Click here to return to Internal Forces Home Page
Name___________________________ HW p 1
What Happens At Plate Boundaries?
Continental and Oceanic Crust plates contain both continental and oceanic crust!
When discussing collisions, plates are often described as oceanic or continental plates. An oceanic plate has a colliding edge that consists of dense oceanic crust. A continental plate has a colliding edge that contains less dense continental crust.
Keep in mind that all plates contain oceanic crust and most plates contain continental crust. Also: a plate may act as a continental plate in one collision and an oceanic plate in another.
Differences between the two types of plates:
Oceanic plate: 1. denser than continental crust
2. heavier than continental crust
3. made from chiefly 'basic' rocks (basalt)
4. the older oceanic crust becomes, the more dense it becomes
5. thinner than continental crust
6. younger than continental crust and is constantly being destroyed and replaced.
Continental plate:
1. less dense than oceanic crust
2. lighter, cannot sink
3. made chiefly of 'acidic' rocks (granites)
4. thicker than oceanic crust
5. older than oceanic crust
6. Is permanent
Ocean- Continent collisions (converging plates)
A continental plate rides over the edge of an oceanic plate because the continental plate is less dense. The oceanic plate is subducted (sucked under) into the trench that forms the plate boundary.
HW: Answer this question: Compare the continental crust with oceanic crust. Write a paragraph using the above information. Title your paragraph: "Continental and Oceanic Crust Compared". Write this in your OWN words.
What Happens At Plate Boundaries ?
Name________________________ HW p2
Ocean-Ocean Convergent Boundary
When two oceanic plates collide, the older oceanic plate is subducted under the younger. Plates grow denser as they cool, and older plates have had longer to cool. Thus older plates are denser. The plate being subducted melts. Molten rock then rises up and breaks through the surface. As a result, a string of volcanoes erupts on the ocean floor along the trench. In time, this string of undersea volcanoes may rise above the ocean's surface as a string of islands. Because the islands usually appear in a curved line, they are called island arcs.
Features: deep trench, island arcs
Examples: Japan, Aleutian Islands, Indonesia, Philippines
Earthquake activity: violent to less violent
Volcanic Activity: violent
What Happens at Plate Boundaries?
Name_____________________________ HW p 3
When an oceanic plate collides with a continental plate, the continental plate rides over the edge of the oceanic plate because the continental plate is less dense--even though it is thicker. The oceanic plate is subducted (sucked under) into the trench that forms the plate boundary. This action pushes up and folds the continental crust on the edge of the continental plate. This forms folded mountain ranges and volcanoes.
Features: folded mountains, volcanoes
Examples: Andes Mountains of South America, northern Cascades of North America, Mt. St. Helen's
Earthquake activity: violent
Volcanic Activity: violent
What Happens at Plate Boundaries?
Name_________________________________ HW pg 4
When two continental plates collide, the edges of the continents fold upward to form large mountain ranges. At first, some of the oceanic crust that lies beneath the continental crust on one of the plates may be subducted. Because continental crust is too light to sink into the Earth, it is scraped off the oceanic crust and piles up at the boundary. But the continental crust cannot pile up forever. Eventually, the boundary jams up and disappears.
In some collisions of continental plates, the plates have approximately the same density. In such collisions, neither plate is subducted, and the edges of the continents thicken greatly and push upward as they are forced together.
Features: mountain belts, folding, faulting
Examples: Appalachian Mountains eastern US., Himalayan Mountains(still happening!)
Earthquake activity: violent, sometimes occurring mid-continents
Volcanic Activity: rare
What Happens at Plate Boundaries?
Name________________________ HW pg 5
The Earth's plates can move apart from one another. When this happens in two oceanic plates, it is called ocean-floor spreading. New material is being created by lava flowing into a huge crack at the plate boundaries. The crack is called a mid-ocean ridge. Molten rock, or magma, immediately rises to fill any possible 'gap', forming new ocean crust. The Atlantic Ocean is widening by about 9 centimeters a year, which means that the Americas are moving away from Eurasia and Africa at about the rate your fingernails grow!
Features: mid-ocean ridge, gentle volcanoes, less violent earthquakes, islands, transform faults
Examples: Mid-Atlantic Ridge, Iceland, Azores
Earthquake activity: less violent
Volcanic Activity: undersea volcanoes
What Happens at Plate Boundaries?
Name___________________________ HW page 6
New divergent boundaries may form in the center of continents. The formation of the new boundary begins when rising magma heats and weakens an area of a continental plate. The area cracks and sections slip down to form a rift valley. Eventually ocean water fills in the widening gap between the newly formed continents. Lava erupting from the rift forms new ocean floor. After millions of years, there is a mature ocean where there was once dry land.
Features: rift valleys, transform faults, block faulting
Examples: East Africa-rift valley
Earthquake activity: less violent
Volcanic Activity: rare
What Happens At Plate Boundaries?
Name_________________________________ HW pg 7
At a strike-slip boundary, two plates grind together and slip past each other horizontally. No new plate material is made and no new plate material is destroyed. But earthquakes can occur if the plates 'stick'.
The San Andreas Fault which passes through California, marks the junction of the Pacific and North American Plates. The American Plate moves more slowly than, and at a slight angle into the Pacific Plate. Instead of the plates slipping past one anther, they tend to 'stick', like a machine without oil. When sufficient pressure builds up, one plate is jerked forward. These shock waves caused Earthquakes in San Francisco in 1989 and in 1906, when the ground moved by 6 meters!
Features: faulting, earthquakes
Examples: San Andreas fault
Earthquake activity: violent
Volcanic Activity: rare
