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saturnbutton1.JPG (21728 bytes)Earth Questions

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Q1. What is the appearance of Earth when it is viewed from distant space? Answer




saturnbutton1.JPG (21728 bytes)Earth - Interior Questions

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Q1.  What observations give us information about the structure and material of the interior of Earth? Answer

Q2.  Describe the structure and material of the interior of Earth. Answer

Q3.  What is the difference between an S and a P wave? What does their arrival (or lack thereof) tell us about the interior of Earth? Answer

Q4.  When referring to the average density of Earth, what does the concept of uncompressed density refer to? What does its value tell us about Earth? Answer

Q5.  How can we tell that the outer core of the Earth is liquid? Be sure to explain any terminology or concepts you use in your answer. Why do we believe that the inner core is solid? Answer

Q6.  Why do we suspect that the very center of Earth is solid even though the outer layers of the core are liquid? Answer

Q7.  How can earthquake waves tell us whether a planet has a molten core? Answer

Q8.  Describe the contents and properties of the interior of Earth. Mention such properties as size, mass, and density of different regions, what they are made of, and whether they are solid or liquid. Answer

Q9.  Explain in general why convection occurs? Describe the process of convection in the context of Earth’s mantle. Answer

Q10.  How do earthquakes help us learn about the interior of Earth? What is the basic structure of Earth’s interior? Answer

Q11.  What are the sources of energy which keep the center of Earth warm? Answer

Q12.  Describe the structure of the core and mantle of Earth. Answer

Q13.  Why is it important to have a network of seismic stations to study the structure of Earth’s interior? Answer

Q14.  The observed density of Earth is 5.5, while the uncompressed density is only 4.5. What is an uncompressed density? Why is it less than the observed density? Answer

Q15.  What are the two types of seismic waves that travel through the Earth? How can we use them to discover that the interior of Earth contains a liquid region? Answer

Q16.  Describe the interior structure of the Earth. Compare this structure to that of Mercury and the Moon. Answer




saturnbutton1.JPG (21728 bytes)Earth - Surface Questions

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Q1.  How is crust created and destroyed on Earth? Answer

Q2.  How do we believe the Hawaiian Islands were produced? In what way do they provide evidence in support of the concept of plate tectonics? Answer

Q3.  Describe and explain the variety of geologic processes which occur when two plates collide. Answer

Q4.  How are mountain ranges formed on Earth? Why are they often associated with earthquake zones? Answer

Q5.  What are the two different (although related) processes that create mountains on Earth? How do they build mountains? Long after the mountain is formed and its shape altered by other processes, how could we tell which process was responsible for its creation? Answer

Q6.  For each of the four processes that can shape the surface of a planet, describe how it has affected the current appearance of Earth. Answer

Q7.  Why are the continental plates higher than those which make up the ocean floor? Answer

Q8.  Why is there horizontal plate motion on the surface of Earth? Answer

Q9.  What is the evidence for horizontal plate motion on the surface of Earth? Answer

Q10.  How is Earth's crust being recycled? Discuss both the destruction and the creation of the crust. Answer







saturnbutton1.JPG (21728 bytes)Earth Answers

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A1.  When Earth is observed from a great distance, it appears primarily as a blue, white ball. The blue color comes from the world’s oceans while the white comes from the clouds in our atmosphere. Land masses are relatively inconspicuous, but appear as dull reddish-brown areas.




saturnbutton1.JPG (21728 bytes)Earth - Interior Answers

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A1.  We learn about the interior of Earth by studying the arrival times of earthquake waves at many different places around the surface of Earth. When, and even whether, the waves arrive at a particular location tell us how the waves traveled through the interior of Earth. For example, if only P (or pressure) waves are observed at one place (meaning the S or shear waves were absorbed somewhere in between the earthquake and the station), that tells us that liquid material must exist on the path the waves followed.

A2.  Earth contains a core of dense material, probably iron, that occupies about half the radius of the planet. This core is solid on the inside and liquid on the outside because of the increasing pressure at the center of Earth. A less dense mantle of silicate rocks surrounds the core.

A3.  Pressure or P waves are waves that oscillate in the direction the wave is traveling in, while shear or S waves oscillate perpendicular to the direction of travel. S waves cannot travel through a liquid. Since there is always a shadow zone in which S waves are not received from earthquakes, this observation suggests that there is a liquid portion to the core of Earth. The size of the shadow zone tells us how large this liquid core is.

A4.  The uncompressed density of a planet is the density it would have if its interior were not compressed by the force of gravity. It is, therefore, a better measure of the properties of the material from which the object is made than the observed density of the planet. The uncompressed density of Earth is about 4.5 times the density of liquid water, significantly higher than the average density of rocks found at the surface of Earth. This result tells us that Earth must contain significant amounts of materials that are denser than rocks.

A5.  S-type earthquake waves (waves whose vibrations are perpendicular to the direction of motion of the wave) can not penetrate through a liquid medium. This type of earthquake wave is not received by stations on the far side of Earth — which indicates that a liquid region is located somewhere between the station and the site of the earthquake. By mapping precisely those regions that do not receive S-type waves, geologists can pinpoint the size of the liquid core of Earth.

While we do not have direct observations to prove that the center of Earth’s core is solid, our models and computations indicate that it is. As one approaches the center of the Earth, both the temperature and the pressure increases. With increasing pressure, the minimum temperature required to melt the iron core also increases — at a faster rate than does the actual temperature. At some distance from the center, the actual temperature becomes less than the minimum temperature to melt iron and the core freezes into solid form.

A6.  Deeper layers of Earth are subjected ever greater temperature and pressure. We suspect the inner core is solid because of the greater pressures that exist there. The properties of iron indicate that iron subjected to such conditions of high pressure and high temperature returns to the solid state in spite of the high temperature.

A7.  Of the two types of earthquake waves, only P (pressure) waves can pass through a liquid. The other type, S or shear waves, cannot pass through liquids. Hence if only one type can penetrate through the center of a planet we know that there must be liquid somewhere inside the planet.

A8.  Earth’s interior consists of a dense iron core and a less dense silicate mantle. The radius of the core is about half the radius of Earth, but accounts for only 17% of its volume and 33 % of its mass. The core consists of two sections – a solid inner core and a liquid outer core – both composed mostly of iron.

A9.  Convection occurs when heat is added to a fluid faster than it can move through the fluid. Eventually a hot bubble of fluid begins to rise through its cooler surroundings and deposits its heat at the top of the fluid. In Earth’s mantle, heat from the core is added to the base of the mantle. Hot bubbles of rock rise to the top of the mantle, spread out horizontally, and then fall back toward the center of Earth.

A10.  Earthquakes cause vibrations in the Earth which can pass through the interior and be detected on the opposite side of Earth. By studying exactly when different kinds of waves arrive at different locations on Earth, we can learn about the material they passed through on their way. For example, if there is a significant delay in the arrival of waves at one station compared to a nearby station, it tells us that some material in the path of those waves slowed down their passage but did not exist along the path the other waves took. The analysis of many earthquakes allows us to gradually piece together a three-dimensional picture of Earth’s interior. It consists of a two part iron core that is solid on the inside and liquid on the outside, a large mantle of less dense rocky material surrounding the core, and a very thin crust on the top.

A11.  The interior of Earth is warm because of the energy trapped there during the formation of the planet and because of the release of energy by radioactive materials which are concentrated there.

A12.  The core of Earth is composed of nearly pure iron. It occupies about half the radius of Earth, but only 17% of the volume. Because it is very dense, it accounts for about one third of the mass of Earth. The inner portion of the core is solid while the outer portion of the core is molten. The less dense, rocky mantle which surrounds the core accounts for most of the rest of the material of Earth.

A13.  When an earthquake occurs, its waves travel through Earth in all directions. These waves can be recorded at a network of seismic stations around Earth. The arrival times and characteristics of the waves tell us how fast the waves traveled through different parts of the interior. Without a network of stations we would not be able to deduce where the waves originated or what path they followed through the interior of Earth.

A14.  For any large object, gravity squeezes the material of the object to a smaller size than the material would naturally have. This compression increases the observed density of the object. If you have a good model of the interior of the object, it is possible to predict what the density of the object would be without the natural compression due to gravity. This predicted density is the uncompressed density. It is smaller than the observed density because the uncompressed object (without the effects of gravity)is larger than the observed object.

A15.  All waves, including seismic waves, are one of two types: transverse waves which oscillate in a direction perpendicular to the direction of the wave motion and longitudinal waves which oscillate in the same direction as the wave motion. For seismic waves, transverse waves are referred to as S waves while longitudinal waves are called P waves. S waves are not able to move through the body of a liquid. Since S waves are not observed to penetrate the central core of Earth even though P waves do, we can conclude some part of the core is liquid.

A16.  The interior of Earth consists of a solid inner core and a liquid outer core, both mostly iron, surrounded by a mantle of silicate rocks. Mercury's core is a much larger fraction of the total planet than is Earth's, but the Moon has little if any differentiated core. The cores of both Mercury and the Moon are probably solid because the small bodies have lost their heat content.




saturnbutton1.JPG (21728 bytes)Earth - Surface Answers

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A1.  New crust is created on Earth where two plates of old crust separate. Molten material from the interior of Earth wells up to fill the vacancy left by the separating plates, cools, and becomes new crust. When two plates of crust collide, one is pushed down into the interior of Earth while the other is crumpled as it rides up on top of the other plate.

A2.  The Hawaiian Islands, and the undersea mountains to their west, represent a long chain of volcanic mountains that we believe have been created by a single hot spot in the mantle of Earth. Different mountains are produced as the crust of Earth moves over this stationary hot spot. This idea is supported by the observation that each island in the chain is older than the preceding one as you move from the eastern end of the chain toward the west.

A3.  When two plates collide, one is usually driven downward under the other, its material to be re-absorbed into the mantle. The other plate is crumpled at the point of collision, forming folded ranges of mountains along the boundary. Cracks and weak spots in this folded plate allow hot material from inside Earth to rise to the surface in volcanoes. Motion inside Earth as the plates slip past each other cause earthquakes.

A4.  Mountain ranges are formed on Earth when two plates collide. One plate is subducted while the other rides on top and is buckled to form mountains. As the two plates slide against each other, stress and pressure are occasionally released by jerky motion of the plates -- what we experience as an earthquake.

A5.  Mountains can be created either by volcanic processes or by the collision of two tectonic plates. When a volcano erupts, molten rock form the interior flows onto the surface. It may build up in a pile forming a mountain. When plates collide, one side rides on top of the other. The buckling of this plate creates tall ridges that become mountain ranges. We can tell the difference between these types of mountains from the type of rock present. The cooled lava is a distinctive type of rock, easily recognized long after the end of the volcano.

A6.  Cratering has had only a minimal effect on Earth. Volcanism has covered only relatively localized regions of Earth, often in association with tectonic activity. Plate tectonics through both the creation of new crust at spreading centers and the destruction of crust during plate collisions has had a profound effect on Earth’s surface. Erosion by air and water has also had a profound effect on changing Earth’s surface appearance.

A7.  The continental plates are both thicker and less dense than the plates which make up ocean floors. Since all the plates are floating on the top of the mantle, the less dense ones float higher just like a light piece of wood floats higher in the water compared to a denser object.

A8.  Horizontal plate motion occurs because of the convective currents in the Earth’s mantle. When hot rising mantle material reaches the top of the mantle, it spreads out and moves horizontally before sinking back into the deep mantle. Pieces of the crust are carried along on this horizontal motion of the mantle.

A9.  There are many lines of evidence which support the concept of horizontal plate motion. Among them are the sequential ages of the islands in the Hawaiian island chain (and several similar chains in the Pacific Ocean) as a result of a plate moving over a single hot spot volcano and the similarities of rock strata and fossils between the tips of south America, Africa, and Antarctica from the time that those regions were joined together in Pangaea.

A10.  Because of plate motion of the crust, crustal material is being subducted back down to the mantle where plates collide and one plate is pushed down under the other. Where plates are separating, mantle material rises to the surface to create new crust.