Thursday, May 21 - Earth - Plate Tectonics
Objective: To understand how plate tectonics impact our Earth.
Evidence suggests that about 225 million years ago, the continents were one large land mass. Scientists named it Pangaea. Over time, pieces of the land mass moved apart. Scientists have theorized that the crust and upper mantle are made up of about 20 sections or plates.
The theory of plate tectonics says that these plates float on the heavier material below. The plates move because the currents of partially molten rock in the mantle carry them along. Scientists believe they move about a half inch to four inches a year.
One piece of evidence scientists have for this theory is the fact that the sea floor is spreading. The Mid-Atlantic Ridge is an underwater mountain range about halfway into the Atlantic Ocean. Scientists believe the sea floor is spreading outward along this ridge, caused by the continual flow of magma from cracks in the Earth’s crust and eruptions of volcanoes along this mountain chain. This is pushing North and South America farther from Europe and Africa and is widening the Atlantic Ocean.
When plates move apart from each other, you have divergent boundaries. These mostly happen on the ocean floor, where they create ridges as the crust starts to separate. This leads to volcanic activity and earthquakes. When it happens on land, you get a long narrow valley.
When two plates push into each other, the boundary where they meet is a convergent boundary. You can get oceanic-oceanic crust, oceanic-continental crust, or continental-continental crust pushing into each other.
When two oceanic boundaries converge, the cooler and denser of the two plates slides under the other plate and melts into magma. This is subduction. Some of the magma is forced back to the Earth’s surface and causes volcanic eruptions and may even create chains of volcanic islands.
When and continental plate and oceanic plate converge, subduction also occurs and the oceanic plate slides under the continental plate. This creates an ocean trench and a chain of volcanoes. The volcanoes form under the continental plate and make a mountain range that has lots of volcanoes.
When two continental plates converge, the two edges are pushed together and lifted up, forming a mountain range.
When two tectonic plates slide horizontally past each other, the crust changes shape or is broken. These are called transform boundaries and they create long cracks called faults. Earthquakes take place along these faults.
Most earthquakes occur along the faults in the Earth’s crust. The pushing, pulling and scraping of the Earth’s tectonic plates causes stress to build up along the faults near the plate’s edges. Eventually, the rock breaks. Earthquakes are caused when large amounts of energy are released by the rocks breaking and moving. The larger the fault and movement, the larger the earthquake.
Land volcanoes form at convergent boundaries. Volcanoes also form where two ocean plates diverge. Some volcanoes form at hot sports – a break in the Earth’s crust through which magma erupts. These are usually hot regions of the mantle where columns of magma rise toward the Earth’s surface. The magma then melts the crust and flows onto the surface forming a volcano.