Himalayan mountains (David Mark, Pixabay)
Today, most people know that the landmasses on Earth move around. But people haven’t always believed this. It wasn’t until the early 20th century that German scientist Alfred Wegener suggested that the Earth’s continents were drifting. He called this movement Continental Drift. He was not the first or only person to think this, but he was the first to talk about the idea publicly. Wegener came up with this idea because he noticed that the coasts of western Africa and eastern South America looked like puzzle pieces. He wondered if they might have once fit together and then drifted apart. Looking at the continents he theorized that they had once been joined together as a supercontinent around 225 million years ago. This continent was later named Pangaea.
People now think that other supercontinents may also have existed. These supercontinents are thought to have come before Pangaea. 
Shown are colour maps illustrating the Earth's continents in five different eras. Starting on the top left, the first map is labelled "Pangea, 250 million years ago." Here, the continents are all joined together in one long, lumpy mass down the centre of the map. At the top, North America and Eurasia are joined side-to-side. Below, South America and Africa are also joined side-to-side. These two masses have merged into one long one, with Antarctica and Australia joined at the bottom. The whole mass is labelled "Pangea." To the right, the second map is labelled "Triassic, 200 million years ago." Here, the northern land mass is almost separated from the southern by a wide strip of water. The northern mass is labelled "Laurasia." One thin piece of land joins Asia and Africa. The water to the right of this is labelled "Tethy's Sea." A thin strip of water divides most of South America and Africa. But they are still joined at one spot, just above the ends of each. This mass, is labelled "Gondwanaland." The third map is labelled "Jurassic, 145 million years ago." Here, North America and Eurasia are just touching. As are South America and Africa. The fourth map is labelled "Cretaceous, 65 million years ago." The idea of moving landmasses seems obvious now, but Wegener’s Theory of Continental Drift was not accepted for many years. Why? Well, for one thing, Wegener did not have a convincing explanation for the cause of the drifting. First, he suggested that the continents were moving around due to the Earth’s rotation. This later turned out to be wrong. Secondly, he was a meteorologist, not a geologist. This meant that geologists didn’t think he knew what he was talking about and did not take him seriously. Fossil EvidenceOne type of evidence that strongly supported the Theory of Continental Drift is the fossil record. Scientists have found fossils of similar types of plants and animals in rocks of similar age. These rocks were on the shores of different continents. This suggests that the continents were once joined. For example, fossils of Mesosaurus, a freshwater reptile, have been found both in Brazil and western Africa. Also, fossils of the land reptile Lystrosaurus have been found in Africa, India and Antarctica.
Shown is a map of Earth's southern continents, with colours indicating locations of fossil evidence. The map shows the shapes of present-day South America, Africa, India, Antarctica and Australia. These fit close together, almost like puzzle pieces. The land is white on a pale blue background. A stripe of orange crosses through the width of South America, and into the centre of Africa. This is labelled "Fossil remains of Cynognathus, a Triassic land reptile approximately 3m long." Above the label is an illustration of a reptile with four feet and a long tail. A stripe of beige crosses through most of the width of southern Africa, the middle of India, and into the centre of Antarctica. This is labelled "Fossil evidence of the Triassic land reptile Lystrosaurus." Next to the label is an illustration of a stout reptile with a round head. A stripe of green crosses through the lower parts of South America and Africa and through the tip of India. Then it curves in an L shape through Antarctica and into the lower part of Australia. This is labelled "Fossils from the Glossopteris, found in all of the southern continents, show they were once joined." Next to the label is an illustration of a plant with long, pointed leaves. Finally, a stripe of blue crosses just above the tip of South America, into the tip of Africa. This is labelled "Fossil remains of the freshwater reptile Mesosaurus." Next to the label is an illustration of a reptile with a pointed snout and a long, pointed tail. Plate TectonicsThe Theory of Plate Tectonics builds on Wegener’s Theory of Continental Drift. In the Theory of Plate Tectonics, it is tectonic plates, rather than continents, which are moving. Tectonic plates are pieces of the lithosphere. The lithosphere is made up of the crust and the upper mantle. These pieces float on a layer of partly liquid rock called the asthenosphere. Tectonic plates are able to move because the lithosphere is stronger and more dense than the rock below it. There are currently seven plates that make up most of the continents and the Pacific Ocean. They are:
Shown is a world map overlaid with numbered and coloured sections to illustrate tectonic plates. Section 1 is peach coloured and labelled "African Plate." This covers the continent of Africa, and a wide stripe of the water around it. Section 2 is blue and labelled "Antarctic Plate." It covers the land and ocean along the bottom edge of the map. This plate touches all the others in the Southern Hemisphere. Section 3 is green and labelled "Eurasian Plate." This appear on both sides of the map, touching the east and west edges of the North American Plate. It covers most of Europe, Russia, and Asia. Section 4 is orange and labelled "Australian Plate." It covers Australia, and a wide strip of the ocean around it. Section 5 is beige and labelled "North American Plate." It is in the top centre of the map, and covers North America, Iceland, and the eastern tip of Russia. Section 6 is yellow and labelled "Pacific Plate." It covers a most of the width of large portion of the Pacific Ocean. Section 7 is purple and labelled "South American Plate." This covers South America and a wide strip of ocean to the east of the continent. Small red arrows around the perimeter of each plate indicate its direction of movement. The North American Plate and the Pacific plate move in opposite directions. The Eurasian Plate moves away from the North American Plate. As do the African Plate and the South American Plate. The Australian Plate and the Pacific Plate move towards each other. The Antarctic Plate moves away from all the other plates it borders. Eight other, smaller, coloured sections are labelled, without numbers. These include Filipino Plate, Juan de Fuca Plate, Cocos Plate, Caribbean Plate, Arabian Plate, Indian Plate, Nazca Plate, and Scotia Plate. There are eight other smaller secondary plates. There are also other microplates, which are very small.
You may imagine that these plates are zipping along, but in fact, they are moving VERY SLOWLY! Some move as slowly as 10–40 mm/year. This is about as fast as your fingernails grow. Others move as fast as 160 mm/year. This is about as fast as hair grows. Moving PlatesGeologists came to accept the Theory of Plate Tectonics in the late 1950s and early 1960s after coming to understand the concept of seafloor spreading. Seafloor spreading happens on the seafloor where oceanic plates are moving away from each other. We say that these plates are diverging. When this happens, cracks occur in the lithosphere. This allows magma to come up and cool, forming a new seafloor. The opposite of divergence is convergence. This happens when plates are moving towards each other. Material may push upwards forming mountains or downwards into the mantle. When the material from one plate is pushed on top of another, we call that obduction. When material is pushed downwards, we call that subduction. The material lost through subduction is roughly balanced by the formation of new ocean crust by seafloor spreading. The theory of plate tectonics explained (2015) by Minute Earth (2:36 min.).When tectonic plates meet, some dramatic things can happen. Let’s look at some of these in more detail. Mountains and VolcanoesWhat do mountains and volcanoes have in common? If you said that they are both large, steep landforms made of rock, you would be right. If you said that both are formed when tectonic plates are pushed and pulled, you would also be right! Whether mountains or volcanoes form depends on the type of tectonic plates and where the plates collide. To predict which will happen, you need to know two things.
Let’s look at how tectonic plates form mountains and volcanoes. Let’s see what happens when an oceanic plate converges with a continental plate. Plate Boundaries-Divergent-Convergent-Transform (2020) by MooMooMath and Science (2:52 min.). |
Which of the following explanations best identifies the type of plate boundary and processes occurring at the Mid-Atlantic Ridge shown in diagram below?
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