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Earth’s Internal Processes Plate Tectonics Earthquakes Volcanoes Review • Recall that the Earth is composed of several layers. • Inner Core- Solid iron/nickel • Outer Core- Molten iron/nickel • Mantel- “Plastic” Molten Rock • Crust- Solid Rock QuickTime™ and a decompressor are neede d to see this picture. History • Throughout most of history people believed that the continents were permanent and unmoving. • Alfred Wegener was one of the first to suggest the continents used to be joined together in one Super-continent he called Pangea, meaning “all land” Continental Drift • Wegener proposed the hypothesis of continental drift which suggested that the continents were like icebergs of granite floating in a sea of more dense basalt. • While now accepted as the truth this idea was laughed at until sufficient evidence was found. Evidence • One of the first pieces of evidence to support continental drift was the discovery of fossils. • An ancient freshwater reptile, Mesosaurus, was found in both Africa and South America. • Tropical plant fossils were found in places that they would not normally be able to grow. • Conclusion… the continents are moving. Evidence (cont) • Evidence of glaciers were found in places were they could not have developed. • People also noticed that similar rocks could be found on opposite sides of the Atlantic. • The rocks near the mid-ocean ridge are much newer than the rocks that are farther away. • It was concluded that the seafloor was spreading along the mid-oceanic ridge. High Tech Evidence • Later studies showed that the rocks in the seafloor had alternating bands of magnetism that correlated with reversals in Earth’s Magnetic Field. • Today very accurate lasers have determined that most continents are drifting at a rate of about 1inch per year. Plate Tectonics • The Theory of Plate Tectonics states that the Earth’s crust is divided into sections called plates. • These plates are moved around by convection currents in the mantle. Boundary Issues • If all the plates are moving in different directions how do their boundaries interact. • There are three types of plate boundaries – Divergent – Convergent – Transform Divergent Boundaries • Divergent boundaries exist where two plates are moving apart from one another. • Divergent boundaries in the ocean form mid ocean rifts and ridges. • On continents they form rift valleys like Africa’s Great Rift Valley. Convergent Boundary • When two plates are moving toward each other one of three convergent boundaries will form. • This depends on the density of each of the plates involved Subduction Zone • Where a seafloor plate impacts a less dense continental plate and sinks below it. • The sinking plate brings water with it helping to create volcanoes. • A similar type of convergent boundary occurs when two oceanic plates collide. • Both tend to create volcanoes, earthquakes, and trenches. Going up? • When two continental plates impact, they tend to move upward. • This process created some of the biggest mountains in the world. • The Himalayas were created as the Indian Plate smashed into the Asian Plate. • They are still pushing up at about 0.5cm/year. Transform Fault Boundary • Transform fault boundaries occur where plates are sliding past or along side one another. • These boundaries are known for many powerful earthquakes that release energy that builds up as the plates snag on one another. • The San Andreas Fault is a famous transform fault boundary located under California. Earthquakes • Earthquakes are caused by the motion of Earth’s Crust. • We can measure the seismic waves caused by an earthquake. • We can also see where the waves originated and find the focus or point of origin for an earthquake. • Most foci occur along plate boundaries, also called faults Seismic Waves • Two types of seismic waves radiate out from the focus of an earthquake • Primary waves move particles back and forth like sound waves. • Secondary waves move particles up and down like ocean waves • On the surface these two waves join to give the ground an elliptical motion. • Earthquakes cause massive damage to man made structures. Seismic Waves (cont) • Seismic waves don’t just locate the focus or epicenter. • They can also be used to map Earth’s interior because only primary waves are able to travel through molten rock. • They can show us the location of magma chambers beneath volcanoes. Measuring Earthquakes • Earthquakes can be measured using a seismometer, which traces the affect of the seismic waves on a rotating drum. • These tracings can be used to measure the energy or strength of the earthquake which is described with the Richter Scale. Richter Scale • Is a log base scale. • A 2.00 earthquake is 10 times stronger than a 1.00 earthquake. • Under 4.00 is considered to be minor and over 6.00 is considered to be strong. • Weaker earthquakes are very common while strong earthquakes are very rare. • The strongest recorded earthquake was a 9.5 in Chile in 1960. Tsunami • When an earthquake strikes under or near water the waves it releases can travel through the water generating an enormous wave. • These waves can cause lots of damage when they hit shore. • Dec 26, 2004 an earthquake of the coast of Sumatra generated a tsunami that had a death toll of 230,000 people and traveled across the entire Indian Ocean QuickTime™ an d a GIF decompressor are need ed to see this picture . Volcanoes • Volcanoes are mountains that form as lava and ash from repeated eruptions from a vent build up layer by layer. • The lava comes from magma chambers below the Earth’s surface. • The opening at the top of the volcanoes vent is called the crater or cauldera. Where? • Divergent Boundaries - lava erupts through the rifts and cools. Mid-oceanic Ridge, Iceland. • Convergent Boundaries. Occur as water mixes with lava increasing pressure. Mt. St. Helens, Mt Redoubt. • Hot Spots - Some volcanoes occur as hotspots in the mantle heat portions of the crust enough to melt them and build up pressure. Hawaii, Yellowstone. – Hot spots stay in place as plates move above them QuickTime™ and a decompressor are neede d to see this picture. Types of Eruptions • Eruptions can either be explosive and violent or lava can quietly seep out of a vent. • Magma that has lots of gasses trapped inside tend to erupt more violently than magma with little gas. • Basaltic magma tends to trap less gas than granitic magma because it is more liquid. Pyroclastic Flows • Translated roughly to fire rock these are the result of explosive granitic eruptions. • Mixture of hot gas and rock (tephra) that moves away from the volcano at speeds of up to 450mph and temperatures of near 2000ºF • Often the most deadly aspect of a volcano, a pyroclastic flow was responsible for the creation of the ruins of Pompeii Types of Volcanoes • Shield Volcanoes form when quiet eruptions build up shallow layers of igneous rock. • Cinder Cone Volcanoes form as tephra builds up to form a steep sided mountain. • Composite or Strato-volcanoes form when a volcano regularly switches between quiet and violent eruptions.
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