Plate Tectonics Evolution of the Earth

Earth’s Moving Plates The oceanic crust shows signs of regular movement. This movement is associated with convection currents in the mantle. The rising up, spreading out and sinking of gasses, liquids or molten material create convection currents.

Cracks in the Earth’s Crust The crust, when it is solid, acts as a heat insulator for the hot interior of the Earth. The molten material, magma, below the crust builds up tremendous heat and pressure. The magma creates convection currents and rises to the surface. These currents can crack the crust!

Cracks The lava cools to form new rock for the crust. There is a world wide system of “cracks” in the crust (both oceanic and continental). These cracks separate the crust into plates (huge sections of the Earth that moves relative to each other).

Earth’s crust is broken into about 19 pieces Tectonic Plates Earth’s crust is broken into about 19 pieces These plates move on top of the asthenosphere (between mantle and crust)

How do we know anything about the Earth? Interior structure Volcanoes Earthquakes Tectonic plates Tectonic motion Reconstruction of the Earth’s history

Earthquakes

Earthquakes

Earthquakes Tidal waves or Tsunamis result when a large section of the sea floor suddenly moves and therefore displaces a massive amount of water.

Tectonic Plates

Earthquakes Location of worldwide earthquakes

Tectonic Plates

Tectonic Plates Our first evidence of tectonic motion is based on similar fossils and rock types on opposing sides of the ocean

Tectonic Plates Today plate boundaries are determined by examining the location of volcanoes and earthquakes. Volcanoes result from the friction (heat) of the plates motion. Earthquakes occur where plate rub against one another.

Tectonic Plates Volcanoes

Tectonic Plates Earthquakes

Tectonic Plates Another source of evidence is based on seafloor ages which get younger as we approach sea floor ridges

Based on Motion It turns out that the upper section of the mantle is adhered (stuck to the underside side of the crust) to form what we call tectonic plates.

Plate Boundaries Convergent Boundaries Subduction – when one plate plunges beneath another Continental Collision – plates move toward one another, shoved together Divergent Boundaries – plates move away from each other Transform fault – plate moves sideways from each other, slide past each other

Subduction The downward movement of an oceanic plate into the mantle As the crust enters the mantle, pressure breaks the crustal rock. Heat from friction melts it. It forms a pool of magma. This magma is called andesite lava, which is a mixture of basalt from the oceanic crust and granite from the continental crust.

Subduction

Results of Subduction The lava will travel along channels in the crust, causing great explosive eruptions and form composite volcanoes – steep sided, cone shaped mountains Ocean trenches form at the regions where one plate moves downward beneath another. These trenches are deep, narrow and long.

Continental Collision Occurs if 2 plates carrying continents collide and the subduction is interrupted. Because the continental crust is made of low density material, it does not sink. The crust moves upward, folds and buckles and breaks. Mountain ranges were formed by continental collision.

Convergent Plates

Convergent Plates

Seafloor Spreading As one boundary of a plate is being subducted and destroyed, the other boundary is having new material added to it - Plates move away! (Divergent) Magma rises to the crustal surface and forms mid ocean ridges. As the lava cools it forms new seafloor and features: rift valleys; seamounts; abyssal hills (volcanic peaks) As new material reaches the surface, the plates are pushed apart

Seafloor Spreading cont. Earth’s longest mountain chain 47,000 miles long running down the middle of the Atlantic Ocean (surfacing at Iceland), around Africa, through the Indian Ocean, between Australia and Antarctica, and north through the Pacific Ocean. Running along the top of this chain of mountains is a deep crack, called a rift valley. It is here that new ocean floor is continuously created. As the two sides of the mountain move away from each other, magma wells up from the Earth's interior. It then solidifies into rock as it is cooled by the sea, creating new ocean floor. The speed at which new ocean floor is created varies from one location on the ocean ridge to another. Between North America and Europe, the rate is about 2.2 inches (3.6 cm) per year. At the East Pacific rise, which is pushing a plate into the west coast of South America, the rate is 12.6 inches (32.2 cm) per year.

Seafloor Spreading cont. Mid-Oceanic Ridges  rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep. Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges.

Divergent Plates

Seafloor Spreading cont. Deep Sea Trenches  Convergent boundaries The deepest waters are found in oceanic trenches, which plunge as deep as 35,000 feet below the ocean surface. usually long and narrow, and run parallel to and near the oceans margins. often associated with and parallel to large continental mountain ranges. Like the mid-oceanic ridges, the trenches are seismically active, but unlike the ridges they have low levels of heat flow. Scientists also began to realize that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased. In addition, it has been determined that the oldest seafloor often ends in the deep-sea trenches.

Transform Fault Perpendicular breaks or fractive zones occur when sections of plates slip pass one another This causes shallow earthquakes

Transform Fault

Transform Plates

Transform Plates San Andreas Fault

Why do the Plates Move? No single idea explains everything but we can identify several forces that contribute to the movement of the plates. Slab pull The sinking of the cooled dense oceanic plates pulls on the rest of the plate Ridge rises The material deposited on the top of the ridge slides downs from the rise pushing on the plate Convection Movement within the mantle could be part of the driving force behind the motion of the plates.

The Big Picture

Pangea What is Pangaea? Pangaea was a super continent at one time. 200 mill years ago, plates shifted enough to cause movement. Scientists use the similarity of rock types and fossil types that date to the same age to support their theory that the continents were connected to form a super continent.

Pangea

Pangea The break up of Pangea

Where are we going? We appear to be headed for another super continent as North America, South America, Asia and Australia converge in the ever shrinking Pacific Ocean