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.

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 40 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

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

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

Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

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.

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

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