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Published byBeverly Gibbs Modified over 8 years ago
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A. The theory that pieces of the Earth’s lithosphere, called plates, move slowly on top of the asthenosphere B. Explains the formation and movement of the Earth’s plates
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1. Wegener hypothesized that the continents were once joined in a single supercontinent (Pangaea), which then broke into pieces that moved apart by continental drift.
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2. Problems with the theory: a. Unable to explain how continents plow through solid rock sea floor b. What force could move the continents?
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1. The mid-ocean ridge- chain of underwater mountains 2. Formation of Oceanic Crust by sea- floor spreading- Process by which new oceanic crust is created at mid-ocean ridges as older crust moves away
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a. Subduction- As sea-floor spreading occurs, old oceanic plates sink into the mantle due to differences in density b. Trench- As a plate sinks through the subduction zone, it bends, forming a depression in the ocean floor c. Sea- floor spreading creates new oceanic crust at mid-ocean ridges. Subduction destroys old oceanic crust at subduction zones.
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a. Rock sampling at mid-ocean ridges showed matching stripes on either side of the ridge b. Radioactive dating- rocks near mid- ocean ridge are younger than rocks further away
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1. Plate motions are the visible part of the process of mantle convection 2. Figure 28- Heat flows from Earth’s hot interior toward the cooler surface through large convection currents in the mantle
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1. Dozen major tectonic plates 2. Plates move slowly- 0.1 to 10 cm per year
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a. Divergent- Plates move away from each other, forming new rock b. Convergent- Plates come together or collide, rock is destroyed c. Transform- Plates slide past one another moving in opposite directions, rock is neither created nor destroyed
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1. Geologists found that most mountains form along plate boundaries 2. Example: Himalayas formed from collision of Indo-Australian and Eurasian plate and Iceland formed at a mid-ocean ridge of diverging plates
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