Physical Geology

How the Earth Works

Earth machine

interior fission heat machine, exterior fusion heat machine

inner core, outer core, mantle, asthenosphere, lithosphere, crust

magnetic field

sialic crust, mafic crust

plate tectonics

From our tour of the solar system last time we learned that many planets are heavily cratored and show no sign of having changed for billions of years. These are ‘dead’ planets - changed only by bombardment from outer space. The Earth, in contrast, has few crators and a surface that is constantly changing. The Earth is a living planet - its various layers are in motion and its surface is continually being remade.

What living planets have that dead planets lack is an interior heat supply. As we will see, it is heat rising up through the Earth that drives the Earth Machine. Dead planets have long ago lost their interior heat - without heat to power it, the planet machine stops.

The Earth as a machine

The best way to understand how the Earth works is to think of it as a machine. To understand how the machine works we need to look at both how it is constructed and how it is powered. In fact, the Earth is really two machines - an interior (geological) machine and an exterior (atmospheric) machine.

The interior machine

The most important feature of the Earth’s interior is that it is differentiated into a series of layers. These layers differ in both their chemical composition and in their physical properties (solid, plastic, liquid).

Based on its chemical composition the Earth can be subdivided into three distinct sections:

• a thin outer crust composed of a variety of rocks and minerals
• a thick mantle composed of dense, iron-rich rock called peridotite
• a large core composed mostly of metallic iron, with some nickel

The interior of the Earth is more complex than is indicated by composition alone. From the surface to the center of the Earth both temperature and pressure increase steadily due to the weight and insulation of the overlying rock.

Heat and pressure have opposing effects on materials. Increasing heat tends to cause substances to melt whereas increasing pressure prevents melting. At different depths into the Earth’s interior the influence of one and the other tends to dominate.

At the core, in spite of the great temperature, the tremendous pressure at the center of the Earth prevents the iron metal from melting. However, about the halfway out from the center of the core the pressure drops sufficiently to allow the temperature to dominate and the iron melts. Thus the core has two zones, a solid inner core and a molten (liquid) outer core.

At the boundary between the core and the mantle the Earth becomes solid again due to the higher melting temperature of the mantle rock. The mantle remains solid throughout most of its thickness - a zone called the mesosphere.

At the outer edge of the mantle is a thin zone (app. 150 miles thick) where the pressure is not sufficient to keep all of the material in a solid state. The rock in this zone experiences partial melting which causes it to behave plastically - flowing slowly like thick tar. This ‘weak’ portion of the outer mantle is called the asthenosphere.

Above the asthenosphere the temperature of the mantle drops enough that it again becomes solid. This outermost, solid portion of the mantle is welded to the solid crust above. Together, the two form a rigid outer layer to the Earth called the lithosphere.

The interior power source

The interior machine is powered by the tremendous amounts of heat that are produced in the Earth’s interior. Where does this heat come from? Most of it is created by the decay of radioactive elements that were trapped in the interior when the Earth first formed. These elements (for example Uranium, Thorium, Cesium and many others) spontaneously split apart into smaller elements and release energetic particles in a nuclear process called fission. The energetic particles released by fission collide with other atoms and produce heat.

The Earth can be thought of as a giant fission battery that is slowly running down as it uses up its original charge of radioactive elements. Eventually (in a couple of billion years), the Earth’s interior will cool and the planet will become geologically dead - as the Moon is today.

Two important phenomena result from the way the earth is structured.

Magnetic Field: Because the earth has a liquid, metallic outer core, its rotation causes the conducting liquid metal to move, creating strong electrical currents. The electric currents in turn generate a strong magnetic field. The Earth’s magnetic field is similar to that of a simple bar magnet with opposite north and south polarities.

Plate Tectonics: Because the asthenosphere is soft and plastic, the upper rigid lithosphere floats on the asthenosphere and is able to slide on it. However, the lithosphere does not slide around as a continuous shell. Instead, it is broken up into a series of 11 or so plates, each of which moves independently of the others.

The two types of crust

To understand plate tectonics we need to keep in mind a few facts about the Earth’s crust, mainly that there are two types of crust.

• Continental crust forms the continents and most islands.
• Oceanic crust forms the ocean floors.

Continental crust is enriched in relatively light earth elements, principally Silicon and Aluminum. For this reason we often call it SiAlic crust.

Oceanic crust is enriched in heavier elements such as Magnesium and Iron, so we call it MaFic.

Because of their differences in composition, oceanic crust is denser and heavier than continental crust.

Both types of crust ‘float’ on the underlying, more dense but plastic aesthenosphere, but the lighter continental crust floats higher - forming land masses. Also, oceanic crust tends to be relatively thin and uniform in thickness, while continental crust can be very thick.

Tectonic plates can be made up of either type of crust. Most large plates are a combination of both continental and oceanic crust.

Plate movements

Heat from the core and mantle rises up and causes convection and circulation of the plastic aesthenosphere, which in turn moves the overlying crustal plates.

Where plates are moving apart new ocean floor is created as molten basalt rises from the mantle to fill in the space between the separating plates

Where plates are coming together we have collisions of the crust. Where oceanic crust is colliding, volcanic islands form as one of the two colliding plates is pushed back into the mantle and remelted. If continental crust collides then the crust is thickened, crumpled, and pushed up to form a mountain range.

The exterior power source: Atmospheric movements are driven by the heating of the surface of the earth by the sun. The sun produces energy through the process of nuclear fusion

The Internal and External Heat Engines

The internal heat engine:

Heat from radioactive decay (fission) causes rock in the manlte to flow and melt. This is what drives the movement of the plates. New rock is created at the crust as molten mantle rises to make new seafloor as the plates move. Collisions between the plates push up mountain ranges.

The external heat engine:

Heat from the sun (fusion) falls through the atmosphere and warms the surface, driving circulation within the atmosphere. Wind and water erode rock and wear down the mountains.