World Earthquakes

Earthquakes outline plates Looking at the pattern of major worldwide earthquakes over the past century shows a pattern. Earthquakes are concentrated into areas called “earthquake zones” and most of these zones form bands or lines. Scientists came to realize that these bands represent divisions in the lithosphere and separate it into pieces (plates).

Plate boundaries These divisions within the lithosphere are called “plate boundaries”. It is here that plates interact with each other, resulting in major geologic change, features, and events. Plates may interact in 1 of 3 ways: –Collide, divide (separate), or slide (horizontally past each other).

Plate movement

Types of Plate Boundaries DIVERGENT CONVERGENT

Convergent (compressional) boundaries Plates collide along convergent boundaries. The results of this interaction depends on the types of crust collidiing. Collision zones form where continents collide with other continents. Subduction zones form where continents collide with ocean crust or ocean crust collides with other ocean crust.

Collision zones The worlds largest mountain ranges are formed from colliding landmasses. Continental crust cannot sink into the mantle, so the only place to go is up. Here, the crust buckles, fractures, and thickens to form huge mountains. The fracturing of the crust creates frequent and sometimes violent earthquakes.

plate animations site: collision zone

Convergent (cont-cont) {violent earthquakes; fold mountains}

The Himalayas

Subduction zones Formed by the collision of oceanic crust with either continental or oceanic crust. Ocean crust is dense enough and thin enough to be “dunked” back into the mantle where it collides with less dense crust. This process of ocean crust being absorbed back into the mantle is called subduction.

Trenches A trench is a deep, V-shaped crevice (canyon) on the sea floor. They are created during subduction where the edges of the plates bend downward. They mark the location of a convergent boundary.

Convergent Boundaries

Convergent (oceanic-continental) {violent earthquakes & volcanoes}

Peru-Chile Trench

Aleutian Trench

Puerto-Rico Trench

Volcanoes Some of the world’s most violent volcanoes form in subduction zones, usually as a part of an extended range of mountains and volcanoes called a volcanic arc. These are sparked off by the melting of rock as a plate subducts, heats up, and partially melts.

Convergent (oceanic-continental) {violent earthquakes & volcanoes}

Convergent (oceanic-oceanic) {violent earthquakes & volcanoes}

Divergent Boundary Plates separate (divide) along a divergent boundary. Most divergent boundaries are on the sea floor in the form of mid-ocean ridges. Some form on land in the form of wide rift valleys (Iceland, Eastern Africa). Over time the land splits and a new sea forms. Volcanic eruptions and earthquakes are common along these boundaries.

Divergent boundary {rift valley, mild earthquakes/ volcanoes}

A Rift Valley

Transform Boundary Plates slide past each other without much of a collision. A large crack (called a fault) come to the surface, known as a transform (or strike- slip) fault. Motion along the fault generates frequent and sometimes severe earthquakes. The San Andreas fault in California is an example.

Transform boundary {strike-slip faults, earthquakes}

On the Web Plate Boundaries RAP Musical overview