Earth and the Lithosphere Apologia Physical Science Module 6 The Crust, Mantle and the Core The Crust ▪ The crust is the outermost layer of the earth. ▪ The thickness of the crust depends on where you are.(between 12-55miles thick) ▪ It can be as thin as 5 miles in the oceans ▪ The crust is made of soil, sediment and rock. ▪ Soil is the top layer ▪ Sediment is the next layer ▪ Sand, mud and gravel ▪ The last layer of the crust is rock. Type of Rock ▪ Sedimentary Rock- Rock formed when chemical reactions cement sediment together, hardening them ▪ Igneous Rock- Rock formed from molten rock (magma when cooled) ▪ Metamorphic Rock- Sedimentary or igneous rock changed by great temperature and pressure The Mantle ▪ The next layer of the earth is the mantle. ▪ Because of the metamorphic rock in the crust and the distance into the earth, scientist has never been able to directly observe the mantle. ▪ Scientist use seismic waves to determine what the different parts of the earth are made of. ▪ You will do experiment 6.1 for homework. This experiment allows you to see how sound vibration change depending on the substance it is moving through. The Mantle ▪ Using seimic waves, a scientist name Andrija Mohorovicic found there is a border between the earth’s crust and what is now known as the mantle. ▪ This border is called the Mohorovicic discontinuity or the Moho ▪ The mantle is made up of mostly solid rock but there is a section that is made up of plastic rock. ▪ Plastic rock is rock that behaves like something between a liquid and a solid ▪ When plastic rock is subjected to a force it hardens. Once the force is removed, it liquifies. The Earth’s Core ▪ Once again scientist used seismic waves to determine the different layers of the earth. ▪ The Gutenberg discontinuity is the border between the mantle and the outer core. ▪ The Lehmann discontinuity is the border between the outer and inner core. The Earth’s Core ▪ The core is divided into two parts: the inner core and the outer core. ▪ The outer core is made mostly of molten iron. ▪ Molten means liquid ▪ The inner core is mostly solid iron. ▪ Why is the inner core a solid and the outer core a liquid? ▪ Haven’t we learned that a liquid metal is hotter than a solid metal? Inner Core ▪ Even though the inner core is deeper than the outer core, it is a solid. ▪ The reason it is a solid is because there is a great amount of pressure. This pressure causes an effect called pressure freezing. ▪ Pressure freezing is when a liquid is under such extreme pressure than the molecules are pushed together to make a solid. This effect counteracts the melting effect of the high temperature. Earth’s Magnetic Field ▪ The earth’s core generate a magnetic field ▪ It is sort of like a big magnet ▪ We know how the magnet field behaves but we don’t exactly know why it behaves that way. ▪ We do know that electric current in the core is responsible for the earth’s magnetic field, but scientist can’t agree on why there is an electric current in the core. Theories of the Earth’s Magnetic Field ▪ There are two theories ▪ The dynamo theory- the motion of fluid in the outer core is caused by temperature differences in the outer core as well as the rotation of the earth. This motion causes the motion of electrical charges in the core which creates electrical current. This causes the earth to be magnetic. This is theory most scientist agree with. ▪ The rapid-decay theory- the electrical current of the earth is a consequence of how it was created. It is slowing down over time. Theories of the Earth’s Magnetic Field ▪ How can we tell which is correct? Should we just assume that the one that most scientist agree with is correct? ▪ No, we must use indirect observations. We can measure the magnetic field that is a result of the core. ▪ We have observed that the strength of the magnetic field is decaying. Which theory does that fit? Both but rapid-decay fits better. ▪ The earth’s magnetic field has reversed a few times. We can see this is certain rock formations in the crust. Certain materials in the crust are magnetic and always go towards the direction of the magnetic field. In certain rock layers we see these materials in the opposite direction. Which theory fits this? Both but the dynamo theory fits better but this would indicate that the change would happen slowly. Scientist have observe the change in 15 days which lead to the rapid-decay theory. ▪ Final observation- Other planets magnetic fields. Only the rapid-decay theory can predict the magnetic field or lack of in other planets. Theories of the Earth’s Magnetic Field ▪ So it seems like we should be following the Rapid-decay theory but why do most scientist follow the dynamo theory? ▪ Problems scientist have with rapid-decay theory▪ In order to explain a magnetic field reversal, there has to be a global, cataclysmic event. (the flood). Most scientist don’t want to accept that. ▪ Young earth (less than 10,000 years) Plate Tectonics ▪ The theory of plate tectonics states that the lithosphere broken up into several plates. These plates are floating on the plastic rock of the asthenosphere. ▪ These plates behave like islands floating on the ocean. Sometimes they interact violently. When this happens earthquakes can occur, mountains and deep trenches can form. Plate Interactions • When plates separate magna leaks up between them and creates new crust. • This is the only interaction that creates crust. • These interaction occur mostly on the ocean floor. Plate Subduction • When plates collide, one can slide under the other. This creates a mountain on one side and a trench on the other. • In this interaction, the crust is actually melting into the mantle. • This is only interact where crust is destroyed • This interaction is occurs mainly at the edge of a continent. Plate Collision • When plates collide, but they don’t slide under each other instead they buckle into mountain. • In this interaction, crust is neither created or destroyed. • The Himalaya Mountain were made because of this type of interaction. Plate Sliding • When plates slide against each other, the edges scrape against each other. • In this interaction, crust is neither created or destroyed. • This interaction can cause severe earthquakes. Pangaea ▪ There is theory that the earth was once a giant super continent that has been separated because of the tectonic plates separating. ▪ Watch the attached link to see how this could have occurred. ▪ https://www.youtube.com/watch?v=ALBUX31tW4 M Earthquakes ▪ Definitions- ▪ Earthquakes - vibrations of the earth that result either from volcanic activity or rock masses suddenly moving along a fault. ▪ Fault- The boundary between two sections of rock that can move relative to one another. If there is a fault, there is the possibility of an earthquake. ▪ Focus- the point where an earthquake begins ▪ Epicenter- the point on the crust directly above an earthquake Earthquakes ▪ Definitions- ▪ Earthquakes - vibrations of the earth that result either from volcanic activity or rock masses suddenly moving along a fault. ▪ Fault- The boundary between two sections of rock that can move relative to one another. If there is a fault, there is the possibility of an earthquake. ▪ Focus- the point where an earthquake begins ▪ Epicenter- the point on the crust directly above an earthquake Elastic Rebound Theory ▪ The rock masses on either side of a fault push against each other. As they move they get caught on each other and bend. Eventually the pressure gets to be too much and the rock masses break free and rebound back to their normally shape. ▪ Think about breaking a popsicle stick ▪ This is a good video to explain earthquakes and the elastic rebound theory ▪ https://www.youtube.com/watch?v=z0gesxAeY7w ▪ Don’t forget that although earthquakes occur most frequently along faults, they can also occur in other places.
