Unit 5 – The Dynamic Earth Chapter 17 – Plate Tectonics Major focus for the Regents Exam! ESRT pages we will study in this unit: Page 5 – Tectonic Plates Page 10 – Inferred Properties of the Earth Page 11 – Earthquake wave travel time & crust composition

Plate Tectonics Plate tectonics is a proven “theory” that describes the formation, movement, and interaction of lithospheric plates over a plastic asthenosphere. Lithosphere – is broken into plates that ride on the asthenosphere. (rigid mantle and crust) Asthenosphere – is the “plastic” mantle. (movement mechanism for lithosphere)

Drifting Continents What caused people to consider that the earth’s surface might be moving?   Shape of South America and Africa Fossil evidence And much later…bands of reverse magnetism in igneous rock on the ocean floor.

Principles of Plate Tectonics The lithospheric plates are made of both continental and oceanic crust, and cover the entire earth surface. As plates move…so do the continents. Plate tectonics explains how new crust forms and why earthquakes and volcanoes are located in concentrated belts.

Evidence of Plate Tectonics Evidence that we use today to observe plate tectonics for minor crustal movement: Folds Faults Displaced fossils Displaced strata  

Evidence Folds – occur when rocks are deformed slowly due to pressure.

Evidence Faults – happen when rocks are deformed quickly due to pressure of tension.

Evidence Displaced fossils – Marine fossils found high above sea level. Uplift of land (Grand Canyon, Alps, Rocky Mountains Fossils of land organisms (plants and animals) found below sea level. Subsidence of land (Gulf of Mexico)

Evidence Displaced Strata – Sedimentary rocks found above sea level due to uplift.  

Evidence Evidence that we use today to observe plate tectonics for major crustal movement:   Movement of plates (G.P.S.) Mountain Ranges Rift Zones

How does it work? How can we assume that the processes that occur today…formed the “evidence” that we see from events of the past?   Uniformitarianism – the laws of nature have not changed (ex. Gravity, weathering, erosion and deposition) …”the present is the key to the past”

Start of Plate Tectonics Where did the theory of plate tectonics start?   Observations 1.    Similarities in the shape of continents. 2.    Mesosaurus (lived about 270 m.y.a) fossils are found in both South America and Africa. Theory of Continental Drift – proposed by Alfred Wegner in 1912 -        He used observations above to propose plate movement -        Downfalls of his theory…was he could never explain the mechanism for movement.

5 Pieces of Evidence to explain plate tectonics: 1.  Fit of Continents (modified from old theory of continental drift ~ Wegner)   2.  Glaciation – evidence of glaciation in lower latitudes. (ex. Scratches, glacial erratics, till) 3.  Correlation of rocks and mountain ranges on both sides of the Atlantic Ocean. 4.  Age of ocean floor (Mid-Ocean Ridge) 5. Magnetic Reversals (Mid-Ocean Ridge)

Theory of Plate Tectonics Plate Boundaries – tectonics plates touch at locations called plate boundaries. There are 3 main types of plate boundaries, based on how the plates are moving in relation to each other. Convergent boundaries Divergent boundaries Transform boundaries

Convergent Boundaries Locations where two plates are moving towards one another. There are 3 specific types of convergent boundaries Continent – Continent boundary Ocean – Ocean boundary Continent – Ocean boundary

Continent – Continent A location where two plates of continental crust are coming together.

Ocean - Ocean A location where two plates of oceanic crust are coming together.

Continent - Ocean A location where continental crust is colliding with oceanic crust

Subduction Why does one plate usually sink below another plate? The more dense plate sinks below the less dense plate. Continental crust = 2.8 g/cm3 Oceanic crust = 3.0 g/cm3

Divergent Boundaries A location where two plates are moving apart, due to magma rising up and creating new crust. Sea-floor spreading.

Transform boundary A location where two plates are sliding past one another.

Animations of Plate Boundaries http://www2.wwnorton.com/college/geo/egeo/flash/2_6.swf

Causes of Plate Motion Still much to be discovered in this area, but we have a better explanation of a mechanism for plate movement. Mantle Convection

Convection explained… Convection: the transfer of thermal energy by the movement of heated matter. Heated matter will expand. Matter that expands becomes less dense. Less dense matter rises in more dense matter.

Convection Current Matter is heated = density is decreased = matter rises Matter is cooled = density is increased = matter sinks

Mantle Convection The mantle is heated unevenly by deeper layers of the asthenosphere. As the lower parts of the mantle are heated they start to rise. As they rise, they enter colder regions of the mantle and begin to cool. This cooler mantle material will begin to sink…thus creating a convection cell.

Mantle Convection These convection cells in the mantle can move the lithosphere (plates) in several ways.

Types of Movement Push – the weight of the uplifted rock at the ridge will push the crust away from the ridge. Pull – the weight of a subducted plate pulls the rest of the plate down into the Earth.