The earth is hurtling through space in its orbit around the sun at a speed of some 67,000 MPH, and rotates on its axis at a speed of over 1,000 MPH at the equator. Yet, someone standing on the equator would hardly be aware of this rapid movement, except for the changing positions of the sun, moon, and other celestial bodies. In fact, except for the occasional earthquake or volcanic eruption, the earth seems to be a solid, unmoving ball of rock.
If one could, however, see an X-ray motion picture of the earth’s interior, a radically different picture would emerge. On both a human and a geologic scale, the earth’s interior, as well as parts of its surface, is in constant motion. Like the earth’s age of 4.5 billion years, which is hard for the human mind to comprehend, the geological processes taking place within the earth’s interior are difficult to grasp.
A close study of geology, however, can help to clarify some of what does occur.
Earth’s Constant Internal Movement
One of the most common processes that takes place constantly in the earth’s interior is the movement of the plates that make up the earth. Movement of these plates relates to earthquake activity, but even in the absence of tremors that can be felt, they are constantly moving; pulling away from each other, diving under or climbing over each other, or sliding past each other.
Those points where the plates move apart are called Divergent Boundaries. The point where one plate dives beneath another is called a Convergent Boundary. The boundary where plates slide past each other is called a Transform Boundary.
At a Divergent Boundary, as plates pull apart, new crust is created by magma swelling up from the earth’s mantle, or that portion of the interior between the crust and the core. The action is like that of two giant conveyor belts, facing each other but moving in opposite directions. One of the best known Divergent boundaries is the Mid-Atlantic Ridge, which is a submerged mountain range which extends from the Arctic Ocean past the southern tip of Africa. This rate of divergence along this ridge is an average of 2.5 centimeters per year, or about 25 kilometers in a million years. While slow by human standards, this spreading is what has caused the Atlantic Ocean to grow from a thin stream between Africa, Europe and the Americas into the vast ocean it is today, a process that took some 100 to 200 million years. The tiny island nation of Iceland is bisected by this ridge, and is slowly pulling apart.
One would assume that this constant creation of new crust, even over millions of years, would result in an increase in earth’s size. But, this has not happened. The creation of new crust is balanced by the destruction of an equal amount at Convergent Boundaries, where crust is destroyed as one plate dives beneath another. Earthquakes are also caused by movement at these convergent boundaries, some of extremely high magnitude, but because many of them are deep beneath the ocean, they don’t cause appreciable damage. The collision of plates can cause significant geological change. The Himalaya’s and the Tibetan plateau, for instance, were caused by the collision of the Indian and Eurasian plates.
The zone between two plates that slide horizontally past each other, or a Transform Boundary, is also called a fracture zone or fault. Most of these faults are found on the ocean floor, but some, such as California’s San Andreas Fault, are on land, and often cause shallow earthquakes.
Looking Below the Surface
The earth is made up of three zones; the crust, the mantle, and the core. The crust, or outer layer, which makes up about one percent of mass and two percent of volume, is the thinnest layer, extending down from the surface to an average depth of 35 kilometers to the mantle.. It is, however, the richest in elements essential to humanity’s survival relative to the other two layers, in large part due to the constant activity in those regions. The crust is not a solid sheath over the inner layers, but a series of plates that float independently on the mantle. These plates are in constant motion, moving a few centimeters each year due to the effect of convection in the mantle.
The mantle, or second layer, is the largest, with about 82 percent of volume and 68 percent of mass. It extends downward 2,900 kilometers to the earth’s core. The mantle acts as a conveyor of the core’s heat to the earth’s crust through a region called the asthenosphere, where the reaction between hot magma or molten rock and cooler magma creates numerous convection currents.
Earth’s innermost layer, the core, accounts for 16 percent of its volume and nearly 31 percent of its mass. It is divided into two regions, a solid inner core believed to be mostly metallic iron alloyed with nickel, and a liquid outer core which is believed to also contain small amounts of sulfur and oxygen.
The Movements We See and Feel
This activity within the earth’s interior sometimes result in phenomena that is apparent to humans, such as volcanic eruptions, earthquakes, tsunamis, landslides and floods.
Volcanic eruptions occur when molten rock reaches the surface through a crack or vent in the crust. These eruptions typically occur near plate boundaries where plate motion has created spaces through which the magma can flow.
An earthquake occurs when the strain in a rock mass builds up to the point where there is a sudden rupture. This rupture is often deep below the surface, with the waves from the point of focus radiating up and out in a release of energy.
Tsunamis occur after underwater tremors, often traveling thousands of kilometers at great speeds, building towering and destructive waves when they hit land.
As can be seen from this brief introduction, earth is not a quiet, unmoving mass, but a cauldron of constant activity. While some of that activity is destructive, it is this constant activity that makes life on this planet possible in the first place.