Part 1: Earth’s Dynamic Interior Unit 5: Plate Tectonics Part 1: Earth’s Dynamic Interior

Earth’s Structure and Composition The structure and composition of Earth’s interior can be determined through observations and sampling. Scientists have synthesized the results of investigations to develop models of Earth’s interior. The two basic models are the structural model and the composition model. Earth’s compositional layers: crust, mantle, and core-are distinguished by their chemical compositions, the minerals and rock they are made of. Earth’s structural layers: lithosphere, mesosphere, asthenosphere, outer core, and inner core-are distinguished by physical properties such as temperature, physical state, and whether the layers flow or behave rigidly.

Earth’s Structure and Composition

Earth’s Structure and Composition The model shown above is simple, and assumes that each layer is homogeneous, or the same throughout, with clear-cut boundaries. Earth’s interior is much more complicated. The exact composition varies from place to place and some boundaries are blurry and uneven. Like many scientific models, as scientists gather more evidence and information through direct observation and measurement, models can be refined, updated, and changed based on the new information.

Outer Layers The outermost compositional layer of the Earth is called the crust. The crust along with part of the mantle below make up Earth’s outermost structural layer called the lithosphere. Our understanding of the lithosphere is based on evidence from rock and rock formations, remote sensing of subsurface features, and laboratory experiments. Scientists can examine the rocks, mineral crystals, and gases that result from volcanic eruptions to gather evidence about the chemical composition and physical properties of the crust and mantle below.

Outer Layers

Outer Layers Data from earthquakes can be used to infer physical properties and boundaries within the lithosphere. Seismic waves-waves caused by earthquakes-reflect off boundaries and refract (change speed and direction) when they move from one material to another. A zone relatively near Earth’s surface where seismic waves refract indicates a transition between two layers. This zone marks the boundary between the crust and mantle.

Seismic Data When an earthquake occurs, rocks bend, break, and snap back suddenly. This causes waves of energy called seismic waves to move out in all directions. Two types of seismic waves move through Earth’s interior: P-Waves and S-Waves. The behavior of the waves depends on the properties of the medium that the waves travel through. Seismic waves typically move faster through denser, more rigid materials. They also reflect off different layers.

Seismic Data Scientists analyze seismic waves-how they change speed and direction-to infer the density and composition of rocks, thickness of rock layers, and the physical state of layers.

Middle Layers Earth’s density is about 5.5 g/cm3. The crust has a density of less than 3.0 g/cm3. This tells us that there must be materials beneath Earth’s crust, and those materials are denser than Earth’s crust. Most of the materials-83% of Earth’s volume-is found in the middle compositional layer called the mantle. The mantle is denser than the crust because it is made of a higher proportion of heavier elements and because it is compressed by the crust above. The mantle can be divided into three structural layers based on their physical properties. The part of the mantle just above the core is called the mesosphere.

Middle Layers

Middle Layers Evidence from rocks about the mantle is gathered from inferences made about rocks we can examine on the surface, geophysical measurements, and laboratory experiments. Most of our understanding of Earth’s structure (whether layers are solid or liquid, how hot they are, and where the boundaries are) come from analyzing data from earthquakes. Meteorites from space provide information about Earth’s composition as the materials in both have similarities. Laboratory experiments, such as testing the melting points of rocks, can give more insight into Earth’s interiors.

Inner Layers There is no direct evidence for the composition and physical properties of the core. The core includes Earth’s innermost layers. Seismic data shows there are two internal layers: an inner core and outer core. The inner core is solid and the outer core is liquid. The composition of meteorites supports the idea that there is a large amount of iron in Earth and that it is concentrated in the core. Earth’s magnetic field supports the claim that Earth has an iron-rich core. Earth’s magnetic field can change over time.

Earth’s Dynamic Interior We know from direct observations that Earth has a magnetic field. Think: Compass? Measurements show that the strength of the field and the location of the poles changes slightly from year to year. Earth’s magnetic poles reverse every few hundred thousand years. Because minerals lose their magnetism at high temperatures, it is not possible that a permanent magnet exists deep within Earth. Temporary magnetic fields can change in strength and orientation over time, depending on changes in the current. Geologists have inferred that Earth’s changing magnetic field is from the motion of liquid iron in the outer core.

Earth’s Dynamic Interior

Earth’s Dynamic Interior The flow of the solid rock in the mantle is driven by differences in density, which are caused by differences in temperature. Cool, dense materials sink and hot, less dense materials rise. The motion continues as the cooler materials re-heat and the hot materials cool. Evidence from features on Earth’s surface indicate this motion occurs in convection cells. Convection occurs in the mantle at a rate of a few centimeters per year.

Earth’s Dynamic Interior

The Lithosphere The lithosphere is cold and rigid and does not experience convection in the same way the mantle below does. It does move slowly and change over time, partly because of mantle convection. Many features we see on Earth’s surface and the processes we witness are evidence of this motion. Earth’s processes are not only shaped by processes such as weathering and erosion that are driven by solar energy, but also by processes that are driven by Earth’s internal energy.