Plate Tectonics Introduction



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Plate Tectonics
Introduction


  • Earth is a dynamic, ever-changing planet

  • If we could go back in time 200 million yours ago there would be no Grand Canyon or Himalayas

  • More over we would find landmasses with unfamiliar shapes

  • Earth scientist have shown that the landmasses are not fixed but slowly migrate across the globe

  • Large landmasses have split apart resulting in the formation of oceans

  • While the floor of the ocean has been recycled back into the Earth’s interior

  • Separate landmasses collided and joined to form a larger continent

  • The movement of earth outer rigid layer generates earthquakes

  • Volcanoes, mountains, new ocean basins

  • Moreover, the changing positions of the continents helps explain ancient climates and distribution of fossils

  • The scientific theory which describes the mobile Earth is called plate tectonics


Continental drift

  • A revolution in the Earth sciences begin in the early part of the twentieth century when a radical proposal was mad that the continents drifted about the face of Earth

  • The development of this revolutionary hypothesis, known as continental drift, is credited to Alfred Wagener, a German meteorologist and geophysics

  • Wegener proposed that a supercontinent, which he named Pangaea

  • In addition, Wegener suggested that this supercontinent began breaking into smaller landmasses, which drifter to their present position

  • Fit of the Continents

  • Fossil evidence

  • Evidence from Rocks

  • Paleoclimatic Evidence

  • Wegener first suspected that the continents might have been joined when he noticed the similarities of the coast line of Africa and South America

  • South America and Africa


Fossil Evidence

  • Scientist have discovered that fossils of the same ancient plants and animals were found on widely separated continents

  • Why ore fossils of Mesosaurus found on both sides of the South Atlantis but nowhere else in the world

  • Three ideas were proposed to explain the distribution of the fossils

  • Rafting, stepping stone islands, and land bridges

  • Wegener had a better explanation- he proposed the continent of supercontinent Pangaea


Evidence from rocks

  • When working a jigsaw puzzle every

  • The picture must be continuous as well

  • The picture that must be matched in the continental drift puzzle is represented by ricks of similar types

  • And mountain belts

  • Mountains of about the same age

  • The mount ranges such as the appellation mountain


Paleoclimatic evidence

  • Earth science have learned that between 220 and 300 million years ago vast ice sheets covered extensive portions of earth’s landmasses

  • Evidence called till glaciers leave behind

  • An unsorted mixture of many different sediment sizes

  • Below these layer of till lay bedrock that was scratched and polished by glaciers armed

  • Most glaciers are in an area that you wouldn’t expect

  • Scientist have rejected the idea of the equator becoming cold because during this period large tropical swamps existed in the Northern Hemisphere

  • The lush tropical of these lush swamps eventually became the major coalfields of the eastern United States and Europe.

  • If the coal is under enough pressure the coal will turn in to a diamond

  • Fossils from the coal fields indicate that the environment trees had large fronts

  • Scientist realized that the area containing swamp areas and glacier areas when put together are all in one spot


The Great Debate

  • Wegener’s continental drift began to attracted attention in 1924

  • The continents didn’t break through the ocean floor

  • Harold Jefferson said that the ground under water got softer and moved little by little


Continental drift and Paleomagnitism


  • The imputes for renewed interest in continental drift came from studies conduct to determine the intensity of direction

  • Any one who has used a compass knows that Earth’s magnetic fields has a north pole and a south pole that align closely but not exactly

  • Earth’s magnetic field is similar to that produced by a simple bar magnet.

  • The technique used to study ancient magnetic field relies on the fact that some rocks contain iron rich minerals serve as fossil compass

  • Magnetite is found in basaltic rich rocks

  • Magnetic alignment in older lava fields was found to vary widely

  • In The past 500 years the magnetic pull has moved

  • Either the poles had migrated or the continents shifted

  • Poles have remained fixed yet the plates are moving

  • The polar magnetism this alone proved that Pangaea did exist because the lava magnetism created


Seafloor spreading

  • One reason that Wagener’s continental drift hypothesis was not widely accepted because there was not much known of the ocean floor

  • The war ships gave echolocation in 1950-1960 as new echolocation sounding technology was developed

  • An echo sounder measures the sound in time and can tell how far is the ocean bottom

  • 1500 meters per second

  • Extensive sampling of the ocean floor did not recover material that was older than 200 million years in age

  • Ridges proposes that ridges are located above upwelling portions of large convection cells in the mantle

  • As rising material spreads laterally seafloor is carried

  • As the crust is pulled apart, molten rock intrudes the fractures

  • Older portions of the seafloor are gradually consumed as they descend into the mantle


Plate tectonics: the new paradigm

  • According to the plate tectonics model, the uppermost mantle along with the overlying crust, behaves as a strong, ridged layer known as the lithosphere

  • This rigid outer shell overlies a weaker region in the mantle known as the asthenosphere

  • The temperature and pressure regime in the upper asthenosphere results in a very weak zone that permits the lithosphere to be effectively detached from the layers below.

  • Thus the weak rock within the upper asthenosphere allows Earth’s rigid outer shell to move

  • The lithosphere is broken into numerous segments, called plates, that are in motion with respect to one another, thereby continually changing shape and size

  • Seven major lithospheric plates are recognized

  • South American plate

  • North American plate

  • Pacific plate

  • Australian- Indian plates

  • Antarctic plate

  • Eurasian plate

  • And African plate

  • The largest is the Pacific plate which encompasses a significant portion of the Pacific Ocean basin

  • Most of the large plates include an entire continent plus a large area of ocean floor.

  • Intermediate size plate include

  • Philippian plate

  • Arabian plate

  • Scotia plate

  • Caribbean plat

  • Nazca plate

  • Cocos plate

  • And Juan de Fuca plate

  • Interactions of the plates sre n there boundaries

  • There are three types of plate boundaries

  • Divergent plate boundaries where two plates move apart resulting in upwelling of material from the mantle the create new seafloor

  • Convergent plate boundaries: where two plates move together, resulting in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantel

  • Although the total surface area of Earth does not change

  • The individual plate may diminish or grow in size

  • The Antarctic plate is mostly bounded by divergent boundaries and is growing larger

  • The pacific plate is shrinking

  • The African plate will eventually split

  • Every plate has all three plate boundaries



Divergent plate boundaries

  • Most divergent plate boundaries are located along the crest of oceanic crust


Oceanic ridges

  • Along a well developed divergent plate boundaries the sea floor is elevated forming the oceanic ridge

  • The oceanic ridge represents 20% of Earth’s surface

  • The ocean ridge can be 1000 to 4000 km wide

  • Along the axis of some ridges segments is a deep down faulted stretch called a rift valley

  • The average spreading is 5 cm slow is 2cm

  • The oldest ocean floor is 200 million years old

  • The ocean is growing symmetrically


Continental rift

  • Spreading centers mostly under water

  • Continental rifting =extensional forces, up warping

  • Faulting the rift is up warping the rift valley creating a continent like the Red sea

  • Also like lake Victoria



Earth’s interior 2/6/13

  • Heat from the Earth’s interior is the major source of energy for the movement of Earth’s outer shell

  • Scientist have determined the Earth’s interior is divided into four layers

  • The inner core is a solid iron rich sphere having a diameter of 2432 kilometers (1511 miles)

  • The outer core is a liquid metallic layer that is about 2270 km (1410 miles)

  • Earth’s magnetic field is thought to be generated by vigorous churning of the iron-rich material in the hot, fluid outer core. Movement of outer core creates the magnetic field.

  • The mantle is a solid rocky layer having a thickness of about 2885km (1789 miles.

  • Over 82 percent of Earth’s volume is contained within the mantle, a thick shell of rock composed of silicate materials that are rich in iron and magnesium

  • The outer crust is a very thin outer layer that ranges from 7-70km

  • The crust is made up of two crusts oceanic (basalt) and continental (granite)

  • The continental crust is about 35-70 km

  • Oceanic crust is about 7km (5 miles)

  • Continental rocks are less dense than oceanic crust

  • There is an Asthenosphere and a lithosphere

  • Asthenosphere is located within the upper mantle between 100 to 600 km and is composed of hot rocky material that the movment is very slow

  • The lithosphere is part of the crust and goes lower

  • Behaves as a strong ridged layer because it is composed of solid rock

  • Weak rock allows the ridged outer shell to move


Convergent plate boundaries 2/27/13

  • Convergent plate margins occur where two plates collide and the motion is accommodated by on plate sliding under

  • Also called Subduction zones because the lithosphere is descending in to the asthenosphere

  • Deep ocean trenches develop between two convergent plates

  • The oceanic plate is more dense than the continental plate so the plate descend in to the asthenosphere

  • Some oceanic plate descends fairly beep sometimes

  • Density of the plate is what determines which goes under (more compact and temperature change)

  • As it gets older the oceanic plate will more likely sink because it is more cool

  • Older oceanic plate will descend almost 90 degree because of the density

  • as one is decending the plate pulls the continent with it. It changes position.

  • There are three types of convergent boundaries

  • Oceanic and oceanic

  • Oceanic and continental

  • Continental and continental



Oceanic continental convergence

  • When the oceanic lithosphere go 100 km the plate rock starts to melt

  • Peridotite has low water content

  • Different amounts of water in rock effects how it melts

  • Seismic evidence predicts that under average conditions of temperature and pressure the upper mantle consists of solid

  • There are different depths to melt different times

  • 100 km melts wet peridotite at roughly 400 degrees

  • Hot mixture gradually rises toward the surface in tear shape form

  • Basaltic magma separates from the un-melted components and continues to ascent

  • Basaltic magma rises to the surface because it is less dense and hot

  • Magma sometimes melt other continental rock

  • Andesitic magma reaches the surface they erupt explosively generating large columns of ash

  • Andesitic rock name came from the Andes mountains in south America



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