Forces behind change Plate tectonics

Focus Questions How does the movement of the earth’s plates cause land features? What evidence supports the theory of plate tectonics? What are the different types of plate boundaries?

Continental Drift The earth is always changing due to W.E.D. (Weathering, Erosion, and Deposition), but there’s also large scale changes. In 1930 Alfred Wegener had an idea that Drifting Continents were creating changes. He suggested that all continents once fit together like a puzzle into one land mass. He called it Pangaea. Meaning “all land.”

Pangaea The super continent 250 million years ago during the Triassic period.

Evidence of Continental Drift Fit of continents like puzzle pieces. Fossil clues of the same animals and plants. Remains of Mesosaurus in Africa and South America, plant remains like the Glossopteris. Glacial (ice) deposits on different continents. South America, Africa, and India. Rock structures found in North America and Greenland were identical in type and age.

Drift Evidence

Sea floor spreading The continents were moving but what caused the movement? In the 1960s Henry Hess suggested Seafloor spreading was the cause of the movement. As hot, new, less dense magma rises it moves the earth’s crust creating mid ocean ridges. The magma flows sideways carrying the older sea floor away from the newer formed ridge.

Evidence of Sea floor spreading In 1968 researchers found the youngest rocks in new mid ocean ridges. Also, that rocks got older as they moved away from the middle. Carbon dating: rocks on either side of mid ocean ridge were increasing in age. Earth’s magnetic field flows from North to South. But reverses every millions years or so. Magnetic clues: Earth’s magnetic field reversal is recorded in rocks forming at mid ocean ridges.

Sea floor Spreading

Theory of Plate Tectonics Where does the ocean floor go when it spreads? The Earth’s floor moves and interacts on the earth’s crustal “Plates” which float on the molten mantle. Due to convection currents. These plate interactions occur at borders or “Boundaries”. These boundaries explains the locations of volcanoes, mountains, and earthquakes.

Plate Tectonics

Types of Boundaries The three types of boundaries are: Convergent, Divergent, and transform. Convergent boundary: Plates are crashing together. (Collide) Divergent boundary: Plates are moving apart. (Divide) Transform boundary: Plates are moving across each grinding or slipping other. (Slide)

Convergent boundary Subduction zone forms when ocean plates and continental plates meet creating oceanic trench. Mountain range forms when continental plate and continental plates meet push upward (uplift).

Divergent boundary Hot, new, low density magma pushes up through crust. Mid ocean ridges and a rift valley forms new crust. Older ridges are pushed away. The start of Sea floor spreading.

Transform boundary Sliding plates are sites of earthquakes, faults and fault zones Boundaries strike and slip across each other in opposite direction.

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