THE STRUCTURE OF THE EARTH

The theories which deal with the origin of the Earth are based on mineral content in the rock, dates of various radioactive decay of gases trapped in tiny rock vesicles, and comparison with rocks from the Moon and Mars. While there is no certainty to the actual origin of the Earth, the information presented in this page and those that follow are based on scientific evidence and deductive reasoning. All students of Astronomy are encouraged to search themselves for the truth and scour available resources before drawing conclusions. In the same manner in which Martin Luther challenged the teaching of the Church and Nikolaus Copernicus challenged the writings of Ptolemy, you are encouraged to challenge these writings and others, look at the evidence, and think for yourselves.

This page does not offer an explanation for the origin of the Solar System or the Big Bang. For that information, you can look at the appropriate links. Here we look at what happened to the Earth after it condensed from the swirling debris that was surrounding the collapsing sun.

Early on, the entire mass of the Earth was molten from the intense heat generated by collisions and also by the gravitational collapse of the sphere. Additional heat was generated by the radioactive decay of certain elements like thorium deep in the core. Shortly after formation of the sphere, the entire earth was homogenous, its chemicals and rocks being distributed evenly throughout the entire surface and interior. Because this material was so hot from the gravitational collapse, gravity forced all the material toward the absolute center, resulting in a spherical shape. At some point within the first several hundred million years this homogenous mixture was separated by the mass of different materials into huge layers in an event called the "Iron Catastrophe." The iron/nickel elements that are heavy and dense collected in the center, while lighter silicate rock was circulated out from the center toward the surface. The collection of the heaviest elements in the core, including heavy radioactive thorium molecules, gave rise to great internal heat ... generated by relentless gravitational pressure and radioactive decay. The surface cooled and solidified, but only a thin shell was hardened, while the hotter interior portions remained relatively liquid in the deeper areas, or "plastic" in the upper regions. The earth became zonally stratified.

There is an inner solid core and outer liquid core of iron/nickel metals, a very thick mantle of silicate rock, and an outer crust of granite and gabbro. This image shows the thickness of the layers, although not to scale.

 

 

 

 

The contents and temperatures of these layers is shown in this diagram. Present in the core is a fair amount of nickel and relatively small amounts of heavy elements, some of which are radioactive. The deepest hole ever dug into the Earth is a 12 km drilling in Kamchatka, Russia. This is barely a scratch compared to the entire Earth radius. A journey to the Earth's center was only a fictitious writing of Jules Verne. No one has ever found an ancient lava tube and spelunked deep into the core to tell us what is there. The drilling site in Russia reveals very little either. So, how do we know about these layers?

 

 

We know of the existence of these layers by the manner in which seismic waves travel through the earth from the epicenter of a quake to listening posts around the globe. Some forms of seismic waves travel through all mediums at constant velocities, some are bent by liquid and altered by liquid/solid boundaries. P waves (primary) move in the same manner that a stretched slinky will carry a vibration ... in a compression wave. S waves (secondary) vibrate perpindicular to the direction of motion much like a stretched rope that is snapped. P waves travel the fastest and arrive at a seismograph first, followed later by the S waves, and lastly by surface waves. By measuring the exact timing of the arrival of particular waves arriving at the different listening posts after a quake, scientists can determine the depth at which boundaries between earth's layers occur. For more information about these waves, click on the image to your left.

 

Okay ... so the Earth has layers, but what is that to us? The interior is of great interest to scientists studying the way Earth recycles itself, but most of us are just interested in what goes on at the surface. So, what caused the surface to have these mountains, valleys, and oceans? It is most probable that the earliest Earth was repeatedly struck by debris flying around in the early Solar System. The repeated impacts melted the exterior and probably vaporized any water trapped in the rock. A dense layer of aqueous atmosphere blanketed the planet, trapping solar radiation and heating the surface some more. Thus, the surface was molten or dry early on. Many cosmologists believe that comets "rained" down on the planet, depositing relatively large amounts of water. Oceans accumulated as rock and comet impacts vaporized aqueous rock, sending the materials into the atmosphere and then raining the particulate matter back to the surface, or allowing water droplets to form and remain in the air. It is reasonable to conclude that most of the original earth water was airborne in a gaseous state while the earth was being bombarded and heated. Only with the cooling of the crust and trapping of internal earth heat, coupled with the reduction in bombardment, did the oceans rain down from the atmosphere and collect in regions that are geographically lower in elevation. Some scientists hold to the theory that this early water vapor reached escape velocity, and the bombardment of comets was solely responsible for returning the water to the earth over a gradual timeframe. More likely, the combination of comets and atmospheric rain accounts for the present water supply.

You might ask, where did all this water come from? You may also discover it interesting when you now learn that water is exceedingly abundant in space, but rare on Earth. Consider that stars are full of Hydrogen gas, and all fuse that hydrogen into heavier elements like Helium, Carbon, Oxygen, etc. Over time, as stars explode and release their contents into space, vast clouds of hydrogen, helium, carbon, and oxygen form. Since Oxygen is so incredibly electronegative, it steals electrons from anything it can. The presence of Oxygen and Hydrogen quickly results in the formation of water, with the Oxygen grabbing the electrons of the Hydrogens and holding tightly to them. Indeed, water may be the most abundant compound molecule in the Universe. Huge clouds of water exist in galactic nebulae whose trademark blue color when backlit by stars reveals its presence. For a planet like Earth with a diameter of 12,756 km to have 71% of its surface covered by water seems like a lot, but when you consider the total volume of Earth and the thin veneer of ocean cover, the water is actually trivial.

Secondly, Earth has just the right amount of mass to generate sufficient gravity that water does not reach escape velocity upon vaporization. Mars certainly has had water in its past, and may even have some frozen under its surface layers. But whenever water liquefies on Mars, the low atmospheric pressure causes the water to boil and it escapes into space because the smaller mass of Mars does not create sufficient gravity to hold gaseous water. Earth's gravity, however is great enough to hold water vapor, so we receive the benefit of a continual recycling of water without mass loss. Interestingly, the gravity of the Earth is less than that of Neptune or Jupiter. These planets hold on to gaseous Hydrogen, and if Earth were to do so, life on this planet would probably never exist.

The interior is made of hot Iron. The mantle is made of plastic silicate rock with aluminum and potassium. The crust is made of brittle rocks called Gabbro (Basalt) and Granite. Once again, click on this site to learn about the effects of these two rocks on the underlying plastic mantle.

The crust is made of two silicate-bearing rocks: Basalt is the most common by far (3.3g/cm3 in density - dark in color) forms a thin sheet roughly 8 km thick; Granite is less common (2.7g/cm3 in density - lighter in color) forms sheets 40-60 km thick. I use the terms Basalt and Granite instead of Gabbro because basalt is the most common oceanic rock form, and the density of basalt is greater than the 2.9 g/cm3 given for Gabbro at the University of Michigan site. The basalt plains are more dense and therefore sink deeper into the plastic mantle underneath. These lower elevation plains were a perfect collecting area for the rains which fell from the atmosphere, and thus became home for the oceans. The oceans are not deep because the water is heavy, but are deep because the underlying basalt is heavier than the surrounding granite highlands. The granitic masses, due to their relatively lower density, floated higher on the plastic mantle underneath, and thus remained above the level of the ocean surface, thus becoming the earliest continents. These earliest continental regions were called cratons, and there appears to have been only three or four original cratons amidst a vast, vast ocean. Minnesota and much of central Canada are part of one of the original cratons.


*There is a place just southwest of Morton, Minnesota where you can walk on rock that is 3.9 billion years old. This small quarry of metamorphic granite called "Morton" gneiss is just behind a small motel on Hwy 19, prior to crossing the river. Immediately behind this motel is the quarry, and you can walk on the most ancient rock in the western hemisphere, and even collect a sample from the slag pile for your personal pleasure. We have put a nice piece in our family fireplace:)

For specific information about the Earth's Interior, click on Core, Mantle, Crust or you can learn more about the crust in particular in the next section of this course.

With our knowledge of the Structure of the Earth somewhat complete for this class, you are now wondering where does all of the trapped internal heat go? To learn about the forces which shape the surface of the Earth, please move ahead to Plate Tectonics, or return to the Earth Introduction, the Syllabus, or the Home Page.


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