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Geology LibreTexts

7.2: The Earth's Structure

We are half way through the course, and we have discussed the atmosphere and the hydrosphere.  We will focus on the lithosphere for the next two weeks and discuss the formation and composition of our home planet, and discuss the processes that shape it.  The earth is one of nine planets that orbit our sun. It is believed that the sun and the planets formed around the same time from a cloud of dust and gas comprised mostly of hydrogen and helium (the two lightest elements).  This is called the nebular hypothesis.  Around 4.5 billion years ago, for reasons that are not well understood, this enormous cloud of dust and gas began to contract under its own gravitational influence, and the planets and the sun were formed.  The sun was comprised mostly of the lighter elements:  hydrogen and helium.  The heavier elements are believed to have stuck together forming small particles which collided forming bigger particles, eventually creating rocky masses that gained their own gravitational force and developed into the nine planets of our solar system. 

Soon after the earth was formed, the decay of radioactive elements, coupled with the heat resulting from the energy of the colliding particles produced some melting of the interior of our planet.  The heavier elements (iron and nickel) sunk while the lighter elements floated, forming the dense iron-nickel core, and light, silica rich crust.

We do not know for certain how the solar system or the earth formed.  The above is simply a hypothesis which provides an explanation for our present day observations of the planet, and tries to answer the geographer’s question of how. How did our planet form?

The earth as we know it (land and ocean) is only a thin shell which covers the very dense, hot interior of our planet.  The earth is composed of a dense iron-nickel core which has a radius of approximately 1,216 kilometers (756 miles).  The core of the earth is very hot (3000º-6650ºC) and it is so compacted that it is nearly five times as dense as the rocks on the surface of the earth.  Atop the inner core of the earth is the outer core, which is where our magnetic field is believed to be generated from.

The liquid lower mantle sits atop the outer core, and is composed of iron silicates.  The liquid upper mantle is also composed of iron magnesium silicates, and is relatively thin (compared with the layers below it) at only about 400 kilometers in thickness.  The next layer in the earth is called the athenosphere, which is a little less than 200 kilometers thick.  The athenosphere is semisolid, and deforms ‘plastically’ or like silly putty.  Atop the athenosphere is the crust of the earth.  You might want to look for figures that demonstrate the relative thickness of the earth’s crust.  If the center of the earth were Halifax Nova Scotia, the core and mantle would stretch to the California/Nevada border.  The crust would barely reach from Livermore to San Francisco. 

There are two types of earth crust:  continental crust (or what we call ‘land’) and ocean crust (the bottom of the sea).  Continental crust is about three times as thick as ocean crust, and it is also much lighter than ocean crust.  Continental crust is composed mostly of relatively light silicate rocks, while the ocean crust is composed of rocks formed from iron-magnesium silica compounds. 

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