Geology 12

Divergent Boundary

http://www.wwnorton.com/college/geo/animations/basic_plate_boundaries.htm

Plates move apart.

The crust cracks and magma bubbles between the plates, rises up to form ridges and solidifies.

This is known as seafloor spreading.

Divergent Boundary When two continental plates

diverge, a valleylike rift develops.

This rift is a dropped zone where the plates are pulling apart.

As the dropped zone widens and thins, valleys and volcanoes form.

Early in the rift formation, streams and rivers flow into the low valleys and long, narrow lakes can be created

Eventually, the crust may become thin enough that a piece of the continent breaks off, forming a new tectonic plate.

At this point, water from the

ocean will rush in, forming a new sea or ocean basin in the rift zone.

Convergent Boundary – Type 1Oceanic Plates meets

Continental Plate Oceanic crust tends to be denser

and thinner than continental crust.

The oceanic crust gets pushed under the continental crust. This is called “subduction”.

This forms a subduction zone. The sinking crust creates a deep

oceanic trench, or valley, at the edge of the continent.

The crust continues to be forced deeper into the earth, where high heat and pressure the crust to melt & then rise.

When this magma finds its way to the surface through a vent in the crust, the volcano erupts.

An example of this is the band of active volcanoes that encircle the Pacific Ocean, often referred to as the Ring of Fire.

Type 1 Convergent Boundary

Convergent Boundary – Type 2Two Oceanic Plates Meet

When two oceanic plates collide, the older plate is forced below the younger plate.

This creates another subduction zones.

This creates a chain of volcanic islands known as island arcs.

Examples include the Mariana Islands in the western Pacific Ocean and the Aleutian Islands, off the coast of Alaska.

Colliding Plates – Tsunami Makers

The collision and subduction of plates creates large, powerful earthquakes.

Earthquakes generated in a subduction zone can also give rise to tsunamis.

A tsunami is a huge ocean wave caused by a sudden shift on the ocean floor.

If the wave reaches land, it can cause incredible destruction, like the Asian Tsunami, which killed more than 200,000 people in 11 countries across the Indian Ocean region in December 2004.

Convergent Boundary – Type 3Two Continental Plates Meet

A collision between two continental plates crunches and folds the rock at the boundary, lifting it up and leading to the formation of mountains and mountain ranges.

An example of this mountain-building process is the Himalayan range in southern Asia.

The Himalayas were formed by the collision of the Indian and Eurasian Plates.

Its best known peaks, Mount Everest and K2, are among several mountains that measure over 8,000 meters high at their summits.

Since the Indian Plate is continuing in its northward movement into Asia, the Himalayas continue to grow higher each year by small amounts (5 to 20 mm or 1 inch per year).

Transform Boundary Two tectonic plates grind past each

other in a horizontal direction. This kind of boundary results in a

fault — a crack or fracture in the earth's crust that is associated with this movement.

Faults and Earthquakes

Faults produce many earthquakes.

As the plates grind past each other, the jagged edges "lock" together for a time.

Stress builds up at the fault line. A lot of energy is released when the

plates suddenly slip into new positions.

The sudden movement is what we feel as the shaking and trembling of an earthquake.

Transform Boundary

To recap:Boundary Types Geological Phenomenon

1Divergent Boundary

In Oceanic CrustNew crust formationSea-floor spreading

2Divergent Boundary

In Continental CrustRift valley, volcanic activityNew ocean if plate portion breaks away

3Convergent Boundary

Oceanic meets ContinentalSubduction Zone – Deep sea trenchVolcanoes in mountain range

4Convergent Boundary

2 Oceanic Plates

Deep sea trench at subduction zoneVolcanic Island arcsTsunamis (due to underwater earthquakes)

5Convergent Boundary

2 Continental PlatesMountain range

6 Transform Boundary Faults & Earthquakes