Geology 12

Divergent Boundary /animations/basic_plate_boundari es.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 1 Oceanic 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 2 Two 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 3 Two 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 TypesGeological Phenomenon 1 Divergent Boundary In Oceanic Crust New crust formation Sea-floor spreading 2 Divergent Boundary In Continental Crust Rift valley, volcanic activity New ocean if plate portion breaks away 3 Convergent Boundary Oceanic meets Continental Subduction Zone – Deep sea trench Volcanoes in mountain range 4 Convergent Boundary 2 Oceanic Plates Deep sea trench at subduction zone Volcanic Island arcs Tsunamis (due to underwater earthquakes) 5 Convergent Boundary 2 Continental Plates Mountain range 6Transform BoundaryFaults & Earthquakes