygen always contains 16 protons but it can have 16, 17, or 18 neutrons. Oxygen having 16 neutrons weighs less and so evaporates more readily than do the two heavier types. The ratio of oxygen-18 to oxygen-16 decreases 0.02% for each centigrade-degree increase in water temperature. This is used to find how past water temperatures changed with ocean depth and how past temperatures changed with latitude from the equator to the poles.
There are a large number of factors affecting the amount of glaciation on the Earth's surface. The USGS website has information on the extent of glaciation at http://pubs.er.usgs.gov/pubs/fs/fs13399. Mile-thick glaciers cannot accumulate on the ocean surface but can build on mountain tops. As the number of mountain ranges on the Earth varies through time, the total volume of water held in mountain glaciers also varies through time. Continental glaciers can occur whenever a continent is located at a pole during a time of both cool temperature and high precipitation at polar latitudes; glacial buildup most rapidly occurs in the presence of warm equatorial oceans with pole-ward currents. The continental positions, shapes, and groupings also determine if sea currents can carry warm water from the equators toward the poles. The amount of sunlight that reaches the ground to be absorbed by the Earth’s surface depends on how much is reflected back into space. The reflected portion of solar energy depends on the distribution of land and sea by latitude, the percentage of ice-covered and cloud-covered land, the nature of the land surface (seas absorb sunlight while ice and desert reflect it), and the composition of volcanic dust held in the atmosphere. Through the last few centuries of industrialization, we humans have been altering the chemical composition of the atmosphere sufficiently to change its absorptive and reflective properties. Many man-made chemicals, from carbon-dioxide to soot, are contributing to global warming. A presentation of the factors in global warming, including a plot of “Atmospheric Carbon Dioxide Concentrations From Ice Cores 1734 - 1983," is given at www.fsl.noaa.gov/visitors/education/climgraph.The NASA/Goddard Space Flight Center, the SeaWiFs Project and ORBIMAGE, and the Scientific Visualization Studio have created the video SeaWiFS: NASA Carbon Cycle Initiative showing the seasonal changes in global atmospheric carbon-dioxide levels along with changes through the last fifty years and the last one thousand years. To view the clip online, visit http://nix.larc.nasa.gov/info and search for SeaWiFS: NASA Carbon Cycle Initiative. Visit www.pnl.gov/atmos_sciences/snowmovie.html to see a comparison of snow-accumulation with and without a doubling of the atmospheric carbon-dioxide level.
The glaciation found today in the northern and southern poles developed through two different continental movements: the separation of the Antarctic continent and the joining of North and South America. About thirty-six million years ago, the continents of South America and Australia separated from the Antarctic continent, which then drifted toward the southern pole. In addition, a circumpolar oceanic current developed that allows little equator-to-pole heat movement, see http://topex.ucsd.edu/marine_topo/video/acc.mov. The polar location of the Antarctic continent and the circumpolar oceanic current both enabled the southern ice cap to develop. The glacier of Antarctica averages 6,500-feet in thickness (2,000 meters) and account for 90% of all glacial volume. Greenland’s glacier accounts for another 9% of the total volume and mountain-top glaciers account for the remainder. The amount of water held in these glacial regions can be found at http://ga.water.usgs.gov/edu/watercycleice.html. (For a 3-D view of the Martian polar ice cap, see http://ltpwww.gsfc.nasa.gov/tharsis/agu_f98.html.) Mile-thick glaciers can not form on top of the ocean surface. The sea-ice found at the North Pole of the Earth is only three meters (ten feet) thick. During the summer, it becomes 30-centimeters (one-foot) thinner. The sea-ice at the southern pole is only half that thick because it spreads more easily across the wide-open ocean. For more information, visit the NASA websites at http://rst.gsfc.nasa.gov/Sect14/Sect14_14.html and http://svs.gsfc.nasa.gov/stories/arctic. The NOAA website at www.gfdl.noaa.gov/~kd/KDwebpages/NHice.html has an animation of the changes in sea-ice thickness through the last fifty years. Many other animations are found at http://icesat.gsfc.nasa.gov/animations.html. You might also like to check the webcams placed at the north pole, see www.arctic.noaa.gov/gallery_np.html, and the south pole, see www.cmdl.noaa.gov/obop/spo/livecamera.html. The amount of sea-ice occurring at the north pole was indirectly enhanced by the joining of the North and South American continents, which happened about three million years ago, see http://piru.alexandria.ucsb.edu/collections/atwater/emvc/emvc0001.mov. When these two joined, they blocked an east-west flowing ocean current. The gulf stream that developed in its place carries moisture-laden air that increases precipitation near the north pole.
For most of the Earth's history, the average year-round temperature throughout the planet was about 70 degrees Fahrenheit (20 degrees centigrade). There were no areas of year-round ice because wintertime temperatures were not much less than summertime temperatures. In fact, with no polar continents it didn't often get cold enough to snow even during the winter–as occurs today. Under those conditions, each region of the Earth received similar amounts of rainfall so that there were no extensive desert areas. Today, the deserts of the Earth cover about 20% of the surface area and reduce the amount of available farm land. During much of the past, the lack of desert regions meant that a larger portion of the Earth's surface could have been farmed (except that no humans were yet there to do the farming). Today’s year-round temperature is not so comfortable because we are in the middle of an ice age. We might be stuck with this cold weather, with regions of glaciers and deserts, for as long as Antarctica is parked over the south pole and for as long as there is decreased heat flow from the equator to the poles.
We see that there are a large number of variables affecting the Earth’s climate and temperature and that all these variables are simultaneously occurring. Each variable is at some time trying to cool the Earth while at other times it is trying to warm the Earth. If enough of the variables are in a warming phase then the Earth's temperature will increase, otherwise it will decrease; sometimes a temporary balance exists. About ten astronomical cycles are simultaneously contributing to the heating and cooling of the Earth. The two main factors are the heat output of the sun and the heat-holding properties of the Earth’s atmosphere, which raises the Earth’s surface temperature by fifty degrees Fahrenheit (twenty-degrees Celcius) as explained at www.meteor.iastate.edu/gccourse/chem/evol/evol_lecture_new.html. In addition, the reflective and absorptive properties of land, sea, and sky change as continents drift from equatorial to polar locations, as ocean currents allow or do not allow heat movement from the equator toward the poles, as the amount of cloud cover and the proportions of desert- and ice-covered land changes, as the number of dust-emitting volcanoes changes, as the chemical composition of the atmosphere changes and as manmade chemicals are added to the atmosphere. See http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsGHGEmissions.html for an inventory of greenhouse gas emissions and sinks. For information about worldwide carbon dioxide emission, visit www.eia.doe.gov/emeu/international/gas.html#IntlCarbon.
We don’t want to make a too-hasty conclusion about manmade global warming, but at the same time there is no reason to gamble needlessly with the Earth's ecological balance. Recent and cautionary data obtained from glacial drilling experiments show that a global transition from cooling to warming can occur in a time-span of just a few decades. Our environmental impact could go as far as to cause a massive extinction of species. Will the next generation of citizens decide to end pollution by requiring that each factory, home, and car emit nothing at all into the environment? Scientists and engineers would be thrilled to tackle the problem of designing factories, cars, and homes this way. Are factory owners willing to add to the original cost of building a plant and are consumers willing to pay more for products, or should we just spoil our own environment? Zero-emission designs mean that each factory will collects its waste products for use by other industries and that cars will have both a fuel tank and a waste tank. Can you design a tank that collects the exhausts of an automobile engine, and can these exhausts be exchanged at the gas station for further processing? By the way, our car engines consume about fifteen pounds of atmospheric oxygen to burn each pound of gasoline. The combustion products consist of water, carbon dioxide, nitrogen and nitrogen oxides. The atmosphere consists mostly of nitrogen (78%), oxygen (21%), and small amounts of argon, carbon-dioxide, and other gases (1%).
After talking about the effects of us humans on our own environment, let’s talk about the direct effects of our environment on our appearance. We do this by taking a brief digression on physics to help explain the size, shape, and color of objects which are the most effective at repelling excessive heat or cold. A warm object cools by releasing heat through its surface. An object retains warmth if it has lots of volume but little surface area. For this reason, the size and shape of humans varies with climate. At the warm, sunny equator we are tall and thin so that we are more easily cooled. In polar areas, we retain more heat by being shorter and rounder so that we have more volume and less surface area. We humans are also found to have darker colored skin when living near the equator and lighter colored skin when living near the poles. Dark skin helps reflect sunlight–especially ultraviolet light–while light skin absorbs more sunlight. Light skin also avoids the rickets caused by a lack of sunlight–as occurs in northern, cloud-filled latitudes. The color of our skin shows our relationship to the Earth and its latitudes. One anthropologist made a globe of the Earth with each region colored like that of local human skin. Humans are also about 10 percent smaller in height and size whenever we live in year-round highly-humid locations because the decreased sweat evaporation makes a larger volume harder to cool.
Liquids and gasses in the development of life on Earth
Chemical reactions occur more rapidly in a liquid medium than they do when lying on the ground. An energetic environment, as existed in the early periods of the Earth's formation, also enables chemical reactions to occur more quickly. Still, organic compounds have been detected in Halley’s comet, in meteorites, and in interstellar space. Life occurs as organic molecules combine into larger units. Since chemical reactions occur more rapidly in an energetic, liquid environment, the Earth's life began in the ocean. Microbial, pre-life forms, consisting of large numbers of molecules but no cellular structure, are likely to have developed in the Earth's water within some tens of millions of years after its formation. (About four billion years were needed for microbial life to develop into multicellular forms that were large enough to leave easily-found fossil remains.) Since the Earth’s life developed in water, life might also develop on other liquid-covered planets and moons. Water has been found to exist in limited amounts on Mars and on the Moon. If water existed for a few tens of millions of years in an energetic and chemically-rich environment on Mars, then it is possible that microscopic life developed there, too. We already have spacecraft looking for signs of life on Mars and on Saturn’s moon Titan. You might visit the NASA website at http://observe.arc.nasa.gov/nasa/gallery/movie/Solar_System/solar_1.html. It includes a movie of Titan and some images of possible Martian microscopic fossils on a meteorite that originated from Mars but was found in Antarctica. We will soon make expeditions to find out whether microbial life has existed on Jupiter's moon Europa, which might have liquid underneath a frozen surface. Visit
http://jrscience.wcp.muohio.edu/movies/veuropa1.mov to see a video of rotating Europa. For a zooming closeup of its surface, see http://jrscience.wcp.muohio.edu/movies/europa.chaos.19.1.5.mov.
There was little atmospheric oxygen for the first half of the Earth’s history. Four billion years ago, the Earth's atmosphere was mostly carbon-dioxide (the stuff in soda drinks). As plant life occurred, it began converting carbon-dioxide into oxygen, which then accumulated in the atmosphere. Oxygen breathing reactions are more energetic and can power multicellular life forms. The Earth's life forms began to take advantage of oxygen soon after it had become consistently available.
The build up of oxygen is determined from the appearance and disappearance of certain chemical minerals. For example, the two minerals Pyrite and Uranite rapidly convert into other chemical forms when they encounter oxygen. These two minerals stopped occurring about two billion years ago, indicating that oxygen began occurring at that time. The relative amounts of two states of iron, which we'll call iron-2 and iron-3, also provide information about the appearance of atmospheric oxygen because the iron-2 form oxidizes into the iron-3 variety. It is found that iron beds older than two billion years contain more iron-2, while newer beds contain more iron-3. More evidence of increasing atmospheric oxygen is given by the fact that no red oxidized sandstone beds occur before about two billion years ago.
The Earth formed through the gravitational accumulation of the debris that was already in the area. We have seen some evidence for the Earth's moving, tectonic plates and for its fluctuating climate. The main point of this chapter is that the global position of each continent slowly changes through time and that their climate changes as they move from one region to another, or as they block and unblock heat-moving, north-south ocean currents. In turn, this causes changes in each region's plant and animal species.
Now that we have described the Earth's origin and the reasons for its changes in climate, we will next describe its life forms and see how they change as climate changes. The next chapter begins with a description of the physical nature of the molecules of life and ends with a summary of the sequence of animal types that have occurred on the Earth.
Questions
1. The news often discusses global warming. Measurements show that in the last century, the Earth’s temperature has increased by one degree centigrade (two degrees Fahrenheit). What should we measure in order to determine if humans are causing this to occur?
2. What causes ice ages and warm periods? How long do they last? How quickly does an ice age develop or end? How much warmer or cooler is the temperature during these periods than it is today?
3. Can we predict earthquakes?
4. How would the Earth's distribution of plants and animals be different if the continents never moved? Suppose the continents moved completely around the Earth seven times per year while moving in figure-eight patterns. Describe your region's annual temperature changes. How would this affect the type of trees and animals in your region? What if the continents circled the Earth once every one-hundred years?
5. Write down the equations for relativistic plate tectonics. (These would be useful if the continents circled the Earth seven times per second.)
6. How would life be different if we had two-hour days instead of 24-hour days? We never see the backside of the Moon because it always presents the same face toward the Earth. From the surface of the Moon, this means that one side always faces the Earth while the other side always faces outward into space. If the Earth spun about its axis only once during the time needed to make one orbit around the Sun–that is, if each day lasted a year–how would life vary from one side to the other and along the light-dark boundary? How would the Earth's distribution of plants and animals be different if one side of the Earth always pointed toward the Sun throughout the year? What if the Earth then flipped its orientation every 50 (or 500 or 5,000) years so that the opposite pole faced the Sun? If the region of the border between light and dark didn't move as the flips occurred, this region would provide the most stable home. A portion of the observed stars systems have more than one Sun so that days and nights occur in more complicated intervals on its planets than happens on the Earth. How would this change the Earth's life? The output of many stars varies by a factor of two through periods of several months. If our Sun was such a variable star, how would that affect life on Earth? By the way, the planet Mercury has but three days every two years and a sufficient difference between closest and farthest orbital distances that, from the surface of Mercury, the Sun’s apparent size grows and shrinks through the year.
7. Do you believe the rumor that the dinosaurs were wiped out by alien sports hunters using iridium bullets? Do alien zoos still contain any of the Earth's dinosaurs? Do aliens have movies of the dinosaurs? Will we be able to clone dinosaurs? Would a pair of cloned dinosaurs know how to behave if they had no parents to teach them how to be dinosaurs? Were dinosaurs raised by their parents? Are today's reptiles raised by their parents?
8. How will our civilization change when the next glacial advance or retreat occurs? And the next?
9. List some physical, chemical, geological and astronomical things that can affect the rate of appearance of new species.
10. What sort of plants and animals existed in your area one million years ago? 50? 500? 1,000?
11. Each sudden extinction of most of the Earth's species has been followed by an increase in the variations of the remaining species. If we humans are currently causing such an extinction, what sort of species will replace those that will become extinct? How will we live if it occurs that we are the only remaining species?
12. If each species is well-matched to just one climate, what is the climatic range of our crops? How quickly can this be changed?
13. What has enabled us humans to live in each of the Earth's climatic regions?
Primary source for the chapter
A Short History of Planet Earth, Mountains, Mammals, Fire, and Ice by J. D. Macdougall, 1996, John Wiley and Sons New York.
Suggestions for further reading
Geologic Time Don L. Eicher second edition, 1976, Prentice-Hall Inc Engelwood Cliffs, New Jersey.
Paleoclimate and Evolution with Emphasis on Human Origins, Elisabeth S. Vrba et. al., 1995, Yale University Press, New Haven.
Why the Earth Quakes, The Story of Earthquakes and Volcanoes, Matthys Levy & Mario Salvadori, 1995, W W Norton & Co, New York.
Return to www.UsHumans.net Downloads page
Return to www.UsHumans.net Main page
Copyright © 2006 Robert Dalling, www.UsHumans.net