Plate Tectonics: Evidence and Plate Boundaries

Plate Tectonics Plate tectonics is the theory that explains why and how continents move and is the study of the formation of features in the earth’s crust.

How do we know? Mid-Ocean Ridges: undersea mountain ranges with steep narrow valleys in the center. Mid-Ocean Ridges form as magma rises from the asthenosphere and forms new oceanic lithosphere (sea-floor) as tectonic plates move apart. Sea-Floor Spreading: The process by which new oceanic lithosphere forms as magma rises from earth’s interior and forms mid-ocean ridges. https://www.youtube.com/watch?v=GyMLlLxbfa4

Sea-floor spreading at a mid-ocean ridge

Here’s what you might see if the process of sea-floor spreading was sped up…

As you have learned, the earth is made up of layers. The closer you are to the surface, the lower the temperature is. Remember – the lithosphere is the earth’s coldest, most brittle layer. The earth is made up of large masses of rock called tectonic (moving) plates. There are twelve large plates, and several small ones.

And here they are!

Fun Tectonic Plate Facts Most plates are about 50 miles thick. Plates move less than a few inches a year (about the same rate as your fingernails grow!) Even though the plates move slowly, when they collide, catastrophic things can happen.

Plate Boundaries The edges of plates are called boundaries. Different things happen at different types of boundaries. Boundaries can be convergent, divergent, or transform boundaries.

Convergent Boundaries At these boundaries, plates collide into each other (converge) like bumper cars. These can occur three different ways (oceanic-continental, oceanic-oceanic, continental-continental.) Different things happen depending on which way the plates converge. At convergent boundaries, one plate either subducts under another plate, or collides with another plate.

Types of Convergent boundaries Continental-Oceanic: Results in subduction of the oceanic plate under the continental plate into the mantle, resulting in a deep, narrow ocean trench. Oceanic-Oceanic: Forms underwater mountain ranges that can sometimes grow tall enough to emerge from the water and form islands like, for example, Japan! Continental-Continental: When these plates collide, major mountain systems form, like the Himalayas.

Himalayas Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up. “Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!

Think-Pair-Share Think (and write): Why would a convergent boundary between oceanic and continental crust result in subduction? Why would a convergent boundary between oceanic-oceanic or continental-continental result in mountains? Pair with your group Share/discuss with class

Divergent Boundaries In these boundaries, plates move away from (diverge from) each other. (Think divergent = divide) When this happens, magma pushes up from the mantle, cools, and fills in the empty space to form new crust. This is known as sea-floor spreading As this happens, the ocean floor expands, which drives the motion of the plates. Examples of this would be mid ocean ridges like the Mid-Atlantic Ridge, and active zones of rifting, like Africa’s Great Rift Valley https://www.youtube.com/watch?v=GyMLlLxbfa4

Divergent Boundaries Mid Atlantic Ridge Africa’s Great Rift Valley 

Iceland: An example of continental rifting Iceland has a divergent plate boundary running through its middle Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature. As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs). With many eruptions over time the island grew out of the sea! Question: Why don’t we have islands like Iceland where ever we get an Ocean Ridge? Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.

Transform Boundaries At a transform boundary, plates move back and forth, or up and down, against each other. This builds up tension that is eventually released in a sudden, violent jerk….. An earthquake! The most well known transform boundary is the San Andreas Fault in the western US. A fault is a crack in the earth where sections of a plate, or two plates, are moving in opposite directions.

So, let’s review: The earth is made of tectonic plates. There are twelve large ones and several smaller ones. Plates only move a few inches a year. There are three types of plate boundaries. Convergent boundaries occur when a plate subducts under another plate. Divergent boundaries happen when two plates move apart. Transform boundaries happen when two plates rub against one another.

What Causes Plate Movement? “Plates” of lithosphere are moved around by convection currents in the asthenosphere. How and Why do tectonic Plates move around? The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells. In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram). This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).

Check for Understanding What type of boundary resulted in this geologic formation? A. Convection Boundary B. Divergent Boundary C. Transform Boundary D. Convergent Boundary

Check for Understanding What type of boundary resulted in this geologic formation? A. Convection Boundary B. Divergent Boundary C. Transform Boundary D. Convergent Boundary

Check for Understanding What type of boundary resulted in this geologic formation? A. Convection Boundary B. Divergent Boundary C. Transform Boundary D. Convergent Boundary