Plate Tectonics: Major and Minor Plates

Lithosphere and Tectonic Plates

• The average thickness of the lithosphere covered by oceanic crust is approximately 50-100 km.
• Tectonic plates move over the asthenosphere due to convection currents, which cause the asthenosphere to flow.
• The movement of tectonic plates is responsible for the Earth's most dramatic geologic events.

Continental Drift Theory

• The scientist who introduced the continental drift theory is Alfred Wegener.
• There are 7 major tectonic plates in the lithosphere.

Evidence for Continental Drift

• The break-up of the large continent Pangaea about 225 million years ago into Laurasia and Gondwanaland supports the continental drift theory.
• The fit of the coastlines of continents on opposite sides of the Atlantic Ocean provides evidence for the continental drift theory.
• The fossil of the extinct mammal-like reptile Mesosaurus, found in both South America and South Africa, supports the continental drift theory.
• The existence of similar mountain ranges of the same age, structure, and rock types in South America and Africa provides evidence for the continental drift theory.

Seafloor Spreading Theory

• According to the seafloor spreading theory, magma rises to the surface and solidifies into new oceanic crust at mid-ocean ridges due to the process of convection.
• The stripes of magnetic material on the seafloor provide strong evidence for tectonic movements, as explained by Frederick John Vine and Drummond Hoyle Matthews.
• The seafloor spreading theory explains the process of plate tectonics, including the creation of new oceanic crust.

Fossil Evidence

• The fossil of the extinct herbivorous, pig-like reptile Lystrosaurus, found in Antarctica, India, and South Africa, supports the continental drift theory.

Mid-Ocean Ridges

• The process of convection, where hot and less dense material from the mantle rises to the surface on mid-ocean ridges, is responsible for the creation of new oceanic crust according to the seafloor spreading theory.