Plate Tectonics

Early Observations Late 1500’s Abraham Ortelius observed the fit of continents on both sides of the Atlantic Ocean His false hypothesis was that the continents had been separated by earthquakes and floods

Early Observations Eduard Suess suggested that the Southern Continents were once joined together in a landmass known as Gondwanaland

Continental Drift Alfred Wegener proposed the hypothesis of continental drift He claimed that all the land on the Earth was a supercontinent called Pangaea Pangaea broke apart 200 Million Years (early Triassic) ago and the continents have been drifting apart ever since

Alfred Wegener (1903) suggested and presented evidence that the continents were once a single supercontinent, called Pangaea, which divided ~ 200 Million years ago into Laurasia and Gondwanaland and later into the continents we see today (“continental drift”)

Continental Drift Evidence: –Similar rock types found on different sides of the Atlantic Ocean –Similar fossils found all over the world –Coal beds found in Antarctica –Glacial evidence found in Africa, India, Australia, and South America

Evidence Wegener gathered most of his evidence from fossils Remains of past organisms were found all over the world Kannemeyerid and Labyhrinthodont were found all over the world and it is unlikely that they swam those great distances

Evidence Other fossils that Wegener studied were: –Mesosaurus – an aquatic reptile –Glossopteris – a seed fern These fossils were found in many different climates and it is unlikely that they could have adapted to such different climates

Fossils of the land-bound lizard Lystrosaurus were found in Africa, Antarctica and India, suggesting that these continents were one landmass during the life-time of this critter

Ancient Climates Wegener also found evidence of past climates all over the world Coal deposits were found in Antarctica Coal is usually restricted to just swamps

Ancient Climates Glacial deposits were found in Africa, India, Australia and South America Glaciers do not exist on these continents Wegner proposed they were once near the poles

Geologic evidence for “continental drift” Distribution of warm-water fossils Rocks of same age cross continents

Continental Drift This hypothesis was initially rejected No evidence to explain what was making the continents move His findings weren’t proven until after his death

Rejection Wegner first suggested that the rotation of the Earth caused this –Physicists disproved this Wegner also said that the continents were plowing through ocean floor –This was rejected because the continents could not push through without fracturing

Seafloor Spreading

Early Beliefs Up until the mid-1900’s everyone believed the ocean floor to be flat It was believed that: –Oceanic crust didn’t change –Ocean crust was much older than continental crust

Technology Two types of technology that helped to study the ocean floor –Sonar –Magnetometer

Sonar Sonar-Uses sound waves to measure water depth Sound waves travel through the water and get bounced back from the ocean floor

Sonar The amount of time it takes for the waves to bounce back can be used to calculate the depth of the ocean floor

Magnetometer A magnetometer is a device that can detect small changes in magnetic fields Magnetometers record the magnetic field strength and are then used to construct a map of the ocean floor

More evidence to support sea floor spreading comes from the study of paleomagnetism, the magnetic properties of ancient rocks. When a magnetometer (a device that detects the magnetic properties of rocks) is towed across the ocean it indicates that there are alternating zones of rock with either normal or reversed polarity. In the green zones bits of magnetic iron in the rocks are pointing to where north is today. They act like countless little magnets trapped in the rock when it solidified from magma. But in the orange zones the bits of iron are pointing in the opposite direction.

Scientists discovered that the ocean floor contained: Mountain chains Ocean ridges Volcanism and earthquakes are very common along the ocean ridges

Deep Sea Trenches Deep sea trenches are elongated depressions in the seafloor and form as a result of these mountain chains –Example: Mariana Trench

Scientists as well made two very important discoveries: –Ages of rocks near ocean ridges were younger than rocks near deep sea trenches –Thickness of sediments increased with distance from the Ocean Ridge

Magnetism Paleomagnetism-The study of the magnetic record Basalt is a good indicator of ancient magnetism

Magnetism Studies of Basalt flows have shown a pattern of magnetic reversals A magnetic reversal is a change in the Earth’s Magnetic Field

Magnetism By studying magnetic data from the ocean floor and comparing it to basaltic flows on land they were able to determine the age of the ocean floor

Sea-Floor Spreading

Isochron Scientists were able to construct an isochron map An isochron is a line on a map that connects points that have the same age

Seafloor Spreading All of this data was compiled and analyzed and an American scientist by the name of Harry Hess proposed the theory of seafloor spreading Seafloor spreading states that new ocean crust is created at ocean ridges and destroyed at trenches

Seafloor Spreading Magma is forced towards the crust Cools once it comes in contact with the water and fills in the missing gaps New ocean floor is created and moves away from the ocean ridge

Seafloor Spreading

Continental Drift Seafloor spreading completes Wegener’s model of continental drift Wegener could not explain what caused landmasses to move or how they move and seafloor spreading helped explain this

Theory of Plate Tectonics The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous slabs called plates These plates interact with each other at areas called plate boundaries

Types of Plate Boundaries Divergent-Places where two tectonic plates are moving apart Most are found near ocean ridges on the seafloor When they occur on the Earth they form Rift Valleys –East Africa

Divergent Plate Boundaries Formation of new ocean crust occurs at Divergent Boundaries This accounts for high heat flow, volcanism and Earthquakes

Divergent Plate Boundaries May cause an ocean basin to grow wider The Atlantic Ocean is spreading at about 2-3 centimeters per year

Divergent Plate Boundaries Divergent Boundary-Area on the Earth where two of Earth’s tectonic plates are moving apart. –Volcanism –Earthquakes

Divergent Boundary The place where two plates move apart. Most occur at the mid- ocean ridge and on land. The boundary forms rift valleys. Example - Great Rift Valley has a 3000 km crack.

Rio Grande Rift Extends from central Colorado to El Paso, Texas.

Convergent Plate Boundaries Convergent Plate Boundary-Area on the Earth where two of Earth’s tectonic plates are moving toward each other –Trenches –Island Arcs –Mountains

Types of Plate Boundaries Convergent plate boundaries are areas where two tectonic plates are moving toward each other

Convergent Plate Boundaries Oceanic-Oceanic-Causes subduction zone and a deep sea trench Oceanic-Continental-Causes a series of volcanoes along the continent Continental-Continental-Form mountain ranges such as the Himalayas

Oceanic-Oceanic Boundary The denser plate is subducted beneath the other This forms a deep sea trench The descending plate is melted and recycled

Oceanic-Oceanic Boundary Some of the magma is forced back to the surface and forms an arc of volcanoes Examples: –Mariana Trench –Aleutian Islands

Oceanic-Continental Subduction occurs as well This forms a series of mountains along the edge of a continental plate Example: –Peru-Chile Trench –Andes Mountains

Continental-Continental The ocean basin between the two continents is subducted The colliding edges of the continents are uplifted and form a mountain range Example: –Himalayas –Appalachian Mountains

Appalachian Mountains

Transform Boundaries Place where two plates slide horizontally past each other Crust is not created or destroyed it is only deformed or fractured Often associated with Earthquakes San Andreas Fault

Convection Current The heated material of the mantle rises because it decreases in density while the cooler part sinks This is called a convection current This is thought to be the driving force of plate movements