Old Earth Ministries Online Earth History Curriculum
Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)
This curriculum is presented free of charge for use by homeschooling families.
NOTE: If you found this page through a search engine, please visit the intro page first.
Chapter 1 - Earth History Overview
Lesson 5: Plate Tectonics
did God create a world that involved moving plates? As you will find
out, tectonic plates are responsible for building mountains, and they are
responsible for volcanic activity. These two methods of building new
land counteracts the erosion that occurs, which removes land. Without
a method of replacing land lost to erosion, our world would be flat and
completely covered in water...great if you are a fish, but not so great for
us humans! The system God put in place ensures that the land will
renew itself, and give us dry land to live on!
Chapter 1 - Overview
When maps were first made of the continental land masses, scientists noted that the continents looked like they could fit together, like pieces of a puzzle. Although this occurred in the 1800's, the first proposal that gained attention was put forth by Alfred Wegener, who was a German meteorologist. In 1915 he authored a work called The Origin of Continents and Oceans. In a series of maps, he showed how the land masses had moved, and had originated from a single landmass, or supercontinent. He called this landmass Pangea (also spelled Pangaea). The name stuck, and geologists today still call the last single landmass by that name. Interestingly, at the time it was proposed, most geologists rejected the idea until the 1960's. To see another animation of the breakup of Pangea, open the link Breakup of Pangea.
There is several good lines of evidence supporting Wegener's original theory.
Paleontologic Evidence. Wegener and others noted that certain fossils found on both sides of the Atlantic ocean were very similar. From this evidence, they reasoned that Africa and South America had once been connected. Marine fossils could have migrated across the ocean, but plants and land-dwelling reptiles could not. Consider the seed fern Glossopteris, which has been found in rocks of the same age in all of the southern continents. Fossils have been found in South America, South Africa, Australia, and India, and even in Antarctica. The seeds of this plant are too large to be carried by the wind. For reptiles, the distribution of Paleozoic and Mesozoic species is even more compelling. For example, the mammal-like reptile Lystrosaurus was a land-dwelling reptile. Its fossils are abundant in South Africa, South America, Asia, and Antarctica. For a diagram of how other fossils support the concept of Pangea, see the diagram at right.
Geologic Features and Rock Types. Some geologic features on continents match, even though they are thousands of miles apart. For example, folded mountain ranges at the southern tip of Africa terminate sharply at the coast, and an equivalent structure of the same age, with the same style of deformation, and the same rock type, appears in Argentina. This is also true of the Appalachian Mountains. They extend northeastward into Newfoundland and terminate abruptly on the coast. They continue on the coast of Ireland and Brittany.
Glaciation. Near the end of the Paleozoic Era, glaciers covered a large area of the southern hemisphere. The deposits left by these glaciers are easily identified, and they are present on parts of South America, Africa, Australia, and Antarctica. Striations (groves left in the rocks from the scraping action of ice) give the direction in which the ice moved, which further confirms the way the continents fit together. For more, see Karoo Ice Age.
Paleoclimates. The climate of the past can be used as evidence of tectonic movement. For example, there are vast coal deposits in Antarctica, which is now a frozen continent. This indicates it once had abundant plant life and a more hospitable climate. Other continents which have ancient deposits of desert sandstones are hard to explain based on their present positions, yet when combined with where the continents were located millions of years ago, they are easily explained.
Why did the geologists of the early 1900's reject the theory of continental drift, when the evidence seemed to support it? They reasoned that there was no mechanism that could cause the such large continental land masses to move. This changed with the study of the earth's ocean basins in the 1960's.
Plate Tectonics is Born
With the study of the ocean floor in the 1960's, geologists noticed features that clearly favored the theory of continental drift. These features were the topography and geology of the ocean floor, and rock magnetism.
Ocean Floor Geology
New devices developed in the 1950's and 1960's enabled geologists to map the topography of the ocean floor. The topography revealed that the ocean basins are divided by long ridges. These features were studied in an earlier chapter. Another tool used by geologists is called core drilling. Using a cylindrical drill bit that is hollow, geologists are able to retrieve sections of the ocean floor to see what it is composed of, and to make other measurements, such as radiometric dating. It was noticed that the younger rocks are near the ridges, and the further you got away from the ocean ridges, the older the rocks. Geologists finally concluded that the ocean floors were spreading from the mid-ocean ridges, and they dived under the continental land masses at the deep trenches that were observed in the topographic maps of the ocean floor. They reasoned that because the mantle was partially molten, convection currents in the mantle had to be the mechanism that caused this movement. Based on the rate of movement, obtained from the dating of the oceanic floor, it was discovered that the oceanic floor completely regenerated itself in 200 to 300 million years.
Also in the 1950's, advances in magnetometers, the tools used to measure magnetism in rocks, brought further evidence of plate movement. The rock type being produced at the mid-oceanic ridges is basalt, which contains iron. As the rock cools, the mineral grains become oriented based on the magnetic field. Because of this, a new branch of study arose, called paleomagnetism, which is the study of magnetism of the past. However, it was not just igneous basalt that could be used. Red sandstone also contains iron, so that its grains become oriented also. By studying the orientation of these grains in basalt and sandstone, geologists are able to put together a history of the earth's magnetic field.
It was discovered that over the millions of years of earth's history, that the earth's magnetic poles actually wander (called polar wandering). You may have already noticed evidence of this. If you look at a map, you may notice that there is a geographic north and south pole, from which the grids of latitude and longitude are determined. The geographic poles are determined from the point of rotation of the earth. The axis upon which the earth rotates indicates the geographic poles. The actual magnetic poles are different from the geographic poles. However, it is even more complicated than that! For more, see North Pole. To see a map of the history of the North Magnetic Pole for the last 400 years, see the image at right.
Another feature that geologists noted is that the grains were oriented in the opposite direction in many rocks. Some would indicate an orientation to the north ("positive" for the magnetic field), whereas others a short distance away were oriented toward the south. Geologists determined that the earth's magnetic field reversed itself from time to time. These magnetic reversals occurred frequently. For example, during the last 76 million years, there have been at least 171 reversals. Our current polarization has been in effect for the last 700,000 years. The periods of alternating polarity are called polarity epochs, and they average about one million years in duration. The changing polarity can be mapped on the ocean floor, which produces a striped pattern of alternating periods of normal and reversed polarity (see picture at right). The striping pattern on one side of the oceanic ridge is a mirror image of the pattern on the other side of the ridge. To read more, click this link.
More evidence for plate tectonics comes from the sediments on the ocean floor. Thanks to core drilling, scientists can also examine the fossils that are buried in a particular section of ocean floor. Fossils with known ranges of existence confirm the dates that we obtain from radiometric dating. For example, if we radiometrically date a section of ocean floor basalt at 100 million years old, then the fossils that are contained just above the basalt should be close to the same age. When we examine them, we find that this is the case.
In addition, observed rates of sedimentation in the open ocean are about 1.0 centimeters of sediment every 1,000 years. If the ocean basins had existed 500 million years ago, the sediment would be at least 5 kilometers thick. However, ocean bed sediment is not this thick. In fact, the thickest sediments are only 300 meters thick. The oldest ocean sediment dates to about 160 million years old. By comparison, the oldest continental crust material is dated to about 4 billion years old.
The earth's crust is made up of tectonic plates that fit together like the pieces of a puzzle. The largest plate is the Pacific plate, which is composed entirely of oceanic crust. It covers about one-fifth of the earth's surface. The rest of the major plates are a combination of both oceanic and continental crust. It is important to note that tectonic plates are not permanent features of the earth's surface. As the earth's surface changes, plates appear and disappear.
Tectonic activity is mostly found at the plate boundaries, where two plates intersect each other. There are three kinds of plate boundaries, each having its own characteristics and causing specific rock deformation.
Divergent Plate Boundary
Also known as spreading centers, these are where a plate splits and is pulled apart. Divergent plate boundaries are characterized by tensional stresses, producing block faulting, fractures, and open fissures at the spreading center. The rock type is typically basaltic magma. For two animations showing this process, see Divergent plate boundary and Divergent boundary.
Divergent plate boundaries can also occur on continents. The Great Rift Valley in Africa is an example (see map). Long, linear valleys, some filled with water, and volcanoes are evidence of continental rifting. The great volcanoes of Mount Kenya and Mount Kilimanjaro owe their origin to continental rifting.
Convergent Plate Boundary
Convergent plate boundaries consist of three types of plate collisions, based on the type of crust colliding and its density. First, when the two colliding plates are oceanic, one plate is thrust under the other. The lower plate dives, or "subducts" underneath the other plate, hence the term subduction zones. Second, subduction zones can also occur between continental and oceanic plates. The oceanic plate always subducts under the continental plate, because continental rock is less dense than oceanic rock, causing it to float over the more dense oceanic crust. Subduction zones are known for their volcanic activity. Mount Saint Helens in Oregon owes its origins to a subduction zone (see map at right). For an animation of oceanic/continental convergence, see Convergent plate boundary. Finally, in a collision of two continental crusts, neither crust can subduct. In this case, one crust may briefly override the other. In most cases, this collision produces mountain ranges, such as the Himalayan range. The India land mass gradually moved northeast until it collided with the Eurasian Plate. It is currently moving at a rate of 67 mm/year, or about 2.6 inches.
Transform Fault Boundary
The third type of plate boundary is the transform fault boundary. Some geologic texts also refer to them as passive plate margins. These are zones where two plates slide past each other without diverging or converging. Another term to describe their relationship is a strike-slip fault. Unlike the other two types of boundaries, there is no volcanic activity with transform faults. You will learn more about these faults in the chapter on earthquakes.
During earth's history several supercontinents have been identified. The youngest of these is Pangea, which existed 200 million years ago. Evidence suggests that another supercontinent, named Pannotia, may have existed about 600 million years ago. Pannotia existed for about 50 million years. The supercontinent Rodinia is believed to have formed about 1.1 billion years ago, and it existed until about 750 million years ago. Some scientists have proposed another supercontinent, called Columbia, between 1.8 and 1.5 billion years ago. There may have been earlier supercontinents as well.
In addition there have been other smaller supercontinents. The most well-known of these is known as Gondwana, which comprised the landmasses of the continents that are now in the southern hemisphere. During the study of the different geologic periods, many of these supercontinents will be discussed.
End of Lesson