Origin and Evolution of the Earth System

 

In order to understand the origin and evolution of planet earth, it is necessary first to introduce the concept of the earth as a system. The term “Earth System” came into prominent use in 1986 after publication of a report by NASA entitled “Earth System Science: A Program for Global Change” ⁴.   Earth System Science regards the Earth as a smaller system within and part of the Solar System. The Earth System is considered to consist of separate but interacting sub-systems – the solid Earth, the atmosphere, the hydrosphere and the biosphere.

Origin and Early History of the Solid Earth

The earth, like most of the other planets, is thought to have grown by accretion, the gradual collection of cosmic dust into a progressively larger mass. This accumulation of cosmic dust no doubt would have led to an initially chemically and physically homogeneous planet. However, we know from the study of seismic (earthquake) waves that the earth is now actually quite heterogeneous, composed of layers: an outer relatively thin (7-50 km) crust, a thick (~2900 km) mantle, and a thicker (3470 km) core, (see figure below.) 

Simplified Structure of the Earth, courtesy John Carpenter and Philip Astwood, “Plate Tectonics for Introductory Geology”

If the earth began as a homogeneous mass and is now heterogeneous, we must somehow account for the re-distribution of material in the earth into a core, mantle and crust. The best explanation for this layering is that the earth must have undergone one or more periods of melting, almost certainly due to heat being released by gravitational energy during the coalescence of the earth. Further, there must have been a significant amount of radioactive elements contained within this early earth. The heat from radioactivity is thought to have assisted in this process.

During the time the earth was (partially?) melting, its interior underwent a chemical differentiation, sometimes called the “primary differentiation” of the Earth, separating into a core, a mantle and a primordial crust. But now, we need to examine the interior structure in more detail:

• Crust – Briefly and simply put, the crust is the thin rocky veneer of the earth, composed of a wide variety of sedimentary, igneous and metamorphic rocks and minerals of low density (2.5 – 3.0 g/cc). The crust today is approximately 6 km thick under the oceans and up to 30 km thick under the continents. The earth’s primordial crust was chemically similar to today’s crust, with some exceptions. The primordial crust was almost certainly much more compositionally uniform than the crust is today. The primordial crust was also much smaller in volume than is the crust today, which is continually growing due to the ongoing addition of volcanic lava rising from the mantle. The oldest crustal rocks found date to approximately four billion years old, but they could be much older.
• Mantle – The mantle is thought to be a thick shell, hotter and more chemically and mineralogically homogeneous than the crust. It is almost certainly composed of a few rock types more dense (3.0 – 3.5 g/cc) than most of the rocks of the crust. The mantle is more chemically homogeneous than the crust. Although hot throughout, the mantle temperature is greatest near its base. Nearest the surface the mantle is physically very heterogeneous. The uppermost part of the mantle, called the lithosphere, is approximately 100 km thick and behaves like a rigid solid. Beneath the lithosphere, the next lower part of the mantle is known as the asthenosphere, which extends from approximately 100 km and to a depth of approximately 700 km. The asthenosphere, while still chemically similar to the lithosphere, behaves like a plastic solid in that it can be permanently distorted without actually breaking. Still deeper, the mantle behaves as a rigid body from approximately 700 km below the surface to the boundary with the outer core.
• Core – The core is extremely hot with highest temperatures of about 6000^o C. It is thought to be composed of metallic iron and nickel, and is believed to be liquid in the innermost core and solid in the outer parts of the core.

Origin and Early History of the Earth’s Atmosphere

During the “primary differentiation,” while the more dense elements contracted into the solid earth, the less dense, gaseous elements, like nitrogen, carbon dioxide and argon were released, forming the primordial atmosphere. Hydrogen and helium escaped from the solid earth into space. Additional material for the early atmosphere is thought to have come from the release of gases contained in extra-terrestrial material that has been impacting the earth.

Early in the history of the Earth System, volcanic activity in the mantle that produced magma for the crust also produced a significant amount of other gaseous material, primarily water vapor, carbon dioxide and sulfur that accumulated in the Earth’s atmosphere. Continuing volcanic activity in the mantle contributes to a changing composition of the earth’s atmosphere over geologic time.

Notice that there has been no mention of molecular oxygen, O₂, in the atmosphere. That is because O₂ only became an abundant component of the atmosphere approximately 2 billion years ago. The process by which O₂ became abundant will be discussed in a later chapter.

Origin and Early History of the Earth’s Hydrosphere

The origin of water in the oceans remains one of the most significant controversies in the geosciences and astrophysics. However, most experts in the field agree that the primordial hydrosphere is thought to have formed shortly after the formation of the primordial atmosphere.

As the solid earth and its primordial atmosphere cooled, at about 4 billion years ago, eventually atmospheric water began to condense on the earth’s surface, forming the primordial hydrosphere. Also at around this time, the primitive mantle began melting producing magma. It is believed that at about 3.6 billion years ago, the chemical makeup of submarine eruptions of magma showed evidence of surface water being incorporated into the magma. Some water from the primitive mantle is thought to have been released by the volcanic activity.

While some continuing volcanic activity in the mantle throughout geologic time has contributed to a slightly greater volume of ocean water, the volume of the oceans is thought not to have increased significantly over time.

But how did the oceans become salty? It is now thought that the salinity (saltiness) of the oceans is primarily due to the weathering of crustal rocks.   Rain falling on the surface is slightly acidic due to chemical reactions within the atmosphere and because of its acidity, the rain not only physically erodes the rock, the acids chemically break down the rocks and carry salts and to the streams and rivers and then to the ocean. Gases released by volcanic activity in the oceans and chemical activity in and near hydrothermal vents in the ocean also contribute to the oceans’ saltiness.

Evolution of the Earth System

Geologists have known for over 200 years that the earth is evolving, undergoing constant change. We can see rocks changing into soil through the processes of chemical and physical weathering. We have already discussed chemical weathering. Physical weathering processes like cracks forming in surface rocks are another change process. We can observe erosion of soil from the surface of the rock from which it was derived and transported to lake beds or the ocean floor. We can observe new volcanic mountains such as the Paricutin volcano in Mexico which began forming in 1943 and the ongoing growth of the Hawaiian Islands. We can observe huge changes to the Earth’s surface as a result of earthquakes, like the Alaska earthquake of 1964. We can also infer causes of other surface features that we cannot directly observe, such as trees growing out of a crack in a rock and gullies and valleys formed by the erosion of surface material by flowing water.

Many of the most important changes that the earth system has undergone, and is still undergoing, are those thought to be caused by plate tectonics, a theory that ranks as one of the most important discoveries in the earth and planetary sciences.

Plate Tectonics

Plate tectonics is actually an outgrowth of the old theory of continental drift, which was based on the complementary nature of the shapes of the continents, e.g. South America and Africa, and other evidence. The theory of continental drift was eventually discarded primarily because there was no physically viable explanation of how and why the continents might have moved. In plate tectonics, we now believe that the entire surface and near-surface of the Earth are composed of relatively large, but also relatively thin pieces of the earth’s crust and uppermost part of the mantle. These large segments of the earth are called plates (see figure below) These plates are composed of continental types of crustal rocks such as granite, limestone, marble, etc., and oceanic types of crustal rocks, composed mostly of volcanic basalt overlain by consolidated and unconsolidated marine sediment. Under both continental and oceanic crustal rocks is found a relatively thin, rigid part of the mantle, known as the lithosphere.   Together, the crust and lithosphere form the tectonic plates. These plates are all constantly moving relative to one another. Major plates of the Earth are shown in the image below. Plate boundaries are shown by dark lines; continental boundaries are shown in lighter lines. Arrows indicate direction of movement of individual plates. Note that plates can contain both continental material and oceanic material.

 Plates of the Earth, courtesy John Carpenter and Philip Astwood, “Plate Tectonics for Introductory Geology”

Movement of plates takes place due in large part to convective currents in the asthenosphere (see figure below).   Based on much evidence, we also believe that asthenosphere behaves like a plastic solid, able to flow. It is believed that temperature increases with depth in the interior of the Earth. Thus, the lower part of the asthenosphere is hotter than the upper part of the asthenosphere. If so then, it is believed that the hotter lower asthenosphere is less dense than the cooler, upper asthenosphere and in places begins to move upward toward the surface due to convection. When it reaches the base of the lithosphere, this warm, slightly less dense asthenosphere begins to move laterally. As it does, it carries the plates of the solid lithosphere with it.

Convection in the Upper Mantle, courtesy NASA

As these plates move, they interact with each other, producing many of the major Earth features. Plate interactions are categorized as:

• Convergent – where plates are moving toward each other. Features like the folded and faulted Himalaya and Appalachian mountains are produced, accompanied by earthquakes, when the continental portions of two plates converge on one another. Volcanic mountains like the Andes in South America and the Aleutians in Alaska, accompanied by earthquakes and deep oceanic trenches, like the Marianas Trench in the Pacific Ocean, are produced when the oceanic part of one plate converges with and is pushed under another plate.
• Divergent – where plates are moving away from each other. Features like the Mid-Atlantic Ridge, and the Great Rift Valley of eastern Africa are produced by this type of interaction. Where these plates diverge, the mantle beneath is subjected to lower pressure and responds by partial melting. The lava so produced rises and is incorporated into the crust.
• Transform – where plates move past one another without major convergent or divergent components. This kind of interaction produces faults, like the San Andreas Fault in California. Faults are breaks in the surface along which large blocks of crust move more or less horizontally past one another.

Other processes changing the Earth’s surface include:

• weathering – chemical and physical changes that break down surface rocks into soil and move it to a more stable resting place,
• partial melting of the upper mantle that results in the production of lava such as that which is forming the Hawaiian Islands, and
• meteorite impacts, such as the one that formed Meteor Crater in Arizona, and the impact 65 million years ago that was responsible for the extinction of the dinosaurs and thousands of other life forms.
• Climate and related sea-level changes throughout geologic time.

Religious Beliefs

So now we have a brief scientific version of the origin of the universe, solar system and earth, and how they evolved over time. But science has only answered questions related to how and when they came into existence. It cannot answer the question of who or what (if anyone or anything) caused the creation of the universe, and why.

From religion we are told who created the heavens and the earth (God). Many conservative Christians dismiss the scientific story because in their view it conflicts with the Biblical story of creation, which tells us who created the Earth, the stars and galaxies, and the totality of the Universe – God did all that in 7 “days”. (But, there are at least four different uses of the word “day” in the Old Testament.) The Bible does not tell us directly when the “world” was created, however, an Anglican cleric in the 1800s, Bishop Ussher attempted to establish when the world was created, using Biblical references to genealogical lineages. Based on these very inexact data, he concluded that the Earth was formed in 4004 BC.

However, several religious groups have now accepted the scientific evidence that the universe formed about 15 billion years ago and solar system, including the earth, was formed about 4.6 billion years ago. These include such different groups as the Episcopal Church, the Presbyterian Church (USA), the Roman Catholic Church and Islam.