Chapter 10 Plate Tectonics

Continental Drift One scientist who looked at the continents as pieces of a puzzle was Alfred Wegener. He was the first to advance the idea of mobile continents in 1912.

Continental Drift Wegener believed that all continents were originally joined into one supercontinent called Pangaea. Pangaea = All Earth Couldn’t explain how, when, or why the continents moved. Wegener identified several lines of evidence to support the idea that the continents had drifted.

Plate Tectonics: The Scientist Behind the Theory

Evidence for Continental Drift Shape of continents – good fit to form Pangaea.

Evidence for Continental Drift Matching coastlines on different continents.

Rock Clues Some mountain ranges end at the ocean and continue on a continent directly across the ocean. (Appalachians and Caledonian)

Rock Clues Rocks are similar in age and history across continents. Parts of the Appalachian Mountains of the eastern United States are similar to those found in Greenland and western Europe. Rock structures are similar in eastern South America and western Africa.

Climate Evidence Fossils of warm-weather plants were found in the Arctic Ocean. Fossils of tropical plants and animals have been found in places like Antarctica.

Climate Evidence Similar glacial features have been found on widely separated continents. Evidence of glaciers have been found in the equatorial zone.

Climate Evidence Coal deposits found in temperate and polar regions; however, coal is formed in tropical regions.

Fossil Evidence Similar fossils have been found on continents separated by oceans. Glossopteris Mesosaurus Figure 2-6

When Wegener put forth his theory of continental drift, many scientists would not accept his ideas. Too far-fetched. They thought there wasn’t a force in nature that could move entire continents. The theory of sea-floor spreading helped to prove Wegener’s theory.

Continental drift was reexamined in the 1960s when new information became available. Sea floor features became better known. A technique was developed that enabled scientists to determine the original positions of rocks on Earth (paleomagnetism).

Seafloor Spreading Scientists were suprised to find that rocks taken from the ocean floor were much younger than those found on the continents. The youngest rocks were those nearest the underwater ridge system (a series of mountains that extend around the world, stretching more than 40 thousand miles).

Seafloor Spreading Harry Hess envisioned new sea floor being created at the mid- ocean ridge and destroyed in deep ocean trenches.

Mid-Ocean Ridges are where Sea Floor Spreading Occurs. Sea floor spreading is the process by which new oceanic lithosphere is created as older materials are pushed away (like a conveyor belt). As tectonic plates move away from each other, the sea floor spreads apart and magma rises to fill the gap. They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!)

Evidence for Seafloor Spreading Age of the sea floor matches pattern predicted by sea floor spreading: Youngest sea floor is at mid-ocean ridge. Sea floor is older with increasing distance from mid-ocean ridge. Oldest seafloor is less than 200 million yrs old.

Evidence for Seafloor Spreading When rocks cool at Earth’s surface, they record Earth’s magnetic field (normal or reversed polarity).

Evidence for Seafloor Spreading Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge. Pattern was created by seafloor spreading.

The Theory of Plate Tectonics The proof of sea-floor spreading supported Wegener’s idea that continents move. Both oceanic and continental crust appear to move, so a new theory was devised to explain both continental drift and sea-floor spreading: The Theory of Plate Tectonics.

Plate Tectonics According to the theory of plate tectonics, the earth’s outer shell (lithosphere) is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness. Plates interact with each other along their edges (called plate boundaries). Plate boundaries have a high degree of tectonic activity (mountain building, earthquakes, active volcanoes). Tectonic plates and Plate boundaries Tectonic plates, Earthquakes, and Volcanoes

Plate Tectonics Plate Tectonics: An Introduction Plate Tectonics: Further Evidence

Evidence for Plate Tectonics Pattern of worldwide tectonic activity (left) matches plate boundaries (right) Actual movement of land masses have been measured in Iceland.

The 3 Types of Plate Boundaries 1. Divergent (Constructive) 2. Convergent (Destructive) 3. Transform Tension Compression Shearing

Divergent Plate Boundaries Boundary between two plates that are moving away from each other. Sea floor spreading occurs forming Valleys or Ridges.

Divergent Plate Boundaries If oceanic crust is pulling apart from oceanic crust then new crust will be made. This means that in some places the sea floor is actually growing! Small chains of islands are created in some places - as new crust is created it builds into a small volcano that sometimes breaks the surface of the water. If continental crust pulls apart from continental crust then the same process occurs. As magma erupts to the surface to fill the gap a volcano is created.

Example - Divergent Plate Boundaries The Mid- Atlantic Ridge is a divergent plate boundary where sea- floor spreading occurs.

Example - Divergent Plate Boundaries Iceland sits atop a divergent plate boundary where continental rifting occurs.

Divergent Plate Boundaries Formation of an ocean basin by rifting and sea floor spreading.

Convergent Plate Boundaries Two plates moving towards each other collide. There are three types of convergent boundaries with different results. Subduction Zone – Place where one plate is subducted under another plate. The subducted plate heats and eventually melts causing pressure. The magma rises to the surface causing volcanoes.

Convergent Plate Boundaries Convergent plate boundaries vary depending on the type of crust. a. Ocean-continent b. Ocean-ocean c. Continent-continent

Oceanic-Oceanic One plate (the older one/more dense one) bends and slides under the other forming a subduction zone. A deep sea trench is formed and new magma rises to form volcanoes, or Island Arcs. (Islands of Japan)

Oceanic-Continental Here the dense ocean plate sinks under the less dense continental plate. Mountain ranges that contain volcanoes occur here. (Cascade Mtns)

Continental-Continental Plates collide and crumble up. No subduction zone (both have the same density). Formation of mountain ranges by folding. (Himalaya Mtns) Earthquakes are common at these converging plates.

Transform Plate Boundaries Two plates slide past each other. Can be moving in opposite or same direction but at different speeds. Many earthquakes occur at these boundaries. (San Andreas Fault)

Transform Plate Boundaries Transform plate boundaries occur between segments of the mid-ocean ridge. Can also occur on land.

Mountain Maker, Earth Shaker

The world as it may look 50 million years in the future

The world as it may look 250 million years in the future

What type of plate boundary is constructive?

No subduction zone (both have the same density).

Formation of mountain ranges by folding.

Two plates slide past each other.

Sea Floor Spreading occurs.

What type of plate boundary is destructive?

Boundary between two plates that are moving away from each other.

Two plates moving towards each other collide.

Mid-Atlantic Ridge.

Can be moving in opposite or same direction but at different speeds.