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