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Plate Tectonics Chapter 17. Early Observations. In the late 1800s, Eduard Suess , an Austrian geologist, hypothesized that the present southern continents had once been joined as a single landmass that he named Gondwanaland. Continental Drift.
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Early Observations In the late 1800s, Eduard Suess, an Austrian geologist, hypothesized that the present southern continents had once been joined as a single landmass that he named Gondwanaland.
Continental Drift • In 1912, Alfred Wegener, proposed that in the distant past, Earth’s continents were all joined as a single landmass. • He called this supercontinent Pangea • He said that the continents have separated and collided as they have moved over Earth’s surface for millions of years.
Continental Drift • Wegener proposed that if the land areas were brought back together, the move would line up ancient mountain ranges, similar continental rock formations, and evidence of ancient glaciers. • There are even similar fossils on both sides of the Atlantic.
Continental Drift Animation: Plate Movement
Evidence: Rock Formations Some of the rocks of the Appalachian Mountains in the United States shared similar features with rocks in Greenland and Europe.
Evidence: Fossils • Similar fossils of several different animals and plants that once lived on land had been found on widely separated continents. • Reptiles that are only found in fresh water would not have crossed ocean waters. • Land-dwelling animals would not have swum that far of a distance. • Fern plant had been found on many continents that today, have very different climates.
Evidence: Climate Change Coal deposits had been found in Antarctica indicated that this land once had a temperate, rainy climate. Glacial deposits in Africa, India, Australia, and S. America suggested that these areas had once been covered by thick ice caps.
A Rejected Hypothesis • Wegner’s hypothesis had two flaws: • 1. He could not explain what was causing the continents to move. • He suggested that the rotation of Earth could be possible, but physicists were able to show that this force was not great enough. • 2. He could not explain how the contents were moving. • He suggested that the continents were plowing through a stationary ocean floor, but geologists argued that continents could not push through the ocean floor without fracturing and there was no evidence of such fracturing.
The Earth’s Layers • The Crust • Varies from 5-60 KM • In most places, a thin layer of sedimentary rocks covers the mostly granite-like rocks of the continental crust • The oceanic crust, under layers of marine sediments, is composed of darker and denser rocks similar to basalt
The Earth’s Layers • The Mantle • Extends to a depth of about 2900 KM. • Earthquake waves travel faster in the mantle than they do in the crust. • Composed mostly of dense, the dark mafic minerals olivine and pyroxene.
The Earth’s Layers • The Core • Composed of iron and nickel. • Outer Core is thought to be liquid because S-waves are unable to pass through the outer core. • The Inner Coreseems to be solid.
Seafloor Spreading • Advancement in technology allowed scientists to study the ocean floor • Sonar: use of sound waves to measure water depth • Magnetometer: device that can detect small changes in magnetic fields
Evidence from the Oceans • In the 1950’s both fossils and the analysis of radioactive material showed the age of the oceanic crust increases with distance from the mid-ocean ridges. • Some oceans were growing wider from the middle. • Scientists also used magnetic measurements of the oceanic crust.
Ocean Floor Topography • Ocean ridges: underwater mountain chains • Mid-Atlantic Ridge, East Pacific Rise • Trenches: narrow, elongated depression in the seafloor with very steep sides • Mariana Trench (deepest)
Ocean Rocks & Sediments • 1. ages of rocks vary in different places • Age of oceanic crust consistently increases increases with distance from a ridge • Rock near ocean ridge = younger • Rock near trenches = older
Ocean Rocks & Sediments • 2. the thickness of ocean-floor sediment is much less than expected • Ocean floor sediments = a few hundred meters thick • Continents = up to 20kilometers thick
Critical Thinking • Question: • Why aren’t seafloor sediments as thick as continental sediments? • Answer: • thickness of sediments increases with distance from an ocean ridge • Sediments are thicker closer to trenches b/c the sediment supply is greater than it is near ocean ridges, which are often located in the middle of the ocean (hence, not as thick).
Magnetism • Paleomagnetism: the study of Earth’s magetic field record . • Basalt is rich in iron-bearing minerals, therefore, provides an accurate record of ancient magnetism. • As basalt cools, the iron-bearing minerals become oriented parallel to Earth’s magnetic field.
The Geomagnetic Time Scale • Earth’s magnetic field periodically reverses (magnetic reversal) • Different ages provide a record of when the Earth’s magnetic field reversed its polarity.
Magnetic Symmetry • Scientists noticed that the positive and negative areas formed a series of stripes that were parallel to ocean ridges. • Scientists were able to determine the age of the ocean floor from magnetic readings and create isochron maps.
Isochron Map Isochron: a line on a map that connects points that have the same age.
Seafloor Spreading • Harry Hess proposed a theory called seafloor spreading which states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. • Magma is forced up toward the crust along an ocean ridge and fills the gap that is created. • When magma hardens, a small amount of new ocean floor is created
The Missing Link • Seafloor spreading was the missing link needed by Wegner to complete his model of continental drift. • Provides the answer of how these landmasses move
Plate Tectonics • The surface of Earth is composed of about a dozen major rigid, moving crustal plates and several smaller plates. • These plates contain areas of light continental rock and dense oceanic bottoms.
Plate Boundaries • Tectonic plates interact at places called plate boundaries. • Each type of boundary has certain geologic characteristics and processes associated with it.
Convergent Boundary • Convergent boundaries occur from converging plates • plates that are moving toward each other.
Convergent Boundary • Subduction occurs when a dense oceanic plate dives beneath a lighter continental plate. • Subduction forms ocean trenches that are linear fractures and are the deepest parts of the oceans.
Three Types of Convergent Boundaries • 1) Oceanic-Oceanic • One of the two plates is subducting beneath the other • Creates a deep-sea trench • The subducted plate descends into the mantle and melts which is then forced back to the surface • forms and arc of volcanic islands • ie: Mariana Trench/Islands & Aleutian Trench/ Islands
Oceanic-Oceanic Examples Mariana Trench Philippine Plate subducting under the Pacific Plate Aleutian Trench Pacific Plate is being subducted under the North American Plate
Three Types of Convergent Boundaries • 2) Oceanic-Contintnetal • Denser oceanic crust is subducted • Creates trench & volcanic arc… insead of arc of volcanic islands, a series of volcanoes erupt along the edge of the continental plate • Mountain range with many volcanoes • ie: Andes Mountains & Peru-Chile Trench
Oceanic-Continental Example Peru Chile Trench Nazca Plate is being subducted under the South American Plate
Three Types of Convergent Boundaries • 3) Continental-Continental • Colliding edges of the continents are crumpled and uplifted to form a mountain range • ie: Himalayas Indian Plate is colliding with the Eurasian Plate. (no subduction)
Convergent Boundaries Plate Boundary Movement
Divergent Boundary • A divergent boundary (rift) is found at the mid-ocean ridges where upwelling material creates new crust that moves away from the ridge in both directions.
Divergent Boundary • Over millions of years, seafloor spreading along a divergent boundary may cause an ocean basin to grow wider. • Atlantic Ocean = 2-3 cm/yr Atlantic Ocean Growing Animation • Some divergent boundaries may form rift valleys: when continental crust begins to separate, the stretched crust forms long, narrow depression. • East Africa -
Plate Boundaries • When a plate slides horizontally past another plate, they meet at a transform boundary.
Transform Boundary • Characterized by long faults, sometimes hundreds of km long, and by shallow earthquakes • Most offset sections of ocean ridges • Rarely do transform boundaries occur on continents. • San Andreas Fault is the exception
What Moves the Plates? • The transfer of thermal energy by the movement of heated matter is called convection. • The heating of matter causes it to expand and to decrease in density. • The warmed matter then rises. • The cooler part of the matter sinks. • This up-and-down flow produces a pattern of motion called a convection current.
Push & Pull • How is mantle convection related to the movement of tectonic plates? • The weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone called ridge push. • The weight of a subducting plate helps pull the trailing lithosphere into the subduction zone in a process called slab pull.
Earthquakes • Any vibrating, shaking, or rapid motion of the Earth’s crust. • Most occur when stress builds along a zone of weakness or a break in the rock known as a fault. • When the crust shifts, energy is released. • The energy radiates in all directions through vibrations.
Hot Spots • In several places on earth, hot plumes of magma pierce the crust. • As a crustal plate moves over this source of magma, volcanoes form at the hot spot. • This movement of a plate leads to the formation of a chain of volcanoes of differing ages. • One example is the Hawaiian Islands.
Volcanic Hazards • When volcanoes erupt they may spew hot lava, hot ash, and/or toxic gases. • The lava and ash can bury cities, and the toxic fumes can suffocate people. • Volcanoes can also provide fertile soil that is composed of weathered volcanic material.
Earthquakes • The place underground where the break occurs is the focus of the earthquake. • The epicenter is the location at the Earth’s surface just above the focus.
Measuring Earthquakes • Magnitude (Richter Scale) • Seismographs are the most reliable measures of earthquakes. • Each increase in one unit of magnitude means a ten-fold increase in shaking. • Intensity (Mercalli Scale) • Based upon the reports of people who experienced the earthquake and observed the destruction.
