The Earth's crust is divided into separate pieces known as tectonic plates. These plates are free-floating and move independently. Earthquakes and volcanoes directly result from the movement of these plates along fault lines. Plate tectonics, the scientific theory explaining the movement of the Earth's crust, is widely accepted today.
Fig. 7.14. Map of the Earth’s major tectonic plates with arrows indicating movement direction.
The locations of Earth's major landmasses today differ significantly from their past positions. Over millions of years, they have gradually moved, combining into supercontinents and separating in a process known as continental drift.
Evidence of Continental Drift
The shapes of continents offer clues about their past movements. For instance, the west coast of Africa has an indentation that matches the bulge along the east coast of South America. Some fossils also provide evidence that continents were once closer together. Fossils of marine reptiles like Mesosaurus and land reptiles like Cynognathus have been discovered in both South America and South Africa. Similarly, the fossil plant Glossopteris is found in India, Australia, and Antarctica.
Fig. 7.19. The shapes of the continents and fossil distribution provide clues about past continental movements.
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Evidence for continental drift is also found in the types of rocks present on different continents. There are matching rock belts in Africa and South America when the continents are joined. Mountains of similar age and structure are found in northeastern North America (Appalachian Mountains) and across the British Isles into Norway (Caledonian Mountains).
Paleoclimatologists study evidence of prehistoric climates. Glacial striations in rocks show that 300 million years ago, large ice sheets covered parts of South America, Africa, India, and Australia. These striations indicate that glacial movement in Africa was toward the Atlantic Ocean basin, while in South America, it was from the Atlantic Ocean basin. This suggests that South America and Africa were once connected.
Plate Boundaries and Movement
Convection currents in the Earth's mantle drive the movement of tectonic plates. Where these currents rise towards the crust's surface, tectonic plates move away from each other in a process called seafloor spreading. Hot magma rises to the crust’s surface, cracks develop in the ocean floor, and the magma pushes up and out to form mid-ocean ridges.
Fig. 7.23. Subduction occurs when two tectonic plates meet and one moves underneath the other.
Seafloor spreading gradually pushes tectonic plates apart at mid-ocean ridges. When this happens, the opposite edges of these plates push against other tectonic plates. Subduction occurs when two tectonic plates meet, and one moves underneath the other. Oceanic crust, being denser than continental crust, slides below it. This collision can result in the formation of volcanoes.
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Continental collision occurs when two plates carrying continents collide. Because continental crusts are composed of the same low-density material, one does not sink under the other. During collision, the crust moves upward, and the crustal material folds, buckles, and breaks. Many of the world’s largest mountain ranges, like the Rocky Mountains and the Himalayan Mountains, were formed by the collision of continents resulting in the upward movement of the earth’s crust.
Volcanoes and Hot Spots
Some volcanoes form near plate boundaries, particularly near subduction zones. However, some volcanoes form over hot spots in the middle of tectonic plates, far from subduction zones. A hot spot is a place where magma rises up from the Earth’s mantle toward the surface crust. When magma erupts and flows at the surface, it is called lava.
A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. Rift valleys are found on land and at the bottom of the ocean, where they are created by the process of seafloor spreading. Tectonic plates are huge, rocky slabs of Earth's lithosphere-its crust and upper mantle.
Many rift valleys are part of “triple junctions,” a type of divergent boundary where three tectonic plates meet at about 120° angles. Two arms of the triple junction can split to form an entire ocean. The third, “failed rift” or aulacogen, may become a rift valley. The Atlantic Ocean, for instance, is a result of a triple junction that started in what is now the Gulf of Guinea on the west coast of Africa.
Africa Is Tearing Apart Much Faster Than Expected
Rift valleys can also form at transform faults, where tectonic plates are grinding past each other.
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The African Plate
The African Plate is the 4th largest plate tectonic boundary on Earth, moving at an average rate of about 2.5 centimeters per year. The west side of the African Plate straddles the South American Plate and the North American Plate. Along the east edge of the boundary, the Somali Plate is positioned along the East African Rift zone. The west side of the African major plate diverges from the North American plate. With an area of about 61,300,000 km2, the African Plate includes most of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south.
The African plate includes several cratons, stable blocks of old crust with deep roots in the subcontinental lithospheric mantle, and less stable terranes, which came together to form the African continent during the assembly of the supercontinent Pangea around 250 million years ago. The cratons are from south to north, the Kalahari Craton, Congo Craton, Tanzania Craton and West African Craton.
The African plate is rifting in the eastern interior of the African continent along the East African Rift. This rift zone separates the African plate to the west from the Somali plate to the east.
It is pushing closer to the Eurasian plate, causing subduction where oceanic crust is converging with continental crust (e.g. portions of the central and eastern Mediterranean). In the western Mediterranean, the relative motions of the Eurasian and African plates produce a combination of lateral and compressive forces, concentrated in a zone known as the Azores-Gibraltar Fault Zone.
Passive Margins and Seismic Activity
The coast of West Africa, once the site of active tectonics, is now a passive margin, far from an active tectonic plate boundary. Despite being considered seismically quiet, the region is experiencing a growing number of earthquakes. Research suggests that the continental margin is undergoing a mix of thrust and strike-slip deformation, influenced by stresses transferred from the oceanic crust and from tectonically active northern Africa.
Types of Coasts
The tectonic setting significantly influences the character of a continental shelf and coast. Collision coasts, facing a plate boundary, are tectonically active with earthquakes and volcanic activity. Trailing edge coasts, far from plate boundaries, lack tectonic activity and can be further classified into Amero-trailing, Afro-trailing, and Neo-trailing edge coasts, each with distinct characteristics.
Trailing Edge Coasts
- Amero-trailing edge coasts:Occur in the middle of a tectonic plate and face a spreading center.
- Afro-trailing edge coasts:Face a major spreading center, and the opposite coast also faces a spreading center.
- Neo-trailing edge coast:Face relatively young plate tectonic spreading centers.
Marginal Sea Coasts
Coasts of this character occur along continental coasts facing an island arc. They are sheltered from the conditions of the open ocean by other landmasses, such as island arcs, which were created by the collision of tectonic plates.
Geologic History of Africa
The African continent consists of five ancient Precambrian cratons-Kaapvaal, Zimbabwe, Tanzania, Congo, and West African-formed between about 3.6 and 2 billion years ago. These cratons are bounded by younger fold belts formed between 2 billion and 300 million years ago. The oldest rocks are of Archean age (about 4.6 to 2.5 billion years old) and are found in the granite-gneiss-greenstone terrains of the Kaapvaal, Zimbabwe, and Congo cratons.
After the Precambrian, Africa’s geologic history is characterized by the formation of fold belts in the Paleozoic Era (about 541 to 252 million years ago) in South Africa, Morocco, and Mauritania; voluminous basaltic volcanism some 230 to 200 million years ago in South Africa, Namibia, and East Africa; the formation of a young mountain belt in northwestern Africa some 100 to 40 million years ago; and the development of the East African Rift System during the Cenozoic Era (i.e., roughly the past 66 million years).
The East African Rift is one of the most biodiverse regions in Africa, featuring a narrow corridor of highland forests, snow-capped mountains, savannas, and chains of lakes and wetlands.
Here's a table summarizing the key characteristics of different types of coasts:
| Type of Coast | Tectonic Activity | Continental Shelf | Depositional Features | Examples |
|---|---|---|---|---|
| Collision Coasts | Frequent earthquakes and volcanic activity | Narrow | Few | West coasts of North and South America |
| Amero-Trailing Edge Coasts | None | Wide | Numerous (marshes, barrier islands, spits, mangroves, deltas) | East coast of North America, east coast of South America, India |
| Afro-Trailing Edge Coasts | None | Variable (wide to narrow) | Variable (many to very little) | East and west coasts of Africa, coast of Greenland, parts of coastal Australia |
| Neo-Trailing Edge Coasts | Some volcanism, seismically active | Very narrow | Very few (pocket beaches) | Gulf of California |
| Marginal Sea Coasts | None | Wide | High sediment influx, deltas | Coasts of mainland China on the South China Sea, the East China Sea, the Sea of Japan, and the Yellow Sea |
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