AP Notes - Plate Tectonics
PLATE TECTONICS
Theory arose out of two separate geologic observations: continental drift and seafloor spreading.
The Continental Drift Theory
1915 – Alfred Wegener proposed that all present day continents originally formed one land mass (Pangaea). Wegener believed that this super-continent began to break up into smaller continents around 200 million years ago. He based this theory on five factors:
1. Fossilized tropical plants were discovered beneath Greenland’s icecaps.
2. Glaciated landscapes occurred in the tropics of Africa and South America
3. Tropical regions on some continents had polar climates in the past, based upon paleoclimatic data.
4. The continents fit together like pieces of a puzzle.
5. Similarities existed in rocks between the east coast of North America and South America and the west coasts of Africa and Europe.
Continental drift gained acceptance in the 1960s when the theory of plate tectonics provided a mechanism that would account for the movement of the continents.
Seafloor Spreading Theory
During the 1960s, alternating magnetic properties were discovered in the rocks found on the seafloor. Similar patterns were discovered in either side of the mid-oceanic ridge near the center of the oceanic basins. Dating rocks indicated that as one moved away from the ridge, the rocks became older. This suggested that new crust was being created at volcanic rift zones.
The lithosphere (crust and upper mantle, approximately 62 miles (100 km) thick) is divided into massive sections/pieces known as tectonic plates. These float and move on the viscous asthenosphere.
There are about a dozen or so plates in the lithosphere that move independently of each other. The plates are made of both mantle and crust. The majority of the land on earth sits above six giant plates, the remainder of the plates lie under the oceans as well as the continents. Some plates consist only of ocean floor (the Nazca plate). Some contain both oceanic and continental material (North American Plate). The largest plate is the Pacific Plate. The plates move slowly over time. They sink in areas of volcanic island chains, folded mountain melts, and trenches. They rise up from ridges and rift valleys. These plates move in relation to one another. The edges of the plates are called plate boundaries. The three types of plate boundaries are , divergent, and convergent.
Transform Boundaries
Occur where plates slide past each other.
The friction of the plates sliding past each other causes earthquake (common at these boundaries)
Example: The San Andreas fault (western coast of North America) is where Pacific Plate and North American plate move past each other such that the Pacific plate is moving northwest with respect to North America.
Transform Boundaries
Occur where plates slide past each other.
The friction of the plates sliding past each other causes earthquake (common at these boundaries)
Example: The San Andreas fault (western coast of North America) is where Pacific Plate and North American plate move past each other such that the Pacific plate is moving northwest with respect to North America.
Divergent Boundaries
Occur where two plates slide apart from each other with the space that was created being filled with molten magma from below forming new crust.
Examples of Oceanic divergent boundaries: Mid-Atlantic Ridge and East Pacific Rise
Examples of Continental divergent boundaries: East Africa Great Rift Valley.
Can create massive fault zones in the oceanic ridge system and are frequent areas of oceanic earthquakes.
Occur where two plates slide apart from each other with the space that was created being filled with molten magma from below forming new crust.
Examples of Oceanic divergent boundaries: Mid-Atlantic Ridge and East Pacific Rise
Examples of Continental divergent boundaries: East Africa Great Rift Valley.
Can create massive fault zones in the oceanic ridge system and are frequent areas of oceanic earthquakes.
Convergent Boundaries
Occur where two plates slide toward each other, forming either a subduction zone (if one plate moves underneath the other) or an orogonic belt (if two plates collide and compress).
Occur where two plates slide toward each other, forming either a subduction zone (if one plate moves underneath the other) or an orogonic belt (if two plates collide and compress).
When a denser oceanic plate moves underneath (subducts) underneath a less-dense continental plate, an oceanic trench is produced on the ocean side and a mountain range on the continental side. An example is the Cascade Mountain range (extends from California’s Sierra Nevada Mountains and includes Mt. St. Helens.
When two oceanic plates converge, they create an island arc – a curved chain of volcanic islands rising deep from the seafloor and near a continent, They are created by subduction processes and occur near the continent side of the subduction zone. Their curve is generally convex toward the open ocean. A deep undersea trench is located in front of such arc where the descending plate dips downward. Examples include Japan and the Aleutian Islands of Alaska.
When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upward. Examples include the northern margin of the Indian subcontinental plate being thrust under a portion of the Eurasian plate, lifting it and creating the Himalayas. Other examples include the Urals, the Alps, and the Appalachian Mountains.
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