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Plate Tectonics Tectonics. The Problem with Continental Drift. While Wegener was able to find evidence of continental drift, there were 2 major problems with his ideas: Wegener could not explain what could cause the continents to drift - PowerPoint PPT Presentation


Plate Tectonics

Plate Tectonics Tectonics

The Problem with Continental DriftWhile Wegener was able to find evidence of continental drift, there were 2 major problems with his ideas: Wegener could not explain what could cause the continents to driftGeologists did not believe that the continents could simply plow through the oceanic crust as they moved.

Wegener suggested tidal pull of the moon and Earths rotation, both of which were dismissed. 2The Plate Tectonic ModelThe lithosphere is broken into rigid pieces called plates.Earths surface contains 7 major plates along with many other smaller plates. Each plate floats and moves around on the plastic-like layer of the mantle known as the asthenosphere.


The 7 major plates are the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic Plates.What causes the plates to move?The transfer of heat in Earths interior causes convection to occur. Convection currents in the mantle provide the energy for plate motion.

A model for convection inside the EarthWhile scientists agree convection is responsible for plate motion, there are several alternative models based on where the process of convection occurs inside the Earth. The image shown above is a deep layer model which includes convection currents deeper inside the Earth than many other models and shows the currents moving like a lava lamp on a lower setting. 63 Types of Plate BoundariesDivergent Boundary two plates are moving apart.Convergent Boundary two plates are moving together.Transform Boundary two plates slide past each other.

Divergent BoundariesNew lithosphere is produced as molten rock from the mantle rises to the surface at divergent boundariesContinental Rifting

Occurs when plates diverge within continents forming rift valleys Can break apart land + form new bodies of waterThe East African Rift Valley may eventually break up Africa

This kind of plate movement would result in the breakup of Pangaea. For the East African Rift Valley, scientists debate whether this could be the beginning of the breakup of a continent. Tremors from plate movement felt near this rift valley as recently as 2007. 9

Mid-Ocean Ridges

Occur when plates move apart on the seafloor and create rift valleys. New seafloor is created at ocean ridges by the process of seafloor spreading.

The ocean ridge system is the longest topographical feature on Earth --- 70,000+ km in total length. Sound waves and sonar were used to map the seafloor for the first time in the 1940s and changed what we know about what the structure of the seafloor looks like---series of ridges, mountains, and valleys---similar to continents. Two big questionswhere did these features come from and why are rocks near the ridges so young? 10Seafloor SpreadingTheory put forth by Harry Hess in the 1960s. As plates separate on the seafloor, molten rock from inside the Earth (magma) rises to the surface. The magma cools when it reaches the Earths surface and creates new oceanic crust. As a result, the youngest rocks on the seafloor are found at mid-ocean ridges. The further away from the ridge you get, the older the rocks will be. Typical rates of seafloor spreading are between 5-15 cm (2-6 inches) per yearabout the same rate as your fingernails grow. Explain how Harry Hess theory explains underwater features of the seafloor.11Convergent Boundaries

Old lithosphere is destroyed as plates descend into the mantle at convergent boundaries. Relate expansion at divergent boundaries to destruction at convergent boundaries. Convergent boundaries result in destruction of material, whereas divergent boundaries are creating new material at Earths surface. Thus the overall surface of the Earth is not expanding in size.12Subduction ZonesSubduction occurs when a less dense continental plate collides with a more dense oceanic plateThe more dense oceanic plate subducts or goes down under the less dense continental plate.

Continental Volcanic ArcsWhen subduction occurs, rock can melt forming magma. Less dense magma rises to the surface and in some cases can lead to volcanic activity. Include Andes Mountains in South America and the Cascade Mountains in the western United States.

The Cascade Mountain range runs through Washington, Oregon, and California and into Canada and includes Mount Rainier and Mount St. Helens. Explain how the ocean trench in the picture forms during subduction.14

Formation of MountainsCan happen from the collision of 2 continental plates.The ongoing collision of India with Asia started about 45 million years ago and produced the Himalayan Mountain Range.

Lower density = less subduction and more uplift of rock15Transform Boundaries

Plates slide past each other without creating or destroying lithosphere at transform boundaries.

Transform FaultsThe San Andreas Fault connects two divergent boundaries, the Juan De Fuca ridge and the Gulf of California. Plate movements along this transform fault are responsible for most of Californias earthquakes. Earth in 50 million years?