Plate Tectonics

Contents Earth’s Layers Continental Drift Seafloor Spreading Plate Tectonics

Earth’s Layers

Earth Layers Crust: oceanic and continental, Lithosphere: crust and upper mantle Asthenosphere: upper mantle, plastic-like consistency, lithosphere floats on it Mantle (mesophere): solid silicate rock, below asthenosphere and above the outer core Outer core: liquid iron and nickel Inner core: solid iron and nickel

CONTINENTAL DRIFT

OBJECTIVES Describe one piece of early evidence that led people to suggest the Earth’s continents may have once been joined. Discuss evidence of continental drift. Explain why continental drift was not accepted when it was first proposed.

DRIFTING CONTINENTS Early observations Map-makers (Abraham Ortelius) noticed how continents fit on either side of the Atlantic. Thought continents had been separated by floods and earthquakes Edward Suess proposed continents had once been joined together as supercontinent known as Gondwanaland.

PANGAEA

CONTINENTAL DRIFT Wegener proposes continental drift, calling his supercontinent Pangaea, which means “all Earth” in Greek Proposes supercontinent began breaking apart about 200 million year ago. Begins collection of scientific evidence to support his theory

EVIDENCE FROM ROCK FORMATIONS Wegener hypothesized that the same types of rock formations should exist on both sides of the Atlantic. Noticed rocks in the Appalachian mountains shared features with rocks in Greenland and Europe. All rocks were dated older than 200 million years; therefore found together before continental drift began.

EVIDENCE FROM FOSSIL RECORDS Fossils of the same animals and plants were can be found on separate continents. Kannemeyerids and Labyrinthodonts fossils can be found in Africa, North America, South America, Antarctica, Greenland, and Northern Europe. Glossopteris plants can also be found in many continents, strengthening the hypothesis that these continents shared the same climate.

Lystrosaurus Glossopteris Mesosaurus Cynognathus

FOSSIL EVIDENCE SUPPORTING PANGAEA

ANCIENT CLIMATIC EVIDENCE Wegener studied sedimentary rocks and found evidence of widespread climate change. Coal deposits in Antarctica suggest that plants once lived in Antarctica; therefore Antarctica was once closer to the equator.

GLACIAL EVIDENCE Glacier deposits can be found in Africa, India, Australia, and South America. This suggests that these areas were once cold enough for glaciers to form.

OPPOSITION TO THE HYPOTHESIS Wegener could not explain why the continents had moved throughout geologic time. Scientists doubted that such forces could exist on Earth to move entire continents. Wegener could explain how continents could move through the ocean floor to new locations on Earth. Wegener continued to collect evidence until his death in 1930.

SEAFLOOR SPREADING

OBJECTIVES Summarize the evidence that led to the discovery of seafloor spreading. Explain the significance of magnetic patterns on the seafloor. Explain the process of seafloor spreading.

TECHNOLOGICAL ADVANCE People before the 1900’s assumed the seafloor was essentially flat. Advances in technology in the 1940’s and ’50s led to new ideas. Sonar can map surfaces by bouncing high frequency sound waves in order to calculate distances (elevation)

TECHNOLOGICAL ADVANCES Sonar revealed ridges had corresponding trenches Mariana Trench is over 11 km deep.

OCEAN FLOOR TOPOGRAPHY Use of magnetometer to detect variations in magnetic fields allow scientists to map the seafloor. Images revealed underwater mountain ranges, called ocean ridges. Ridges form the largest continuous mountain range on Earth.

GULF OF MEXICO SEAFLOOR

MID-OCEAN RIDGE

MARIANA TRENCH DEEPEST POINT

QUESTIONS What could have formed the ridges and trenches found on the seafloor? What is the source of volcanism associated with these locations? What forces could be at work to move the entire seafloor and create trenches 6 times as deep as the Grand Canyon?

OCEAN ROCKS AND SEDIMENTS Scientists found predictable patterns in the ages of rock on the seafloor. Rock ages with distance from ocean ridges Ocean floor is dated at its oldest around 180 million years. Continental rocks are dated at 3.8 billion years. Why is the seafloor so young in comparison?

OCEAN ROCK AND SEDIMENT

OCEAN ROCK AND SEDIMENT Thickness of ocean sediments is much thinner than expected. Typically a few hundred meters thick. Continents may have sediments up to 20 km thick. Why are the sediments so thin? Why isn’t seafloor as thick as continental crust? Sediment is found to be thicker the further it is from ridges.

PALEOMAGNETISM Study of the Earth’s magnetic field in iron-bearing minerals is known as paleomagnetism. Seafloor is composed of iron-rich basalt, in which the iron becomes aligned with Earth’s magnetic field as molten rock cools. When rock hardens, the iron orients itself to Earth’s magnetic field at that time

PALEOMAGNETISM Studies in the 1960’s revealed pattern of magnetic reversals over time. Magnetic reversals are changes in the Earth’s magnetic field. Data from these reversals allowed scientists to construct a geomagnetic time scale.

MAGNETIC REVERSAL

THE INVESTIGATION Scientists towed magnetometers behind ships to measure the magnetic field of the ocean floor. Scientists noticed alternating + and – values for the magnetic field readings.

MAGNETIC SYMMETRY Scientists noticed that the + and – magnetic bands were symmetrical, originating from the ocean ridges. Scientists also discovered basaltic lava flows on land matching these magnetic reversals. Age of the ocean floor was ascertained from this data.

ISOCHRONIC MAPPING Isochron is a line on a map that connects points that have the same age. Young rock is found at mid-ocean ridges, while older rock is found further away near deep sea trenches. Why?

ISOCHRON MAP

SEAFLOOR SPREADING After all this data, Harry Hess proposes the theory of seafloor spreading. Theory proposes that new ocean floor is created at the ridges and destroyed at the trenches. Magma rises to the surface and hardens at the ocean ridges. Consecutive bands of seafloor are created in this way.

SEAFLOOR SPREADING

SEAFLOOR SPREADING

SEAFLOOR SPREADING

WEGENER’S THEORY REVISITED Wegener’s theory of continental drift did not account for the forces in action. Seafloor spreading completes the picture and describes the forces that shape the drifting of the continents. Continents are like groceries on the conveyer belt at the checkout line; they don’t push through the ocean floor, they ride on top of it as the ocean floor moves.

THEORY OF PLATE TECTONICS

OBJECTIVES Explain the theory of plate tectonics. Compare and contrast the three types of plate boundaries and the features associated with each.

THE THEORY Why do earthquakes happen? Why do volcanoes erupt? Why do mountains exist? Plate Tectonics is a theory that states the Earth’s crust and rigid upper mantle are broken into enormous slabs called plates. Each plate moves at a particular rate, and measured using a system of satellites and receivers

WORLD TECTONIC PLATES

PLATE COMPOSITION Oceanic plates: basalt Dark (black) and dense rock type composed of silicates, iron and magnesium Continental plates – granite and andesite Light colored (pink, white and gray) and low density rock type composed almost entirely of silicates.

PLATE BOUNDARIES Tectonic plates interact at places called plate boundaries. Divergent Convergent Transform Each boundary has certain geologic characteristics.

DIVERGENT BOUNDARIES Places where two tectonic plates are moving apart. Most are found on the seafloor as ocean ridges (seafloor spreading, ridge push) Example: Mid-Atlantic Ridge) Formation of new ocean crust accounts for volcanism along these locations. A rift valley is formed when a divergent plate boundary is located on continental crust. Example: East African Rift Valley

DIVERGENT PLATE BOUNDARIES AT MID-OCEAN RIDGES

CONTINENTAL DIVERGENT PLATE BOUNDARY: EAST AFRICAN RIFT

DIVERGENT BOUNDARY Map showing the Mid- Atlantic Ridge splitting Iceland and separating the North American and Eurasian Plates.

CONVERGENT BOUNDARIES Places where two tectonic plates are moving toward each other. Crust is destroyed at these boundaries. Three types: Oceanic converging with oceanic crust Oceanic converging with continental crust Continental converging with continental crust

CONVERGENT PLATES

SUBDUCTION AND CONVERGENCE Occurs when a cooler, denser plate descends below a less dense plate. In the event of oceanic and oceanic plate convergence, a trench is formed. Subducted plate melts and becomes magma, rises to the surface, and erupts as a volcano. Ex. Mariana Trench and islands Ex. Aleutian Trench and islands

OCEANIC-OCEANIC CONVERGENCE

OCEANIC-CONTINENTAL CONVERGENCE Denser oceanic plate subducts under less dense continental plate. Forms volcanic mountain ranges. Ex. Peru-Chile Trench and Andes Mountains

OCEANIC-CONTINENTAL CONVERGENCE

NAZCA - SOUTH AMERICAN PLATE BOUNDARY

CONTINENTAL-CONTINENTAL PLATE CONVERGENCE Occurs when two continental plates collide. Produces large folded mountain ranges with little volcanism. Since continental crust is buoyant, crust crumples and folds rather than subducts. Ex. Himalayas

CONTINENTAL-CONTINENTAL CONVERGENCE

HIMALAYAN MOUNTAINS

TRANSFORM BOUNDARIES Place where two plates slide horizontally past each other. Crust is deformed or fractured at these boundaries, but not created or destroyed. Characterized by long faults, hundreds of km in length. Earthquakes. Most transform boundaries are not found on continents. Exception: San Andreas Fault

TRANSFORM BOUNDARIES

SAN ANDREAS FAULT

MANTLE HOT SPOTS Areas of rising magma from deep in the mantle High temperature basaltic lava Occurs intraplate and not at plate boundaries Stationary while the overlying plate passes over Volcanic activity that can form islands and thus island chains Affects primarily oceanic lithosphere and to a lesser degree continental lithosphere Examples: Hawaiian Islands, Yellowstone National Park in Wyoming

HAWAIIAN MID-PLATE HOTSPOT

HAWAIIAN HOT SPOT TRAIL

MECHANISMS OF PLATE MOVEMENT Ridge Push Ridge Push Slab Pull

WHY DO THE PLATES MOVE? No single idea explains everything but we can identify several forces that contribute to the movement of the plates. Slab pull The sinking of the cooled, denser oceanic plates pulls on the rest of the plate as it sinks into the mantle Ridge push The hot molten rock rises through fissure in the ocean floor, cools and hardens to form a ridge, slides down from the rise and thus pushing the plate away from the fissure Convection Currents In the mantle, the hotter (less dense) molten rock rises, cools (becomes denser), and then sinks back into mantle where it is reheated

EARTH’S INTERNAL HEAT SOURCES And what drives the convection currents? The immense amount of heat in the Earth's mantle and core which comes from: heat left over from Earth's formation out of hot gas and dust gravitation friction radioactive decay

THE BIG PICTURE

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