Updates on Earthquakes for Dec

Again, The Changing Earth

The earth has not always looked the way it looks today. In other words, the United States one billion years ago was in a totally different location than it is today!! How does this happen? And why does this happen? Let's take a look. In order for us to get some understanding of how the earth has changed over time, we first need to understand some of the things that took place, and are still taking place, in the earth.

The Earth's Interior

What about the internal structure of the Earth? Our best clues about the interior come from waves that pass through the Earth's material. When earthquakes shake and shatter rock within the Earth, they create seismic waves which travel outward from the location of the quake through the body of the Earth. Seismic waves are disturbances inside the Earth that slightly compress rock or cause it to vibrate up and down. The velocity and characteristics of the waves depend on the type of rock or molten material they traverse.

Studies of seismic waves have revealed two important types of layering in the Earth: chemical and physical. Compositional layering refers to layers of different composition. Physical layering refers to layers of different mechanical properties, such as rigid layers verses "plastic" or fluid layers. 1

Compositional Layering

Compositional layering was the first type of layering recognized. Seismic and other data indicate that the Earth contains a central core of nickel-iron metal. The core is surrounded by a layer of dense rock, called the mantle, that extends most of the way from the core to the surface. Near the surface, the densities of the rocks are typically lower. The crust is a thin outer layer of lower­density rock about 3 miles thick under the oceans and about 18.5 miles thick under the continents.

The core-mantle-crust structure gives us important clues about the history of the Earth and other planets. First, it shows the importance of differentiation processes - processes that separate materials of different composition from one another. Most geologists believe that the key differentiation process in the Earth was melting of much of the inner rock material after the Earth formed. The source of the heat was radioactive minerals trapped in the Earth as it formed. Gradually those minerals released heat as radioactive atoms decayed. The interior of the Earth was so well insulated by overlying rock that the heat could not escape. The temperature rose until the rock melted. When the rock melted, heavy portions like metals flowed downward toward the center, while lighter, low-density minerals floated toward the surface, where they eventually solidified into a crust of low-density rock.

Note that one of the lowest-density and most common minerals to form in cooling, molten rock is called feldspar. Feldspar should have formed and floated toward the surface. Indeed, we find that surface rocks of both the Earth and the Moon are extremely rich in feldspar. One of the rock types formed from mixtures of minerals rich in feldspar is called basalt. Basalt is the common lava that erupts from volcanoes tapping the crust and upper mantle, and basalt is also common on the Moon's surface.

A still lower-density type of rock is granite, the light colored quartz-rich rock commonly formed from molten materials in continents. In some ways, continents seem to be low-density granite "scum" floating on the denser rocks of the basaltic lower crust and upper mantle.

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