Click to view the video about convergent plate boundaries.
Convergent Boundaries are boundaries where two plates collide. The type of convergent boundary depends on the types of lithospheric plates involved when the plates collide.
|Convergent Boundaries: Three types of Collision|
|Oceanic – Continental||The oceanic lithosphere sinks underneath the continental lithosphere causing a continental volcanic arc to form.|
|Oceanic – Continental||The oceanic lithosphere sinks underneath the other oceanic lithosphere causing volcanic island arc to form.|
|Continental – Continental||The two continental lithospheres collide forming mountain ranges.|
Examples of convergent boundaries are the Andes Mountain range, Mount St. Helens, and the Himalayas.
Please click on the Drilling for Core Samples link to perform the activity. In this activity, you will map the age of oceanic crust.
Divergent boundaries are boundaries where two plates move away from one another. As the plates move apart, magma from the asthenosphere moves to Earth’s surface. Eventually the magma cools to form new oceanic lithosphere. The newly formed oceanic lithosphere is denser, lighter, and taller than the surrounding rocks on the ocean floor. The formation of the additional lithospheric rock is called mid-ocean ridges. Mid-ocean ridges look like undersea mountain ranges. Rift valleys occur at the center of a mid-ocean ridge. An example of a divergent boundary is the Mid-Atlantic Ridge.
Although most divergent boundaries are found along a spreading ocean seafloor; some divergent boundaries create rift valleys when continents move apart. The separation of the African plate and the Arabian plate creates a large rift valley in the Red Sea. These two plates have separated so much that the rift valley has dropped below sea level. The Red Sea covers this rift valley.
The East Africa Rift valley is above sea level and it was also formed when two continental plates were moving away from each other. It contains only a few lakes in certain areas and one day it may be filled completely with water. Rift valleys formed above land can range from 30 -50 km wide. Rift valleys that develop along the ocean floor are less than a kilometer wide. Volcanic and earthquake activities can occur along divergent plate boundaries. Click to watch a video about divergent plate boundaries.
Transform boundaries occur where two plates slide horizontally past one another. This motion is often felt as earthquakes. The plates grind across each other without producing magma. Most of them occur along the ocean floor to produce ocean ridges. Transform boundaries are also known as transform faults. The Sans Andreas Fault in California is an example of a transform fault that can take place over continents instead of in the ocean. It occurs between the North American plate and the Pacific plate. The Sans Andreas Fault is about 1300 km long. Movement of tectonic plates along transform plate boundaries are responsible for the majority of earthquakes that occur in California. Another example of a transform boundary is the Alpine Fault in New Zealand.
Click to watch the movement of transform plate boundaries.
|Plate Boundary Summary|
Description – crust is destroyed when tectonic plates move toward each other and collide
Examples – ocean trenches, mountain ranges, volcanoes, and island arcs
Description – New lithospheric crust is formed when tectonic plates move away from each other
Examples – Rift valleys and mid-ocean ridges
|Transform||Description – Crust is fractured or deformed when tectonic plates slide past each other while moving in opposite directions
Examples – San Andreas Fault in California and mid-ocean ridges