by Neil Sitron and Dan Goldstein
History of the Earth's Layers
The Earth, as we know it today, consists of three separate states of matter: solid, liquid, and gas. They are better known as land, sea, and air. Initially though, during its infancy, the Earth was a part-molten planet with very primitive and less clearly defined forms of each layer. The differentiation that occurred to bring about the modern day Earth took billions of years and involved several complex processes.
One must first examine the formation of land. At first, Earth was a large melt, from which a nickel and iron core developed. Eventually, a solid silicate mantle evolved. The mantle became the foundation from which the crust, or modern day land, was formed. To explain the crust's evolution, it was proposed that the crust of the continents originated from a particular mantle. The average continental crust has properties quite similar to those of granite, but the mantle has properties closer to ocean-floor basalt and chondritic meteorites. Through a process called "chemical fractionation", a concentration of relatively light elements distilled from the basalt mantle to form a granitic residue. This residue comprised approximately 7-8% of the original mantle. As the mantle underwent this process, numerous amounts of light elements including silicon, oxygen, potassium, sodium, aluminum, carbon, nitrogen, hydrogen, helium, and calcium rose to the surface to form a primitive crust. Through these elements, the most familiar rock-producing minerals of today were formed including alkali feldspar (KAlSi3O8 and NaAlSi3O8), quartz (SiO2), and pyroxenes (Ca [Mg, Fe] Si2O6). A complete list of these minerals can be found in Table 1.
(Batten & Dott, 126)
This original continental crust did not become stable until nearly 1 billion years after the separation of the mantle from the core. During its period of instability, Earth was much hotter and saw several times more radioactive decay than today. As a result, some crust would form, melt, and re-form again. As the Earth cooled down, however, this process ceased, and the current crust has been stable for roughly 3.5 billion years.
The evolution of the Earth's atmosphere poses more of a mystery than that of the crust. In an effort to develop an accurate explanation, geologists have decided to compare our present atmosphere with that of large planets, such as Jupiter and the meteorites. Planetologists believe that the larger planets, Jupiter especially, retained the greatest percentage of original gases from the initial atmospheres of the planets. Table 2 lists the components of Earth's atmosphere and compares them to Jupiter's and the gases found on meteorites. The meteorites are widely believed to resemble the early protoplanets, so studying their various gases provides important clues to the Earth's past.
(Batten & Dott, 128)
In analyzing the atmosphere's evolution, two popular hypotheses exist, both assuming that considerable free hydrogen and helium have drifted into space over time because of their small atomic masses. The great mystery that both of these hypotheses attempt to explain is the emergence of free oxygen, something that has not occurred on either Jupiter or the meteorites.
The first hypothesis is called the Photochemical Dissociation Hypothesis. Based on studies of Jupiter, it assumes that the original atmosphere contained the same primary gases that Jupiter does- ammonia, methane and water vapor. The original atmosphere contained no free oxygen nor did it contain ozone. As a result, high-energy, ultraviolet radiation could easily penetrate the atmosphere at will. One property of such intense radiation is its ability to trigger photochemical, or light-induced, reactions. The Photochemical Dissociation Hypothesis states that these reactions caused the evolution of the atmosphere through the following four step process:
Step 1: The ultraviolet lights combined with the water vapor to set
the hydrogen off into space and free the oxygen.
2H2O + uv light energy ----> 2H2 (freed into space) + O2
Step 2: The newly freed oxygen reacted with methane, forming carbon
dioxide and additional water vapor.
CH4 + 2O2 ----> CO2 + 2H2O
Step 3: The oxygen also reacted with ammonia, producing nitrogen and
4NH3 + 3O2 ----> 2N2 + 6H2O
Step 4: After converting the ammonia and methane to carbon dioxide and nitrogen, free oxygen began to accumulate as further dissociation of water vapor continued.
This hypothesis states that once the water vapor reacted with the intense radiation, oxygen was freed, allowing the creation of nitrogen and carbon dioxide. Together, these three elements form the main consituents of the modern day atmosphere.
The Outgassing Hypothesis presents an alternative to the Photochemical Dissociation Hypothesis. It states that most of the atmospheric gases arose from the interior by a process called "outgassing". Through outgassing, a gaseous transfer occurred between the Earth's interior and its surface, mainly through igneous activity. A perfect example of outgassing today comes from volcanoes. When volcanoes erupt, they expel steam, carbon dioxide, nitrogen, hydrogen and carbon monoxide into the air. So, according to the hypothesis, before volcanoes even existed, some other process occurred which launched gases such as nitrogen, carbon dioxide and water vapor into the atmosphere.
In contrast to the Photochemical Dissociation Hypothesis, the Outgassing Hypothesis argues that the free oxygen came from the photosynthesis of primitive organisms which existed 1.5 - 3.5 billion years ago. The oxygen took approximately 2 billion years to become free, but when it did, it formed the ozone layer, eliminating the dangerous radiation and setting up the foundation for a habitable planet.
In either example, the formation of the atmosphere no doubt included the presence of water vapor. This water vapor, in turn, accumulated and liquefied to form the seas and oceans that today comprise over two thirds of the Earth's surface. Geologists do not concern themselves so much with how the water vapor liquefied; instead, they tend to focus on the rate at which the water accumulated over time. Initially, it was assumed that the water grew at a constant rate over time. Others believed that the rate of accumulation of sea water would depend exclusively on atmospheric production. The atmospheric production, in turn, depended on chemical fractionation, based on the hypothesis' link between the Earth's interior and atmospheric gases of nitrogen, carbon dioxide and water vapor. Thus, it was reasoned that the rate of accumulation grew (and still does grow) in direct proportion to an inferred increase of the volume of the continental crust over time.
Geologists have compared estimates analyzing the rate of releases of helium and argon from the interior into the atmosphere. It is believed that if water was released at a comparable rate, then the water would accumulate over a long period of time, but not at a constant rate. Most likely, crustal growth and outgassing were far more rapid earlier in the Earth's history as a result of the immense radioactivity that existed. Table 3 shows a graph of the water accumulating at a constant rate and plots it against the proposed accumulation rate. As a result, the water accumulated at a faster rate as well and has gradually increased at a decreasing rate over the past 2 billion years.
A computer-generated model isolating the Geosphere, picturing the world as if the Hydrosphere did not exist
The exterior of the Earth represents the planet's solid layer. Its base is formed by the uppermost part of the mantle, known as the lithosphere which contains the rigid material of the outer portion of the Earth. The lithosphere possesses a thin, rocky outer layer called the crust. The crust ranges in thickness, from 30-60 kilometers thick beneath the continents to only 5-10 kilometers thick beneath the oceans. Bedrock of the crust is covered by loose rock particles called the regolith that forms when bedrock reacts with the atmosphere and hydrosphere. When nature transports regolith to a different location, the regolith is re-named sediment, which means "settling". The upper portion of regolith creates soil, which permits the growth of plants and crops that are crucial for human survival.
The regions on the Earth where life can evolve belong to the biosphere. The biosphere is defined as all the Earth's living matter as well as all dead plants and animals that have not yet been completely decomposed. The biosphere has a maximum range of 20 kilometers in thickness and ranges from the depths of the oceans to the peaks of the highest mountains. It is primarily composed of various compounds of carbon, hydrogen and oxygen.
Continents, or continental masses, refer to the major land areas of the world. Continental masses comprise roughly 30% of the surface of the Earth and average approximately 800 meters in altitude. The continents are underlain by ancient rocks, dating as far back in time as 3.9 billion years. An interesting point, however, is that the materials which comprise the continents all resulted from the geological cycle that transformed the Earth from its primitive state to its modern one. No remnants of any materials from the Earth's infant stage have ever been found in the continents. Research has shown that the most accurate way to analyze the continents is to organize them into several large regions where the surface morphology and the various rocks display similar traits. These regions are called physiographic provinces. Examples include mountains, plains, plateaus, and shields.
A mountain is a physiographic province defined as a landmass that stands 400 meters higher or more than its surroundings. Cordilleras, a Spanish term meaning "strings", is a pseudonym for the various mountain belts of the Earth. A cordilleras has several individual parts based on morphology, including mountain ranges, mountain systems, and mountain chains.
Plains represent a second type of physiographic province. Because of the minimal variations between their lowest and highest points, plains are quite flat. Beneath the plains are sediments, and the addition of new layers of these sediments causes the surface area of plains to grow. Plains are most often found in coastal regions or large regions within the interiors of the continents. A high-standing area within which the rock strata are generally horizontal is a third physiographic province called a plateau. Plateaus can be viewed as elevated plains. Usually, they result when the sediments underlying a former plains area cement together and subsequently elevate. Not all plateaus are underlain by sediments; some are formed by layers of volcanic rocks. Shields, the fourth physiographic province, are low-lying regions which form when several ancient mountain belts join together. Shield areas are relatively thick and rigid continental blocks. The regions on the Earth where life can evolve belong to the biosphere. The biosphere is defined as "the totality of the Earth's living matter and, in addition, dead plants and animals that have not yet been completely decomposed." The biosphere has a maximum range of 20 kilometers in thickness and ranges from the depths of the oceans to the peaks of the highest mountains. It is primarily composed of various compounds of carbon, hydrogen, and oxygen.
The Liquid Layer
A computer-generated model of the Hydrosphere detailing the average daily temperature of the Earth's water supply
The liquid layer of the Earth is called the hydrosphere and it represents the entire conglomeration of water bodies enveloping the earth's surface. The hydrosphere consists of the rivers, streams, lakes and oceans, while its sub-category, the cryosphere, is made up of the temporarily frozen, solid water bodies such as ice, snow and glaciers. The liquid layer of the earth covers approximately three-quarters of the earth's surface, of which oceans comprise ninety-seven percent. The distribution of the liquid layer of the earth is shown on Table 4.
The daily changes in the liquid layer is explained by the "hydrologic cycle." The Sun's heat evaporates the water of the hydrosphere. The newly-created water vapor rises into the atmosphere, cools, condenses, and becomes precipitation, falling back onto the land and into the oceans, lakes, and rivers. The hydrologic cycle describes the movement of water in its three states: solid, liquid and gas. The evaporation and precipitation of the water, the hydrosphere of planet Earth, recycles and rejuvenates the Earth and its water supply each and every day. Evaporation and precipitation across the Earth is in total balance, though evaporation is greater than precipitation over the oceans and the reverse is true over the land. Water vapor usually averages twelve days in the atmosphere before coming back down to Earth again as rain or snow. This difference is corrected by the flow of rivers into the oceans and balance is maintained.
(Batten & Dott, 131)
The hydrosphere has three main functions which make it so important to the planet Earth. First, water shapes the earth's surface by eroding certain areas and relocating those materials. Ocean waves are a primary instrument of erosion. Surf zone abrasion is an example of this. The sediment carried by the waves smashes into the beach and the constant action wears down the surf zone, smoothing and flattening the land.
Second, the oceans serve as basins for the accumulation of almost all of the substances available on Earth. The rivers of the hydrosphere collect materials and sediment and bring them out to the interconnected oceans. As a result, fine-grained, course-grained and evaporite sediments can all be found in the oceans. Also, terrigenous sediment, materials eroded by waves, carried by rivers, and blown by the wind, combine with pelagic sediment--marine organic materials referred to as deep-sea ooze--dissolve into the ocean waters and collect on the ocean floor. However, the hydrosphere is not just pure water. Through erosion and movement of sediment, several compounds have become dissolved in the water. Three to four percent of ocean water is mineral matter in solution, of which seventy-five percent is sodium chloride (NaCl--common table salt). Most of this mineral matter is contained in the oceans and only a minimal amount exists in the lakes and streams.
Most importantly, the hydrosphere's third function is to be used as an essential building block and sustainer of virtually all life forms. However, one must not ignore the fact that although most people think of this as virtually the only important function of the liquid layer, the importance is actually in the proper operation of all of the functions and the successful interaction of the hydrosphere and the solid and gas layers.
The Gas Layer
The outermost layer of Earth is the gas layer, better known as the atmosphere. The atmosphere is composed primarily of nitrogen and oxygen, but is also comprised of several trace gases such as carbon dioxide (CO2) and water vapor. All of these gases, which together form what is commonly called "air", are held to the Earth by the pull of gravity and each serves an important function. Carbon dioxide gas provides a barrier which keeps heat from escaping the Earth; this is known as the "greenhouse effect." The "greenhouse effect" caused by carbon dioxide in the atmosphere serves to maintain a temperature on Earth great enough to sustain life. Yet, if the amount of carbon dioxide gas were to increase drastically, too much heat would be trapped in the atmosphere, making it too hot for many organisms to survive. This causes a frightful problem wherein a certain balance of CO2 gas must be maintained to ensure that life may be maintained on the planet. Carbon dioxide also serves as the lifeline to most plants, allowing them to breath by taking in the carbon dioxide and emitting oxygen gas. Another gas, ozone (O3) screens out the harmful ultraviolet rays emanating from the sun. One of the most abundant gases in the atmosphere, oxygen, is needed for animals to breath and live. This graph shows the range of predictions made as to the global-mean surface temperature during the next eighty years
The gas layer is constantly moving around the Earth in the form of wind and filling the openings of the solid layer of Earth that have not yet been penetrated by water. The movement of wind is created by the uneven pattern of the Sun's heat on Earth, wherein the heat is greater near the equator than at the poles. This causes the equatorial air to expand, become less dense, and move toward the poles, where it will cool, contract, become more dense and flow toward the equator. However, the Coriolis effect modifies this pattern by forcing the air to clump into wind belts. The Coriolis effect causes any body that moves freely with respect to the rotating solid of the Earth to veer toward the right in the Northern Hemisphere and towards the left in the Southern Hemisphere. The Coriolis effect manipulates the movement of the Earth's gaseous layer and the flow of air becomes curved east in the north and west in the south instead of merely moving north and south between the poles. A satellite image of cloud movements and formations over Europe, with the political boundaries artificially superimposed
The atmosphere works as a geological agent of movement and erosion in two ways: wind and rock weathering. Winds are often extremely violent and can do extensive damage and erode land in a short period of time. Rock weathering, though, also erodes land but in a much slower process. Rock weathering slowly loosens material so that streams, gravity, and other instruments may carry it to the oceans.
The atmosphere's interaction with the hydrosphere, the liquid layer of the Earth, is extremely important, as the gas layer holds water vapor (water in a gaseous state, caused by heat from the sun) and redistributes it back to the Earth's surface. The gas layer's role in the hydrologic cycle is to carry the evaporated water and cool it back down, then release it as precipitation. Without an atmosphere, water vapor would merely escape into space and the Earth's water supply would deplete. The atmosphere is the reason why rain is possible on the planet Earth.
The differentiation process that occurred on Earth was both lengthy and complicated. Over the course of nearly 4.5 billion years, the Earth has transformed from a molten protoplanet to a world with clear, beautiful oceans, majestic mountains, and an atmosphere that can sustain human, animal, and plant life. While the process was tedious, the results are undeniable. Had the Earth not differentiated into solid, liquid, and gas layers, life never would have evolved. But will life as we know it continue forever on Earth? Today's research suggests otherwise. Most scientists believe that several billion years from now the sun will burn out, killing any and all life forms remaining on the Earth. A more immediate problem faces the life-forms on Earth. Because of human negligence, (mainly in the form of pollution), the solid, liquid, and gas layers of the Earth are being threatened. The extent of the damage is unknown at this time. Should humans continue, however, to pollute the oceans, destroy the land and erode the ozone layer, the consequences will be frightening. The differentiation process took billions of years; reversing it may not take as long.
Glossary of Terms
Atmosphere : The gas layer of the Earth, primarily consisting of nitrogen, oxygen, carbon dioxide, and water vapor.
Bedrock : Large quantities of solid rock which form the primary
composition of the crust.
Chemical Fractionation : A concentration of relatively light
elements which distilled to form a granitic residue.
Continental Masses : The major land areas of the world.
Cordilleras : Spanish for "strings". Refers to the
Earth's mountain belts.
Corlolis Effect : Effect that causes any body of air that moves
freely with respect to the rotating solid of Earth to move towards the
right in the Northern Hemisphere and towards the left in the Southern Hemisphere
Cryosphere : A sub-category of the hydrosphere consisting of
the temporarily frozen, solid water bodies including snow, ice, and glaciers.
Greenhouse Effect : Occurs when carbon dioxide gas provides a
barrier which keeps heat from escaping the Earth. It serves to maintain
a temperature on Earth great enough to sustain life.
Hydrologic Cycle : The cyclic movement of water through evaporation,
precipitation, wind transport, stream flow, percolation, and related processes.
Hydrosphere : The liquid layer of the Earth. The entire conglomeration
of water bodies enveloping the Earth's surface.
Lithosphere : The uppermost portion of the Earth's mantle.
Outgassing Hypothesis : Hypothesis which states that atmosphere
gases arose from the interior of the Earth through igneous activity, such
Photochemical Dissociation Hypothesis : Hypothesis which states
that intense ultraviolet radiation triggered photochemical reactions which
caused the evolution of the modern-day atmosphere on Earth.
Physiographic Province : A large continental region where the
surface morphology and various rocks display similar traits to each other.
Plains : A physiographic province with little variation between
its lowest and highest points.
Plateau : A physiographic province. It is a high standing area
whose rock strata are horizontal.
Regolith : A sheet of loose particles covering the bedrock. It
plays a vital role in plant growth by creating soil.
Rock Weathering : A form of erosion which slowly loosens material
so that streams, gravity, and other instruments carry it to the oceans.
Sediment : Regolith which is transported to a different location
Surf Zone Abrasion : One method of shaping the Earth's surface
caused by ocean waves which erode certain areas.
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Landes, Kenneth K. Physical Geology and Man. New York: Prentice-Hall,
Menard, H.W. Geology, Resources, and Society. San Francisco:
W.H. Freeman and Company, 1974.
Porter, Steven C. and Brian J. Skinner. The Dynamic Earth. New
York: John Wiley and Sons, 1989.
Sanders, John E. Principles of Physical Geology. New York: John Wiley and Sons, 1981.