Continental Drift, Plate Tectonics, and Seafloor Spreading Physical Geography 110

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Video – 10 minutes

Theory of Continental Drift First mentioned in 1596, by Abraham Ortelius Fully developed in 1912 by Alfred Wegener The theory proposed the all of the earth’s continents were once a single super continent called “Pangea”. Pangea existed million years ago.

Wegener’s Evidence 1. The way that the continents seem to fit like the pieces of a puzzle (rock formations).

Wegener’s Evidence 2. Glacial deposits from similar places in places that were no longer cold. Striations from glaciers also showed that these places used to be in colder places (near poles).

Wegener’s Evidence 3. The same fossils found in continents that were across the ocean from each other.

Mesosaurus Mesosaurus was a 1 metre long reptile whose fossils have been found in areas of South America and Africa that were once believed to be connected.

Lystrosaurus Therapsid the size of a pig Fossils found in Antarctica, India, and South Africa

Continental Drift Many believed Wegener’s theory was just that, merely a theory. Many said there was no mechanism to explain how the continents could have moved apart. Until.....

Theory of Plate Tectonics Wegener’s work combined with new knowledge from other scientists. Arthur Holmes believed there may be plate boundaries under the sea. Holmes also suggested convection currents within the mantle as the driving force for plate tectonics.

Theory of Plate Tectonics Plate tectonics is the study of how these plates move and effect the earth. The lithosphere (upper layer) is split into plates that are floating on top of the asthenosphere. – Convection current set plates in motion Plates – 100km thick, have a defined boundary, move independently.

Plate Boundaries Notice the location of plate boundaries in the next slide. Commonly associated with land forms such as mountains, volcanoes, faults, and trenches. Also associated with events such as earthquakes. Creates land forms (mountains etc. ), and distributes resources (geological/biological).

Plate Boundaries

Plate Movements Plate movements are responsible for 3 processes. 1.Folding – bending and twisting of rocks commonly found where plates come together. 2.Faulting – rocks moving past each other. 3.Vulcanism – movement of magma above or below the earth’s surface.

The Theory of Plate Tectonics This theory states that the Earth’s crust is broken into many pieces called plates. These plates and a portion of the upper mantle beneath them, called the lithosphere, move around on top of the rest of the mantle, called the asthenosphere. Plates are thought to move around in response to convection currents in the mantle of the Earth.

Why do Plates move – Convection Currents Magma in the Earth’s surface becomes hot in hot spots. Hot magma is less dense therefore it rises. As it rises it cools and spreads out. As it spreads it drags the plates with it. The magma cools and moves back down. These are convection currents.

Convection Currents

Three types of Plate Boundaries 1.Convergent – where two plates meet each other and crash/crunch against each other. Two types are subduction zone (under the ocean) and collision zone. 2.Divergent – where plates split and move apart. 3.Transform – plates slide along or past each other due to shear force.

Convergent Plate boundary

Divergent Plate Boundary

Transform Plate Boundary  San Andreas Fault

Seafloor Spreading Proposed by Harry Hess This theory states that the Atlantic Ocean is spreading/growing from the mid-ocean ridge. The mid-ocean ridge is 10,000km long and 3.2km high above the seafloor. Some scientists believe the Atlantic Ocean will one day grow to be larger than the Pacific. Cuts directly through Iceland.

Mid-Ocean Ridge

Evidence of Seafloor Spreading Oceanic rocks are youngest near the mid ocean ridge. Continental rocks are 4 billion (4,000,000,000) years old while the oldest oceanic rocks are only 180,000,000 years old. Parallel bands of rocks on either side of ridge that had matching but alternating magnetic poles.

Mountain Ranges Below the Sea

Seafloor Spreading with Bill Nye Mid-Ocean Ridges: Aliant Learning Seafloor Spreading