How Has The Earth Changed?
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. 1
Physical Layering
The second type of layering involves layers of different rigidity. This is thus a physical, rather than a chemical layering. Layering by rigidity was created during the cooling of the Earth. The interior stays molten for a long time because it is hard for the heat to get out, but the surface cools fast because it is exposed to the atmosphere and surrounding space, and the heat can easily radiate away. Thus if a planet were melted and left to cool, a solid layer would form on the surface, while the center is still molten liquid. This picture is complicated by the fact that the layers at different depth have different compositions (because of differentiation) and are also at different pressures. They may thus have different solidifying temperatures, and they may actually form alternating layers of solid material, molten material, or partly molten slushy material. A partly molten layer can be visualized as a layer in which some lowmelting-point minerals are not melted.
The solid layer at the surface of the Earth, or any other such planet, is called the lithosphere, and it is underlain by a partly melted layer that is much less rigid and less strong. The underlying partly melted layer is called the asthenosphere . Interplay between the lithosphere and asthenosphere determines the surface features of the Earth, such as mountains, sea floors, and continents.
Scientists now believe the Earth and other planets were fairly hot when they formed. Once each planet formed, its insides cooled by the three processes of conduction, convection and radiative heat loss. Specifically, heat was conducted outward from the hot center through rock; convection may have occurred in some of the more fluid layers where currents can flow; and heat radiated from the surface into space. As the Earth cooled, the surface layers solidified, forming a relatively rigid surface layer of rock - the lithosphere. In the Earth, the lithosphere is about 62 miles deep, and the asthenosphere underlies it, at a depth of 62 to 218 miles. Even though the asthenosphere is not totally molten, it seems to be plastic enough for sluggish convection currents to flow. These currents bring up hot mantle material, creating "hot spots" in the Earth's crust where volcanoes are likely. The current also drag on the underside of the lithosphere, setting up stresses in it and tending to crack it into large pieces.
Just like Arctic explorers crossing ice floes that are floating on the ocean, we are living on a rigid layer that "floats" on a more fluid base. As the asthenosphere shifts, it can stretch the brittle lithosphere only so far before it cracks. This is what we perceive as an earthquake. 1
Plate Tectonics
Tectonics is the study of movements in a planetary lithosphere, such as the movements that cause
earthquakes, mountain building, and so on. For more than a century, geologists studied the Earth's tectonics without recognizing the underlying nature of the forces that cause these movements. Only since the 1960s have geologists pieced together a real understanding of how these principles affect the Earth and how they apply to other planets. This new understanding is called the theory of plate tectonics.
As the asthinosphere drags on the more brittle lithosphere, it cracks the lithosphere into large, continent-scaled pieces called plates. Further asthenosphere movements tend to drag and jostle the floating plates, sometimes pulling them apart from each other and sometimes pushing them into each other. Cracks along the margins of plates are usually the sites of volcanoes and earthquakes. Volcanoes form because molten magma from below can squeeze up to the surface through the cracks. Plate collisions cause stresses - earthquakes - as plates rub together.
These stresses often cause violent fractures of the lithosphere rock layers. The fracturing events are better known as earthquakes. Fractures caused by earthquakes are called
faults. Colliding plate regions are laced with faults like the famous San Andreas fault. Many earthquakes are caused by movements along these faults.
The theory of plate tectonics also explains why rocks older than 1 or 2 billion years are so rare on the Earth. Most older surfaces have been crumpled beyond recognition or driven downward under other plates, to be re-melted, mixed with mantle material, and perhaps re-erupted as new lava.
Now we can see a connection with the landscapes on other planets. Smaller worlds, like Mars and the Moon, do not have well-developed crumpled mountain ranges or plate boundaries because they cooled faster than big worlds. Their lithospheres got thicker in the same amount of time. Thus their
surfaces are more stable and more protected against asthenosphere currents far below. Lava does not so frequently gain access to the surface. Convection can't so easily drive plates apart or cause them to drift into each other. Therefore, the surfaces of Mars and the Moon have much more ancient structures than the Earth does. 1
The surface of the earth is broken into seven large, and many smaller, moving plates -- all of which ride on the asthenosphere. Each plate is approximately 50 miles thick and moves anywhere from 1/2 inch to several inches per year. As plates move, they collide or separate or move over one another. These three basic types of plate movement have been identified and named -- pictured here on the right -- as: extensional movement; compressional movement; and transformational movement.
Here is an exercise for you. Compute the distance in miles a plate moving at a rate of 1 inch per year would travel in 1 billion years.
Plate tectonics, in conjunction with all the others forces spoken of in this section, have caused a change in the location of the continents over the past 4 billion years. Scientists believe that at one time there was one huge land mass on the earth - a land mass we call "Pangea." Over millions and millions of years, that land mass has changed due to plate tectonics and the continents are positioned as we see them today.
Mechanisms of Surface Change
Volcanism is the eruption of molten materials from a planet's interior onto its surface. On the Earth, the
asthenosphere contains pockets of partly melted materials, as indicated by seismic wave analysis. This underground molten rock, called magma, is under pressure, often charged with gas such as steam, less dense than surrounding rock, and highly corrosive. Therefore it tends to work its way to the surface, especially in regions where fractures provide access. When it reaches the surface, it erupts and is then called lava. If enough lava is erupted, it may accumulate into volcanic mountains. During intervals ranging from years to millions of years, volcanism thus creates new land forms ranging from flat lava flows to craters and volcanic peaks.
Space exploration has shown that volcanism is one of the most important processes forming landscapes on other planets. Some planets have huge lava flows and volcanoes. Study of the Earth's volcanoes helps us understand these alien landscapes. Conversely, study of the other planets' volcanic features helps us understand relations we see among terrestrial volcanoes.
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