CHAPTER 10 PLATE TECTONICS

CONTINENTAL DRIFT Early explorers noticed similarities in coastlines. Alfred Wegener, 1912 (German scientist) Hypothesis – Continental Drift – The continents once formed a single land mass called a supercontinent. – The “crumpling” of the crust may have formed mountain ranges. – The single land mass broke up and continents drifted to their present locations. – He thought that the continents plowed through the rock of the ocean floor … on this point he was wrong.

Continental Drift

Fossil Evidence for Continental Drift The same fossils are found on two continents, suggesting that the continents were once connected. – Mesosaur, small reptile: S. America & W. Africa (Lived 270 mya) Not likely the small reptile swam the ocean nor was there any evidence of land bridges. Link

Evidence From Rock Formations The ages and types of rocks in the coastal regions of widely separated areas matched closely. S. America and Western Africa Mountain chains that ended at the coastline of one continent seemed to continue on other continents across the ocean. – The Appalachian Mountains extend northward along the eastern coast of North America, and mountains of similar age and structure are found in Greenland, Scotland, and northern Europe. – When reassembled into one continent, the mountains of similar age line up and fit together.

Similar Rocks

Mountain Ranges

Climatic Evidence Changes in climatic patterns also suggest that the continents have not always been located where they are now. – Layers of glacial debris from ancient glaciers in S. Africa and S. America. – Fern fossils in the Antarctica suggest that tropical or subtropical climates once covered areas that are now much colder. In other words, the continents have moved.

Climates

Mid-Ocean Ridges Undersea mountain ranges through the center of which run steep, narrow valleys. – Sediment covering the sea floor is thinner closer to the ridge than farther away from the ridge. – The closer the sediment is to the ridge, the younger the sediment. – Ocean floor rock is very young (> 175 million years) while rocks on land are as old as 3.8 billion years. – Reason:????????????????????????????????????????

Mid-Ocean Ridges Undersea mountain ranges through the center of which run steep, narrow valleys. – Sediment covering the sea floor is thinner closer to the ridge than farther away from the ridge. – The closer the sediment is to the ridge, the younger the sediment. – Ocean floor rock is very young (> 175 million years) while rocks on land are as old as 3.8 billion years. – Reason: new ocean crust is being formed at the ridges.

Mid Atlantic Ridge

Sea-Floor Spreading Harry Hess, 1950’s (proposed a new hypothesis) The valley at the center of the ridge was a crack or rift, in the Earth’s crust. At the rift, magma rises and moves away from the rift, cools and solidifies, and forms new rock that replaces the rock of the ocean floor (Sea-floor spreading). It suggested that if the ocean floor is moving, then the continents might also be moving. Explained Wegener’s hypothesis.

Mechanism

Evidence from Paleomagnetism The study of the magnetic properties of rock. Earth has North and South geomagnetic poles (it acts as a magnet). As magma solidifies to form rock, iron-rich minerals in the magma align with Earth’s magnetic field. When the rock hardens, the magnetic orientation of the minerals becomes permanent. Geologic evidence suggests that Earth’s magnetic field has not always pointed North (normal polarity). There are periods of time when the Earth’s magnetic field points South (reversed polarity). There is a pattern of alternating normal and reverse polarity (geomagnetic reversal time scale). Supports Hess’ hypothesis of sea-floor spreading and Wegener’s hypothesis of continental drift.

Earth’s crust is classified into: Oceanic crust: Higher density Rock rich in iron and magnesium (Mafic) Continental crust: Lower density Rock rich in silica (Felsic)

Theory of Plate Tectonics 1960’s Earth’s crust and the rigid, upper part of the mantle = lithosphere (thin outer shell of Earth) – Broken into several blocks (tectonic plates) – Plates ride on a deformable layer of the mantle (asthenosphere) o “plastic” rock just below the lithosphere o “plastic” = solid rock that flows slowly

When tectonic plates move, sudden shifts can occur along their boundaries (earthquakes). – San Andreas fault Volcanoes can also form along plate boundaries. – Pacific Ring of Fire

Types of Plate Boundaries Divergent Boundaries – Two plates move away from each other. – Magma from the asthenosphere rises to the surface at these boundaries to form new oceanic crust. The Atlantic ocean is moving at a rate of 1 to 2 cm per year. The Pacific ocean is spreading at a rate of 3 to 8 cm per year. Forms mid-ocean ridges and rift valleys – Mid-Atlantic ridge/rift Most divergent boundaries are on the ocean floor – Red Sea: separates African plate from Arabian plate

Convergent Boundaries – Two plates move towards each other; collide – Three types of collisions Oceanic lithosphere collides with Continental lithosphere – Oceanic crust is more dense and sinks (subducts) under the less dense continental crust. – Called a subduction zone – Forms a deep ocean trench, mantle melts to form magma – Magma rises to form volcanoes. – Continental crust does not subduct Continental lithosphere collides with Continental lithosphere – Both of the same density – The colliding edges crumple and thicken which causes uplift that forms mountain ranges. » Himalaya mountains Oceanic lithosphere collides with Oceanic lithosphere – One plate subducts under the other plate and a deep ocean trench forms – Mantle melts and forms magma – Magma rises to the surface to form an island arc (chain of volcanic islands). An example is Japan

Transform Boundaries – Two plates slide past each other, horizontally. Move in opposite direction Move in same direction at different speeds – They do not slide smoothly, they scrape against each other in a series of sudden jerks that are felt as earthquakes. San Andreas Fault …. N. American plate and Pacific plate. – Do not produce lava

Causes of Plate Motion Mantle Convection – Rising hot material and sinking colder material.

Plate Movement

Reshaping Earth’s Crust Slow movements of the tectonic plates change the size and shape of the continents over millions of years. All continents contains large areas of stable rock called cratons ( > 540 million years old) Cratons represent ancient cores that the continents formed around. Rocks containing Cratons exposed at the surface are called shields.

Cratons

Rifting causes crust to break apart. The East African Rift Valley is splitting a continent. – Terranes are pieces of land that get added to a continent at convergent boundaries… becomes part of a continent. Accretion (added) – Sea mounts, atolls, islands – Himalaya Mtns. formed when India collided with Asia 45 mya. Some continents were once closer to the poles. – Ice once covered Earth’s continents, including the Sahara in Africa (once closer to the South Pole). – As continents rift or as mountains forms, populations of organisms become separated (new species evolve). Madagascar separated from Africa and India

Terranes

The Supercontinent Cycle Several periods of supercontinents … break apart and reform. – Rodina formed 750 mya – Pannotia formed 450 mya Supercontinent PANGAEA formed about 300 mya (Paleozoic). – Appalachian (North America) and Ural (Russia) mountains formed. – Tethys Sea cut into the eastern edge of Pangaea. – Single large ocean = Panthalassa – 250 mya, Pangaea broke into two continents = Laurasia and Gondwanaland – North Atlantic ocean forms and Tethys Sea becomes the Mediteranean Sea. – Gondwanaland broke into Africa and South America. – Further break ups formed India, Australia, and Antarctica. – Slowly the continents moved into their present positions.

Future Movements In 150 million years – Africa will collide with Eurasia – Mediterranean Sea will close – East Africa will separate from the rest of Africa – New ocean will form – North American and South American plates will move westward. – Eurasia and Africa move eastward – Atlantic ocean will become wider – Australia will continue to move northward, and will eventually collide with Eurasia. – Western California will move towards Alaska – In 250 million years, a new supercontinent will form.