Theory of Plate Tectonics
This is the theory that scientists use to describe how continents move throughout the earth’s mantle.
The lithosphere of the earth is broken into pieces, called tectonic plates and these plates move because of the convection currents in the mantle. The convection currents occur when mantle rocks near the core are heated. The rocks become less dense than the upper, cooler rocks, so they rise while the upper rocks sink, creating slow vertical currents which move the mantle rocks centimetres a year. These currents reates pockets of circulation in the mantle called convection cells.
The lithosphere of the earth is broken into pieces, called tectonic plates and these plates move because of the convection currents in the mantle. The convection currents occur when mantle rocks near the core are heated. The rocks become less dense than the upper, cooler rocks, so they rise while the upper rocks sink, creating slow vertical currents which move the mantle rocks centimetres a year. These currents reates pockets of circulation in the mantle called convection cells.
Alfred Wegener's Theory
Theory of Continental Drift
Alfred Wegener’s theory is that the countries had drifted apart and had used to be part of one large mass of land (called Pangaea). Evidence to prove his theory included : - fossils found on island near each other that had drifted away (e.g. the cynognathus was found in both South America and Africa) - landforms from different countries match (e.g. Mountains in South America are similar to ones in South Africa) - fossils of the plant glossopteris had been found in South America, Africa, India, Australia and Antarctica - fossils of the mesosaurus was found in both South America and Africa - fossils of the lystrosaurus was found in India, Africa and Antarctica Another piece of evidence was that rocks formed by magma cooling on the surface of the Earth, all keep a record of the direction and strength of the Earth’s magnetic field at that time. Rocks uncovered at different parts of the Earth did not align with the magnetic field being produced during the time it had been created, telling us that they had moved around. This also proved the continents could be moving. |
Seafloor Spreading
Harry Hess, had proposed that the sea was also moving.
‘Mid-ocean ridges’, huge volcanic mountain ranges in the sea floor, create new rocks out of its solidifying magma (igneous rocks). It was found that the newest rocks were found near the mid-ocean ridges and the rocks got older the further away you got from the ridge. The new seafloor replaces the old which disappears back into the Earth’s interior and ocean trenches. Since the oceanic crust moves, in turn, the continental crust moves with the oceanic crust, supporting the continental drift theory. |
Boundaries
Since the asthenosphere allows the tectonic plates to move around, this allows the plates to come into contact with each other or move away from each other.
The plates all have a boundary between them and can be of 3 types. There are three types of boundaries : Divergent : When plates slide apart from each other (move apart - spreading) - creates rifts and new crust Convergent : When plates collide with each other Transform : When plates scrape against each other (move in opposite directions) - may release built up energy in the form of earthquakes There are two ways the plates can converge, but there are three types of convergent boundaries. This involves continental plates and oceanic plates : - Oceanic to Continental Plate Convergence - Oceanic to Oceanic Plate Convergence - Continental to Continental Plate Convergence The two ways the plates can converge (meet at a point) are: collision and subduction. ~ Oceanic to Continental - Subduction ~ Oceanic to Oceanic - Subduction ~ Continental to Continental - Collision In subduction, one plate goes underneath the other, forming oceanic trenches, while in collision, the plates collide and create mountains. Continental - Oceanic When these two plates collide, the oceanic plate would move underneath the continental plate because of its high density. This collision creates a benioff zone which is an area underground where many earthquakes occur. As well as producing an oceanic trench when the oceanic plate goes under, this convergence can cause mountains and volcanoes. The volcanoes would be created on-shore. Mountains are formed when the continental plate gets lifted over the oceanic plate. Volcanoes are formed when the oceanic crust goes deep into the Earth and melts to form magma. The magma rises and gathers into chambers, creating volcanoes on the continental plate. Oceanic - Oceanic When two oceanic plates collide, one of them slips underneath the other and into the mantle, creating a subduction zone. The plate is then gradually heated up and a benioff zone is formed. The result of this collision is an oceanic trench and a chain of volcanic islands called island arcs. The volcanoes would be created off-shore. The crust that had been pulled under would melt to form magma and this magma rises to the top of the oceanic plates and erupts on the ocean floor. After millions of years, the lava and debris is piled up until it forms a volcano that comes out of the sea, producing a volcanic island. Continental - Continental When these two plates collide, neither of the plates can go under the other because their density is similar. So instead, they start pushing against each other and the rocks on the top of the plates are smashed together and recrystallized because of the intense heat and pressure from the plates. These rocks are compressed and begin the formation of mountains on the Earth’s surface. Often this action would also cause earthquakes. |