Just as the theory of Evolution underpins Biology The theory of Plate tectonics is the basis of Earth Science So how did the theory of Plate tectonics develop?

The Earth is made of layers.

Earth’s Composition Crust outermost layer of Earth The crust is under the ocean as well as under the continents. Mantle extends from bottom of crust to top of outer core Outer core made of iron and nickel and is liquid Inner core made of iron and nickel and is solid

CRUST The first layer consists of about 1o miles of rock and loose materials, scientists call the crust. Underneath the continents, the crust is almost three times as thick, as it is under the oceans.

Mantle Traveling beyond the Earth’s crust, we next encounter the mantle. The mantle extends to a depth of approximately 1,800 miles, as is made of a thick solid rocky substance that represents about 85% of the total weight and mass of the Earth.

Liquid Outer Core Traveling still deeper within the Earth, we next would encounter the Earth’s outer core, which extends to a depth of around 3000 miles beneath the surface. It is believed that this outer core is made up of super- heated liquid molten lava. This lava is believed to be mostly iron, and nickel.

Solid Inner Core Finally, we would reach the Earth’s inner core. The inner core extends another 900 miles inward towards the center of the Earth. It is believed that this inner core is a solid ball of mostly iron, and nickel.

HOW?? The most important thing we use to sense the Earth’s core are seismic waves. Seismic waves are waves of energy caused either by earthquakes, or by massive manmade explosions.

The force that drives plate motion is heat-driven convection currents in the mantle.

Alfred Wegener in 1912, noticed that the continental shorelines on either side of the Atlantic Ocean looked like they would fit together. Wegener thought this could not be coincidental and developed a hypothesis called Continental Drift. Continental Drift stated that continents were once a part of a single landmass, which he named Pangaea. The one large ocean surrounding this continent he named Panthalassa. Wegener believed that 200 million years ago Pangaea began to break apart He believed that the continents drifted to their present positions creating mountains.

Lots of evidence supporting his hypothesis Fossils of Mesosaurus had been found in eastern South American and western Africa. This reptile could not have swum across the Atlantic. The type of rocks in western Africa and eastern Brazil matched closely. Mountain ranges match up if North America and Europe were together. There is evidence of glaciers in southern Africa that today have tropical climates.

New evidence to support continental drift: In 1947 a group of scientists mapped out the Mid-Atlantic Ridge, an undersea mountain range. These scientists discovered that the ocean floor is relatively young compared to the age of the continental rocks. Harry Hess hypothesized that there might be magma welling up from beneath this ridge, making the rocks older farther away from the ridge. This hypothesis became known as seafloor spreading. These scientists needed proof for their hypothesis to be accepted: Rocks on either side of the Mid-Atlantic Ridge were dated and found to get older as they move away from the ridge (this was discovered AFTER Wegener died)

Mid Ocean Ridges:

Sea floor spreading and subduction together explain the young age of the oceanic crust. New oceanic crust forms by sea floor spreading at mid-ocean ridges. This crust is pushed away from the ridges by continuing sea floor spreading. Eventually the oceanic crust subducts below another plate at an oceanic trench and gets melted back into the mantle. Oceanic crust is continually created at mid-ocean ridges, and continually destroyed at oceanic trenches!

Divergent – two plates are moving away from each other. New crust is formed. Most divergent boundaries are on the ocean floor. These are marked by ocean ridges. Transform Fault – two plates grind past each other. They do not slide easily, they stick and slip which creates earthquakes Convergent – two plates are moving towards each other. These are marked by ocean trenches. Plate boundaries – areas where plates are in contact with each other Three types of boundaries:

Paleomagnetism: strips of alternating magnetic polarity at spreading regions. N S today S N then

The theory of plate tectonics explains many important features of Earth’s surface, such as: - mid-ocean ridges, earthquakes, and volcanic activity - ocean trenches - the young age of the ocean floor

Lithosphere composed of 7 major and 13 minor plates A plate can be composed of continental and/or oceanic crust: Continental crust is felsic Oceaninc crust is mafic

Pangaea (“all lands”) Permian 200 million years ago Panthalassa (“all sea”) evolved into today Pacific ocean Tethys sea is preserved in the Mediterranean sea Shading is Permian Glacial deposits

Triassic 180 million years ago Shading shows new ocean floor

Jurassic 135 million years ago

Cretaceous 65 million years ago

Present Day