Chapter 10 Plate Tectonics

10.1 Continental Drift A. Wegener’s idea: Alfred Wegener proposed the idea of continental drift in 1912. Continental drift states that continents have moved to their current locations. Wegener believed that all continents were once connected as one large landmass that later broke apart, eventually forming the continents as we know them today. He named the large landmass Pangea.

Pangea

B. Wegener’s Evidence What did Wegener use as evidence to support his theory of continental drift (and Pangea)? Puzzle-like fit Fossil clues Climate Clues Rock Clues

B. Wegener’s Evidence 1. Puzzle-like Fit: Explanation : Looking at the coastlines of the current landmasses one can see how they might have once fit together in one giant landmass much like pieces of a puzzle fit together to form a whole picture.

B. Wegener’s Evidence 2. Fossil Clues – Example : The reptile Mesosaurus fossils have been found in both South America and Africa, continents that today are separated by many, many miles of ocean. The fossils of the fern Glossopteris were found in Africa, Australia, India, South America and Antarctica areas that Wegener proposed once existed side-by-side.

B. Wegener’s Evidence 3. Climate Clues – Example : Plants normally found in warm-weather have been found on the island of Spitzberen in the Artic Ocean (with a very cold climate). Glacial deposits and grooved bedrock caused by glaciers have been found in southern areas of South America, Africa, India and Australia.

B. Wegener’s Evidence 4. Rock Clues – Example : Similar rock structures were/are found on different continents; parts of the Appalachian Mountains of the eastern U.S. are similar to those found in Greenland and western Europe. South America and western Africa also have similar rock structures. Wegener used these as evidence for connecting these land masses together in Panagea.

10.2 Seafloor Spreading Background: Earth’s layers Outermost layer = Crust Middle, largest, partially melted layer = Mantle Inner-most solid core = Core

10.2 Seafloor Spreading A. Discovery of Mid-Ocean Ridges: Up until the early 1950’s little was known about the ocean floors. Invention of echo-sounding devices allowed for the construction of accurate maps of the ocean floor. Scientists discovered a complex ocean floor that had areas called ridges (like mountains) and dips or valleys.

10.2 Seafloor Spreading A. Discovery of Mid-Ocean Ridges: A system of underwater ridges and valleys was found to extend through the center of the ocean. The scientist began to study what had (was) caused the formation of the mid-ocean ridges.

10.2 Seafloor Spreading B. The cause of the Mid-Ocean Ridges = Seafloor spreading is the theory that hot, less-dense material in the mantle is forced upward to the surface at a mid-ocean ridge. This material then turns and flows sideways, carrying the seafloor away from the ridge in both directions.

10.2 Seafloor Spreading B. Magma, once below the ridge moves upward and flows through the cracks formed in the crust. It solidifies and forms new seafloor. It cools, contracts, and becomes more dense than the mantle layer, called the asthenosphere, below it. The seafloor begins to sink downward, forming trenches. This motion leaves a surface on the ocean floor of ridges and trenches.

10.2 Seafloor Spreading Picture of Seafloor Spreading : (Label Mid-Ocean Ridge)

10.2 Seafloor Spreading C. Evidence of Seafloor Spreading : 1. Rock samples taken from the seafloor were analyzed. Younger rock samples were found directly near the mid-ocean ridges. The further from the mid-ocean ridge, the older the rocks.

10.2 Seafloor Spreading C. Evidence of Seafloor Spreading : 2. Magnetic Clues – Currently, magnetic forces run from Earth’s south pole to the north pole. These forces can be reversed and turn the opposite direction. When this has happened in Earth’s history it has been recorded within rocks on the ocean floor in patterns along the sides of the mid-ocean ridges. These patterns have shown that new rock was being formed at the mid-ocean ridges.

Magnetic Poles

10.3 Theory of Plate Tectonics The theory of plate tectonics shows that sections of the seafloor and continents move around in relation to one another. It’s not just the continents or land masses that are moving like Wegener thought.

10.3 Theory of Plate Tectonics A. Plates and their movement – Earth’s crust and upper mantle (area called lithosphere) are broken into sections called plates that move around on the lower mantle (asthenosphere). Earth’s plates of the lithosphere “float” and move around on the asthenosphere.

10.3 Theory of Plate Tectonics Plate Boundaries are areas where the plates interact. The 3 boundary types are: Convergent, Divergent, Transform.

10.3 Plate Tectonics 1. Divergent Boundaries : Occur when 2 plates are moving apart from one another. These forms rift valleys and increase seafloor surface area (creates new crust).

Divergent Boundary

10.3 Plate Tectonics 2. Convergent Boundaries – Occurs when two plates collide. Boundary occurs when one plate moves under another, cools, becomes denser and sinks. This type of movement causes a decrease in total crust area. There are 3 types of convergent boundaries: Subduction Zones, Ocean-Ocean collisions, Continental Collision.

10.3 Plate Tectonics 2. Convergent Boundaries – A. Subduction Zone : When an oceanic and continental plate collide. The area were an oceanic plate descends into the upper mantle, this creates deep-sea trenches, volcanoes form here. B. Ocean-Ocean Collisions : Occurs when two ocean plates collide, or when seafloor that has become denser due to cooling begins to sink, deep-sea trenches form her and new magma rises and can create island arcs from volcanoes. C. Continental Collision : Usually no subduction occurs sense both plates are less dense than the mantle layer below. The plates form mountain ranges and earthquakes can occur.

Convergent Boundary

10.3 Plate Tectonics 3.Transform Boundary – Occurs when two plates slide past one another and are moving either in opposite directions or in the same direction at different rates. Crust is not formed or destroyed Earthquakes often occur here. Ex: San Andreas Fault.

Transform Boundary

10.3 Plate Tectonics C. What causes the movement of the plates? 1. Convection Currents : Cycle of heating, rising then cooling and sinking of the hot rock in the asthenosphere layer is thought to be the force behind plate tectonics. When the hot, plastic-like rock comes up and reaches the upper layer of the mantle, it will push up and move horizontally, carrying the plates of the lithosphere with it.

Convection Currents

10.3 Plate Tectonics D. Effects of / Features Caused by Plate Tectonics. Interaction of plates creates mountains, valleys, ocean basins as well as volcanoes and earthquakes.

10.3 Plate Tectonics 1. Faults and Rift Valleys : Tension forces associated with diverging plates stretch Earth’s crust. The areas of land (called blocks) around the plates tilt or slide downward. This breaking and downward movement of land forms faults called normal faults. Series of these faults pushing downward and separation of the plates creates rift valleys and mid-ocean ridges.

10.3 Plate Tectonics 2. Mountains, arcs and volcanoes : Formed from compression forces found at convergent boundaries When 2 continental plates collide they form mountain ranges from the folding and faulting of rock layers. These faults are called reverse faults. When two oceanic plates converge, one plate slides under the other. This forms island arcs of volcanoes. When an oceanic and continental plate collide/ converge, the oceanic plate slides under the continental plate. This forms mountains and volancoes.

10.3 Plate Tectonics 3. Strike-Slip Faults: These are found at Transform boundaries where one plate slides past another. The plates stick and then slide causing vibrations. The vibrations are felt as Earthquakes.