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LESSON 3: EARTH STRUCTURE AND COMPOSITION
The crust makes up less than 1 percent of Earth by mass, consisting of oceanic crust and continental crust is often more felsic rock. The mantle is hot and represents about 68 percent of Earth’s mass. The core is mostly iron metal. The core makes up about 31% of the Earth. The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle, rigid solid. The asthenosphere is partially molten upper mantle material that behaves plastically and can flow.
Crust - Earth’s outer surface is its crust; a cold, thin, brittle outer shell made of rock. The crust is very thin, relative to the radius of the planet.
Oceanic crust is composed of magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro. Sediment is thickest near the shore where it comes off the continents in rivers and on wind currents. Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic igneous rocks of the oceanic crust. The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick.
Mantle
The two most important things about the mantle are:
- it is made of solid rock, and
- it is hot
Heat flows in two different ways within the Earth: conduction and convection:
Conduction is defined as the heat transfer that occurs through rapid collisions of atoms, which can only happen if the material is solid. Convection is the process of a material that can move and flow may develop convection currents. Convection in the mantle is the same as convection in a pot of water on a stove. Convection currents within Earth’s mantle form as material near the core heats up.
Core
At the planet’s center lies a dense metallic core. Scientists know that the core is metal for a few reasons. The density of Earth’s surface layers is much less than the overall density of the planet. Scientists know that the outer core is liquid and the inner core is solid because S-waves stop at the inner core. The strong magnetic field is caused by convection in the liquid outer core.
Tectonics is the study of large scale movement and deformation of the earth’s outer layers. Plate tectonics relates such deformation to the existence and movement of rigid “plates” over a weaker, more plastic layer in the earth’s upper mantle.
Plate Tectonics—Underlying Concepts As already noted, a major obstacle to accepting the concept of continental drift was imagining solid continents moving over solid earth. However, the earth is not rigidly solid from the surface to the center of the core. In fact, a plastic zone lies relatively close to the surface. A thin shell of relatively rigid rock can move over this plastic layer below. The existence of plates, and the occurrence of earthquakes in them, reflect the way rocks respond to stress.
Stress and Strain in Geologic Materials
An object is under stress when force is being applied to it. The stress may be compressive, tending to squeeze or compress the object, or it may be tensile, tending to pull the object apart. A shearing stress is one that tends to cause different parts of the object to move in different directions across a plane or to slide past one another, as
when a deck of cards is spread out on a tabletop by a sideways sweep of the hand. Strain is deformation resulting from stress. It may be either temporary or permanent, depending on the amount and type of stress and on the physical properties of the material.
Lithosphere and Asthenosphere
The earth’s crust and uppermost mantle are somewhat brittle and elastic. Together they make up the outer solid layer of the earth called the lithosphere , from the Greek word lithos , meaning “rock.” The lithosphere varies in thickness from place to place on the earth. It is thinnest underneath the oceans, where it extends to a depth of about 50 kilometers. The lithosphere under the continents is both thicker on average than is oceanic lithosphere, and more variable in thickness, extending in places to about 250 kilometers. The layer below the lithosphere is the asthenosphere , which derives its name from the Greek word asthenias , meaning “without strength.” The asthenosphere extends to an average depth of about 300 kilometers in the mantle.
Locating Plate Boundaries The distribution of earthquakes and volcanic eruptions indicates that these phenomena are far from uniformly distributed over the earth. They are, for the most part, concentrated in belts or linear chains.
