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