Earth’s interior After this lesson you will be able to explain how geologists learn about Earth’s interior and describe the features of Earth’s crust, mantle, and core
Vocabulary • Seismic wave • Pressure • Crust • Basalt • Granite • Mantle • Lithosphere • Asthenosphere • Outer core • Inner core
How do Geologists learn about Earth’s interior? • Processes that affect Earth’s surface are often a result of what’s going on inside Earth. But what’s inside Earth? This question is very difficult to answer, because geologists are unable to see deep inside Earth. • But geologists have found other methods to study the interior of Earth. • Geologists have used two main types of evidence to learn about Earth’s interior: direct evidence from rock samples and indirect from seismic waves.
Evidence from rock samples • Geologists have drilled holes as deep as 12.3 kilometers into Earth. The drills bring up samples of rock. These rocks give geologists clues about Earth’s structure and conditions deep inside Earth, where the rocks formed. • In addition, volcanoes sometimes blast rock to the surface from depths of more than 100 km. These rocks provide more information about Earth’s interior. Also, in laboratories, geologists have re-created conditions inside Earth to see how rock behaves. • For instance, they focus laser beams on pieces of rock while squeezing the rock with great force.
Evidence from seismic waves • To study Earth’s interior, geologists use an indirect method. When earthquakes occur, they produce seismic waves. • Geologists record the seismic waves and study how they travel through Earth. The speed of seismic waves and the paths they take give geologists clues about the structure of the planet. • That is, the paths of seismic waves reveal areas inside Earth where the makeup or form of material changes.
Video • http://www.bing.com/videos/search?q=earth%27s+interior&view=detail&mid=04F1CC9DF8474B4BEF6604F1CC9DF8474B4BEF66&first=0
What are the features of Earth’s crust, mantle, and core? • Today, scientists know that Earth’s interior is made up of three main layers. Each of Earth’s layers covers the layers beneath it, much like the layers of an onion. • The three main layers of Earth are the crust, the mantle and the core. These layers vary greatly in size, composition, temperature, and pressure. • Although each layer of Earth has its own characteristics, some properties apply throughout all of Earth. For example, the deeper inside Earth, the greater the mass of the rock that is pressing down from above. • Pressure results from a force pressing on an area. Because of the weight of the rock above, pressure inside Earth increases with depth. The deeper down inside Earth, the greater the pressure. Pressure inside Earth increases much like pressure in the swimming pool increases.
What are the features of Earth’s crust, mantle, and core? • The mass of rock that presses down from above affects the temperature inside Earth. The temperature inside Earth increases as depth increases. Just beneath Earth’s surface, the surrounding rock is cool. But at about 20m down, the rock starts to get warmer. • For every 40 m of depth from that point, the temperature typically rises 1 Celsius degree. The rapid rise in temperature continues for several tens of kilometers. • Eventually the temperatures inside Earth are the result of the great pressures squeezing rock and the release of energy from radioactive substances. Some heat is also left over from the formation of Earth 4.6 billion years ago.
The crust • In the summer, you might climb a mountain or hike down into a shaded valley. During each of these activities, you are interacting with Earth’s crust, the layer of rock that forms Earth’s outer skin. The crust is a layer of solid rock that includes both dry land and the ocean floor. The main elements in the crust are oxygen and silicon. • The crust is much thinner than the layer that lies beneath it. In most places, the crust is between 5 and 40 kilometers thick. It is thickest under high mountains-where it can be as thick as 80 kilometers-and thinnest beneath the ocean.
The crust • The crust that lies beneath the ocean is called oceanic crust. The composition of oceanic crust is nearly constant. Its overall composition is much like basalt, with small amounts of ocean sediment on top. Basalt is a dark, fine-grained rock. • Continental crust, the crust that forms the continents, contains many types of rocks. So, unlike oceanic crust, its composition varies greatly. But overall the composition of continental crust is much like granite. • Granite is a rock that usually is a light color and has coarse grains. Both granite and basalt have more oxygen and silicon than they have any other element.
The mantle • About 40 kilometers beneath dry land, the rock in Earth’s interior changes. Rock here contains more magnesium and iron than rock above. The rock below the boundary is the solid material of the mantle, a layer of hot rock. • The mantle is made of rock that is very hot, but solid. Scientists divide the mantle into layers based on the physical characteristics of those layers. Overall, the mantle is nearly 3,000 kilometers thick.
The Lithosphere • The uppermost part of the mantle is brittle rock, like the rock of the crust. Both the crust and the uppermost part of the mantle are strong, hard, and rigid. • So geologists often group the crust and uppermost mantle into a single layer called the lithosphere. Earth’s lithosphere averages about 100 km thick
The Asthenosphere • Below the lithosphere, the material is hotter and increasing pressure. As a result, the part of the mantle just beneath the lithosphere is less rigid than the rock above. • Over thousands of years this part of the mantle can bend like a metal spoon. But it’s still solid. If you kicked it, you would stub your toe. This soft layer is called the asthenosphere.
The Mesosphere • Beneath the asthenosphere, the mantle is hot but more rigid. The stiffness of the mesosphere is the result of increasingly high pressure. • This layer includes a region called the transition zone, which lies just beneath the asthenosphere. It also includes the lower mantle, which extends down to Earth’s core.
The core • Below the mantle lies Earth’s core. The core is made mostly of the metals iron and nickel. It consists of two parts-a liquid outer core and a solid inner core. • The outer core is 2,258 km thick. The inner core is a solid ball. Its radius is 1,222 km. The total radius of the core is 3,480 km. Earth’s core occupies the center of the planet.
Outer core and inner core • The outer core is a layer of molten metal surrounding the inner core. Despite enormous pressure, the outer core is liquid. The inner core is a dense ball of solid metal. • In the inner core, extreme pressure squeezes the atoms of iron and nickel so much that they cannot spread out to become liquid. • Currently, most evidence suggests that both parts of the core are made of iron and nickel. But scientists have found data suggesting that the core also contains oxygen, sulfur, and silicon.
The core and Earth’s magnetic field • Scientists think that movements in the liquid outer core create Earth’s magnetic field. Because Earth has a magnetic field, the planet acts like a giant bar magnet. Earth’s magnetic field affects the whole planet. • If you place a magnet on a piece of paper and sprinkle iron fillings on the paper, the iron fillings line up with the bar’s magnetic field. If you could surround Earth with iron fillings, they would form a similar pattern. • When you use a compass, the compass needle aligns with the lines of force in Earth’s magnetic field. These lines meet at Earth’s magnetic poles. So the needle points to Earth’s magnetic north pole, which is not the same location as Earth’s geographic North Pole.