PLATE TECTONICS. The Basic Premise of Plate Tectonics

Transcription

1 PLATE TECTONICS The Basic Premise of Plate Tectonics The lithosphere is divided into plates that move relative to one another, and relative to the earth s asthenosphere. Movement occurs at very slow (cm/yr) rates. As a plate moves, its interior remains relatively intact (but it is not totally ridged) while its boundaries become deformed. Relative motion: 30 cm/yr with respect to red star 60 cm/yr with respect to red star 30 cm/yr with respect to blue star

2 Plate: a distinct piece of lithosphere that has boundaries on all sides which are called plate boundaries. Continental margins: : edges of continents where they meet the oceans/seas. These lead to active margins (continental margins that are plate boundaries) and passive margins (continental margins that are not plate boundaries). South American plate active margin passive passive margin margin

3 Major plates : very large tectonic plates Pacific Plate North American Plate South American Plate Eurasian Plate African Plate Australian-Indian plate Minor plates: small to moderate size tectonic plates Juan de fuca Plate Cocos Plate Caribbean Plate Scotia Plate Arabian Plate Iran Plate Philippine Plate Nazca Plate Micro plates: very small tectonic plates that occur as non-ridged segments along plate boundaries (e.g. Tonga Plate)

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5 Plate boundaries are common place for earthquakes to occur. This figure shows the coincidence of areas with frequently occurring earthquakes and plate boundaries. 04_05.jpg

6 Mid-ocean ridges: A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary. Seamount: An isolated submarine mountain. Trench: A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary. Ridge axis: The crest of a mid-ocean ridge; the ridge axis defines the position of a divergent plate boundary. Fracture zone: A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault. Volcanic arc: A curving chain of active volcanoes formed adjacent to a convergent plate boundary.

7 There are three types of plate boundaries: Divergent Convergent Transform

8 04_06.jpg Divergent plate boundaries: : where two plates are moving away from one another. this is where mid-ocean ridges occur.

9 04_06.jpg Convergent plate boundaries: : where two plates are moving towards one another. This is where trenches form in oceanic crust, and are commonly associated with volcanic activity (such as the Cascade volcanoes).

10 04_06.jpg Transform plate boundaries: : where two plates are moving past one another. The San Andreas fault is an example.

11 True divergent plate boundaries occur along mid-ocean ridges. New ocean crust is formed along the ridge in order to fill in the space missing as the two plates move away from each other. As spreading continues, older crust moves farther away from the boundary. Spreading can be equal on both sides of the divergent boundary, or more new crust can be formed on one side then the other.

12 When new crust is created at mid-ocean ridges (divergent plate boundaries) by what is termed seafloor spreading, new sea floor is created by volcanic activity. This volcanic activity is the result of magma rising up from the mantle and forming a magma chamber. This magma is rich in Fe and Mg, and forms several distinct rock types called Gabbro, Diabase, and basalt

13 Gabbro from rocks that are believed to have formed at a mid-ocean ridge

14 Diabase dikes from rocks that are believed to have formed at a mid-ocean ridge

15 Basalt from rocks that are believed to have formed at a mid-ocean ridge

16 03_29.jpg Just like basaltic rocks on the continents, basalts on the ocean record the magnetic polarity of the earth at the time they were erupted. This leads to the stripped pattern that is seen on the modern sea floor. It also allows us to date the age of the sea floor.

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18 04_11.jpg The further away from the plate boundaries you move, the older the ocean floor gets.

19 True divergent plate boundaries occur as mid-ocean ridges; however, rifting is a lesser form of divergent plate boundary. Rifting: The process by which a continent stretches and splits along a belt; if successful, this process separates a larger continent into two smaller continents separated by a divergent boundary. Rifting can lead to true divergent boundary oceanic spreading.

20 Convergent plate boundaries are associated with subduction zones (the region along a convergent boundary where one plate bends and sinks down into the asthenosphere beneath another plate).

21 Convergent plate boundaries can be an oceanic plate being subducted under a continental plate (i.e., the Pacific plate under the South American plate to form the Andes Mountains), or

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23 04_16.jpg Convergent plate boundaries can be an oceanic plate being subducted under a second oceanic plate. This is where volcanic island arcs form. A back-arc basin is a depression formed behind a volcanic arc.

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25 There are a series of terms that go with subduction zones such as: downgoing plate: : plate the is being subducted under the overriding plate (plate that is not sinking/ not being subducted).

26 Accretionary prism: : A wedge-shaped mass of sediment and rock scraped off the top of a downgoing plate and accreted onto the overriding plate at a convergent plate margin.

27 04_15b.jpg Accretionary prisms are not too different than a plow scraping off material as it plows along.

28 Forearc basin: depression in front of a volcanic arc where sediment is commonly deposited.

29 Volcanoes are associated with subduction zones because the down going plate brings water with and within it. This water is then heated up at depth, and is released into the mantle. This commonly happens at km in depth. This water allows the mantle to partially melt, which allows magma to form. Partial melting: : The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.

30 Mt Rainier, and the other Cascade volcanoes are the result of mantle partial melting due to subduction of the Juan de Fuca plate beneath the North American plate.

31 Subduction is a process that continues deep down into the earth s mantle. Also, the top of a downgoing plate is marked by a series of earth quakes. This zone is termed the Wadati-Benioff zone.

32 04_27.jpg Subduction ends when two continental plate collide (called a collision). The density of continental plates is so low that they cannot be subducted, so the former trench becomes a suture (a liner belt of highly deformed rocks), a collisional mountain belt forms where the two plates collided and the downgoing plate breaks off (becomes detached).

33 Transform plate boundaries are where two plates are sliding past each other. A great example of this type of plate boundary is the San Andreas fault, which id the boundary between the North American plate and the Pacific plate. the average rate of movement on the San Andreas fault is ~3.5 cm/yr; however not all parts of the fault are always moving.

34 Transform faults can also be seen offsetting mid-ocean ridges. Where there is active movement between two plates is called a transform fault, and where there is no longer movement it is termed a fracture zone.

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36 Special locations called Triple junctions s are points where three lithosphere plate boundaries intersect.

37 04_21.jpg Hot spots are special locations within plates where deep plumes of magma rises up from the mantle and breaks through the lithosphere.

38 04_23a.jpg It is thought that hot spots do not move, and that they can record plate movement as a plate moves over the plume that rises up from the mantle. As a plate moves over a hot spot a chain of volcanoes is formed. As these extinct volcanoes sink below sea level, they become seamounts.

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40 04_22.jpg How do you explain the bend in the chain of islands formed by the Hawaiian hot spot?

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