Thursday, September 22, 2016

The Layered Earth

Everything we see in geology can be directly or indirectly related to Plate Tectonics. If we played the six degrees of separation game, called it Six Degrees of Plate Tectonics, we would rarely go over three degrees before the answer came back somehow relating to Plate Tectonics. It truly is a unifying theory!

The reason that the earth's plates move is in large part due to the fact that our Earth is layered. It wasn't always layered. When debris first accumulated making the protoEarth, it was a fairly homogeneous rock orbiting the sun. Once it reached a critical mass, however, the gravitational energy accumulated caused the entire earth to melt into a fiery ball of liquid hot magma! This event is called the Iron Catastrophe. Catastrophe in this sense does not refer to a tragic event but rather to the classical, Greek-derived definition which is the culminating act in a drama. The Iron Catastrophe lasted between 100 m.y. and 500 m.y. and was the culminating act in the Earth's drama. It created the layered Earth we have now: a core made of iron and nickel, a rocky mantle that is high in iron and magnesium and poor in silica, and a rocky crust that is rich in silica and other lighter elements such as calcium, carbon, nitrogen, oxygen, sodium, aluminum, etc.

Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of much denser material. Our knowledge of what's inside the Earth has improved immensely since Newton's time, but his estimate of the density remains essentially unchanged. Our current information comes from studies of the paths and characteristics of earthquakes waves travelling through the Earth, as well as from laboratory experiments on surface minerals and rocks at high pressure and temperature. Other important data on the Earth's interior come from geological observation of the Solar System, its gravity and magnetic fields, and the flow of heat form inside the earth.

The planet Earth is made up of three main shells: the very think brittle crust, the mantle, and the core; the mantle and the core are each divided into two parts. The table below lists the thicknesses of the parts. Although the core and mantle are about equal in thickness, the core actually forms only 15% of the Earth's volume, whereas the mantle occupies 84%. The crust makes up the remaining 1%. Our knowledge of the layering and chemical composition of the Earth is steadily being improved by earth scientists doing laboratory experiments on rocks at high pressure and analyzing earthquake records on computers.

The layers crust, mantle, and core refer to the layering by chemical composition as mentioned above. We can also break the Earth into layers based on the physical properties of those layers. The layers are as follows: lithosphere (the brittle outer layer that includes the crust and the uppermost mantle), asthenosphere (the ductile layer below the lithosphere that flows on a geologic time scale), mesosphere (the rigid layer beneath the asthenosphere composed of the the lower portion of the mantle), outer core (this layer is liquid as it is very high in temperature and the pressure is insufficient to force the layer to be a solid), and the inner core (the innermost layer that is under such high pressure that the extremely hot iron-nickel mixture is forced to be a solid.) (Figs. 1 and 2)

Figure 1: The layers of the earth based on physical properties and chemical composition.

Figure 2. The layers of the earth by both chemical composition and physical properties.
The thicknesses of each layer are included.

Layers by Chemical Composition
The Crust
Because the crust is easily accessible to us, it is the layer that is most understood. There are are two types of crust on Earth: 
  • Continental crust - The continents are continental crust. Most of the crust is composed of granite or rocks of granitic composition. Continental rocks are high in silica and contain minerals such as quartz, feldspar, muscovite, biotite, and clay minerals. Averaged together, the density of continental rocks are approximately 2.7 - 3.0 g/cm3. The continental crust ranges from 25 - 70 km thick.
  • Oceanic crust - The ocean floor is oceanic crust. The entire oceanic crust is composed of basalt which grades to gabbro at depth. Basalt is silica-poor and contains mostly ferromagnesian silicate minerals such as pyroxene, olivine, and some hornblende. Some Ca-rich plagioclase is also present. The density of the oceanic crust is approximately 3.0 - 3.3 g/cm3. The thickness of the oceanic crust is approximately 5-6 km.
The boundary between the crust and the mantle is called the Mohorovičić discontinuity (or Moho, for short since we English speakers have such a hard time pronouncing a Croatian name). The discontinuity is named after Andrija Mohorovičić who discovered, through evidence from seismic waves, that the earth is made up of layers. He is the first to describe that there is a seismic discontinuity between the crust and the mantle. Incidently, his name is pronounced (in American English): Ahn'-drea Moh-ho-row'-vitch-itch.

The Mantle
Our knowledge of the upper mantle, including the tectonic plates, is derived from analyses of earthquake waves; heat flow, magnetic, and gravity studies; and laboratory experiments on rocks and minerals. Between 100 and 200 km below the Earth's surface, the temperature of the rock is near the melting point; molten rock erupted by some volcanoes originates in this region of the mantle. This zone of extremely yielding rock has a slightly lower velocity of earthquake waves and is presumed to be the layer on which the tectonic plates ride (Fig. 3). Below this low-velocity zone in the upper mantle; it contains two discontinuities caused by changes from less dense to more dense minerals. The chemical coposition and crystal forms of these minerals have been identified by laboratory experiments at high pressure and temperature. The lower mantle, below the transition zone, is made up of relatively simple iron and magnesium silicate minerals, which change gradually with depth to very dense forms. Going from mantle to core, there is a marked decrease (about 30%) in earthquake wave velocity and a marked increase (about 30%) in density.

Figure 3: The figure on the left shows that rock type 2 has a higher velocity than rock type 1. The opposite is true for the figure on the right: rock type 2 has a lower velocity than rock type 1.
The Core
The core was the first internal structural element to be identified. It was discovered in 1906 by R.D. Oldham, from his study of earthquake records, and it helped to explain Newton's calculation of the Earth's density. The outer core is presumed to be liquid because it does not transmit shear (S) waves (a type of seismic waves that wiggles side to side) and because the velocity of compressional (P) waves (a type of seismic waves that moves in a push-pull motion) that pass through it is sharply reduced. The inner core is considered to be solid because of the behavior of the P waves passing through it.

Data from earthquake waves, rotations and inertia of the whole Earth, magnetic-field dynamo theory, and laboratory experiments on melting and alloying of iron all contribute to the identification of the composition of the inner and outer core. The core is presumed to be composed principally of iron, with about 10% alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements.
Figure 4: Illustration of the P and S wave movement through the Earth. S-waves are not transmitted through liquids; therefore, the S-wave shadow that we see on the opposite side of the Earth of an earthquake epicenter must be the result of a liquid outer core.

Table 1: This table of depths, densities, and composition is derived mostly from information in a textbook by Don L. Anderson*. Scientists are continuing to refine the chemical and mineral compositions of the Earth's interior by laboratory experiments, by using pressures 2 million times the pressure of the atmosphere at the surface and temperatures as high as 20,000°C.

Thickness (km)
Density (g/cm3)
Types of rock and minerals found
Silicic rocks: Andesite, basalt at base
Upper mantle
Rocks: Peridotite, eclogite,
 Minerals: olivine, spinel, garnet, pyroxene, perovskite, oxides.
Lower mantle
Minerals: magnesium and silicon oxides
Outer core
Iron and oxygen, sulfur, nickel alloy
Inner core
Iron and oxygen, sulfur, nickel alloy
*Anderson, D.L., 1989, Theory of the Earth: Boston, Blackwell Publications, 366 pages.

Layers by Physical Property - from the inside out
The Inner and Outer Core
Within the core an inner region exists where pressures are so great that iron is solid despite its high temperature. The solid centre of the Earth is in the inner core. Surrounding the inner core is a zone where temperature and pressure are so balanced that the iron is molten and exists as a liquid. This is the outer core. The difference between the inner and outer cores is not one of the composition (the compositions are believed to be the same). Instead, the difference lies in the physical states of the two: one is a solid, the other is a liquid.

The Mesosphere
The strength of a solid is controlled by both temperature and pressure. When a solid is heated, it loses strength. When it is compressed, it gains strength. Differences in temperature and pressure divide the mantle and crust into three strength regions. In the lower part of the mangle, the rock is so highly compressed that it has considerable strength even though the temperature is very high. Thus, a solid region of high temperature but also relatively high strength exists within the mantle from the core-mantle boundary (at 2,883 km depth) to a depth of about 600 km and is called the mesosphere ("intermediate, or middle, sphere").

The Asthenosphere
Within the upper mantle, from 600 to between 100 and 200 km below the Earth’s surface, is a region called the asthenosphere ("weak sphere"), where the balance between temperature and pressure is such that rocks have little strength. Instead of being strong, like the rocks in the mesosphere, rocks in the asthenosphere are weak and easily deformed, like butter or warm tar. As far as geologists can tell, the compositions of the mesosphere and the asthenosphere are the same. The difference between them is one of physical properties; in this case the property that changes is strength.

The Lithosphere
Above the asthenosphere is the outermost strength zone, a region where rocks are cooler, stronger, and more rigid than those in the plastic asthenosphere. This hard outer region, which includes the uppermost mantle and all of the crust, is called the lithosphere ("rock sphere"). It is important to remember that despite the fact that the crust and mantle differ in composition, it is rock strength, not rock composition, that differentiates the lithosphere from the asthenosphere.

The difference in strength between rock in the lithosphere and rock in the asthenosphere is a function of temperature and pressure. At a temperature of 1300°C and the pressure reached at a depth of 100 km, rocks of all kinds lose strength and become readily deformable. This is the base of the lithosphere beneath the oceans, or, as it is most colloquially termed, the oceanic lithosphere. The base of the continental lithosphere, by contrast, is about 200 km deep.

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