This course covers Continental drift and Plate tectonic theory.
Continental drift was developed in the years 1908-1912 by Alfred Wegener (1880-1930), a German meteorologist, climatologist, and geophysicist. According to him, the continents had all originally been a part of one enormous landmass or supercontinent about 240 million years ago before breaking apart and drifting to their current locations. This supercontinent was called Pangaea.
Over millions of years the pieces of land were separated into two smaller supercontinents, Laurasia and Gondwanaland, during the Jurassic period. Later by the end of the Cretaceous period into the continents of modern period (Figure 2).
Data Supporting Continental Drift Theory
Studies of magnetic rocks in the 1950’s further confirmed Wegener’s theory. As lava cools, or as sediments are deposited, iron tends to align magnetically with the earth’s magnetic field. If the continents were stationary, the iron-bearing rocks would all have the same orientation. Instead, the rocks were uniquely oriented on each continent, suggesting that the continents had moved since the rocks were formed. When the continental shapes were fitted together as one big supercontinent , the magnetic field orientations of rocks on the continents were aligned and the theory gained acceptance.
Wegener proposed that the continents plowed through crust of ocean basins, which would explain why the outlines of many coastlines (like South America and Africa) look like they fit together like a puzzle. Similar rock formations, fossils, and climate data along the coasts of South America and Africa, for example, indicate that these two continents may have once been joined. Magellan and early explorers also noticed that in the map, but Wegener was the first one to explain the concept and advance reasons for it.
Similarly, paleontologists found fossils of similar species on continents separated by great geographic distance. If the continents were in the joined together as one landmass when the fossilized organisms were alive, which was possible through Wegener’s theory, this could be explained.
However, the biggest problem with Wegener’s theory was the failure to identify the force that could move continents.
Although Wegener’s “continental drift” theory was not accepted by many, it was one of the first times that the idea of crustal movement had been introduced to the scientific community. Later, this theory evolved into the widely accepted theory today of plate tectonics.
Plate Tectonic Theory
In the 1960’s, evidence showed how and why the continents move. Ocean exploration revealed mid-ocean ridges and deep trenches in the sea floor. Magnetic evidence from sediment cores taken along the mid-ocean ridge confirmed that the seafloor was slowly moving away from the ridge.
The main features of plate tectonics (Figure 3) are as follows:
- The Earth’s surface is covered by a series of crustal plates.
- The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.
- Convection currents beneath the plates move the crustal plates in different directions.
- The source of heat driving the convection currents is radioactivity deep in the mantle.
The theory of plate tectonics states that the outermost layer of the earth is composed of the lithosphere, which includes the oceanic and continental crust and the rigid top part of the mantle.
This rigid layer is not a single, solid coating surrounding the earth. Instead it consists of several continent-sized pieces of rock, like the well-cracked shell of a hard-boiled egg. The pieces are called tectonic plates. The plates sit atop the asthenosphere, which is the part of the upper mantle just below the lithosphere.
The asthenosphere is solid rock but is hot enough to flow over long periods of time. A solid that can flow is said to be in a plastic state. Convection currents in the mantle are thought to provide enough force to move the tectonic plates across Earth’s surface (Figure 4).
Today, satellites can measure the rate of movement of the tectonic plates and confirm that “continental drift” is taking place. The North American and Eurasian plates are moving away from each other at about 1 inch (2.5 cm) per year.
Tectonic Plate Boundaries
Tectonic plates meet at geologically active regions called plate boundaries. Geologic features, such as mountain ranges, volcanoes, ocean trenches, basins, and ridges form at or near the plate boundaries.
These surface features can change over time as the plates move. The edges of tectonic plates are always interacting with the edges of the adjacent tectonic plates. At these boundaries, there are three possible movements (Figure 5).
These boundaries exist where two plates move toward one another (Figure 6). Subduction zones form when one plate is pushed under another, often due to density differences. A deep trench forms where one plate sinks under the other.
The impact of the colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges or one of the plates may bend down into a deep seafloor trench.
Depth and friction heat and melt the rock deep under the upper plate, causing violent volcanic eruptions inland from the plate edges. The Ring of Fire is a ring of such volcanoes that surrounds much of the Pacific Basin and provides evidence of convergent boundaries.
These boundaries exist where two plates move away from one another. This occurs above rising convection currents. Magma from the mantle wells up to fill in the gaps that form as the plates diverge, creating new lithosphere. At the crest of the uplift, the overlying plate is stretched thin, breaks and pulls apart (Figure 7).
This can lead to formations such as ridges or flatter mountain ranges as basaltic magma wells up through the gap between the two plates. The Mid-Ocean Ridge in the Atlantic, the Rift Valley in Africa, and Greenland are located at divergent boundaries.
These boundaries form when two plates move parallel to one another. Faults, or breaks in the rock, occur at transform boundaries as friction stresses the rock and the plate edges stick and slip (Figure 8).
The San Andreas Fault in California (Figure 9) is an example of a transform boundary. Earthquakes are common along these faults. In contrast to convergent and divergent boundaries, crust is cracked and broken at transform margins.
Away from plate boundaries, upwelling magma can melt a hole through the crust and erupt lava onto the surface. This is called a hot spot. Hawaii is a good example of a hot spot; it is a volcanic island chain right in the middle of the pacific plate.
Another example of a hot spot is Yellowstone National Park. Yellowstone experiences a great deal of geothermal activity because magma very close to the surface heats the surrounding rock.
Any spot where magma is close to the surface, like hot spots or divergent boundaries, can create geological features like geysers. Geysers occur when ground water gets heated to very high temperatures by the surrounding hot rock. The water turns to steam and creates enough pressure to force water up to the surface. Hot springs, which contain water naturally heated by this geothermal activity, are also created in these zones (Figure 10).
The theories of continental drift and plate tectonics are the basis of modern geology. Scientists believe that there were several supercontinents like Pangaea that formed and broke apart over the course of Earth’s . Earth is constantly changing and the continents are still moving.