The Earth’s Crust

Earthquakes Any vibrating, shaking, or rapid motion of the Earth’s crust. Most occur when stress builds along a zone of weakness or a break in the rock known as a fault. When the crust shifts, energy is released. The energy radiates in all directions through vibrations.

Earthquakes The place underground where the break occurs is the focus of the earthquake. The epicenter is the location at the Earth’s surface just above the focus.

Measuring Earthquakes Magnitude (Richter Scale) Seismographs are the most reliable measures of earthquakes. Each increase in one unit of magnitude means a ten-fold increase in shaking. Intensity (Mercalli Scale) Based upon the reports of people who experienced the earthquake and observed the destruction.

Mercalli vs. Richter Scale

Seismic Waves P-Waves Primary (they arrive first), Pressure, or Push-Pull. Material expands and contracts and particles move back and forth in the path of the wave. Sound waves that travel through solids, liquids or gases.

Seismic Waves S-Waves Secondary (arrive later), Shear, or Side-to-side. Material shears out of shape and snaps back. Travels only through solids.

Seismic Waves Surface Waves Travel along the earth's surface. The slowest waves but the ones that damage in large earthquakes.

An Earthquake’s Epicenter Seismologists Scientists who study earthquakes Use the difference in the speeds of P and S-Waves from three seismic recording stations to locate the epicenter

Earthquake Origin Time To find the origin time, a seismologist needs to know the arrival and travel time of the P-waves.

The Earth’s Layers The Crust Varies from 5-60 KM In most places, a thin layer of sedimentary rocks covers the mostly granite-like rocks of the continental crust The oceanic crust, under layers of marine sediments, is composed of darker and denser rocks similar to basalt

The Earth’s Layers The Mantle Extends to a depth of about 2900 KM. Earthquake waves travel faster in the mantle than they do in the crust. Composed mostly of dense, the dark mafic minerals olivine and pyroxene.

The Earth’s Layers The Core Composed of iron and nickel. Outer Core is thought to be liquid because S-waves are unable to pass through the outer core. The Inner Core seems to be solid.

The Earth’s Layers

Earthquake Shadow Zones When a major earthquake occurs, both P-waves and S-waves are received over most of the earth. The opposite side of the earth will receive P-waves but no S-waves. S-waves cannot penetrate the liquid outer core. There is also a region where neither P-waves or S-waves are received. Refraction (bending) of the waves at the mantle-core boundary causes this ring-shaped region known as the shadow zone.

Earthquake P-wave Shadow Zones

Earthquake P-wave Shadow Zones A P-wave traveling through the outer core is labeled K. A bounce off the core is labeled c. A P-wave in the inner core is I. S-waves do not pass through the core, because the outer core is fluid.

The “Ring of Fire” A large number of the world’s volcanoes and seismic events occur around the edges of the Pacific Ocean. Japan, the western coast of the United States are on the Ring of Fire. These areas are damaged frequently by earthquakes and volcanoes.

Seismic Hazards Earthquakes can cause damage by shaking, movement of the crust, or large waves in oceans, called tsunamis. Seismic Risk Level Map for the U.S. Probability of damage in 100 years. Blue = none green = minor yellow = moderate red = major

Volcanic Hazards When volcanoes erupt they may spew hot lava, hot ash, and/or toxic gases. The lava and ash can bury cities, and the toxic fumes can suffocate people. Volcanoes can also provide fertile soil that is composed of weathered volcanic material.

Continental Drift In 1912, Alfred Wegener, proposed that in the distant past, Earth’s continents were all joined as a single landmass. He said that the continents have separated and collided as they have moved over Earth’s surface for millions of years.

Continental Drift Wegener proposed that if the land areas were brought back together, the move would line up ancient mountain ranges, similar continental rock formations and evidence of ancient glaciers. There are even similar fossils on both sides of the Atlantic that would be brought back together by the re-assembly of Pangaea.

Continental Drift

Evidence from the Oceans In the 1950’s both fossils and the analysis of radioactive material showed the age of the oceanic crust increases with distance from the mid-ocean ridges. Some oceans were growing wider from the middle. Scientists also used magnetic measurements of the oceanic crust.

Plate Tectonics The surface of Earth is composed of about a dozen major rigid, moving crustal plates and several smaller plates. These plates contain areas of light continental rock and dense oceanic bottoms.

Plate Boundaries The lines along which plates meet and interact. Convergent boundaries occur from converging plates. As a result of this collision mountains rise as the crust crumples.

Plate Boundaries Subduction occurs when a dense oceanic plate dives beneath a lighter continental plate. Subduction forms ocean trenches that are linear fractures and are the deepest parts of the oceans.

Plate Boundaries When a plate slides past another plate, they meet at a transform boundary. San Andreas Fault in CA

Plate Boundaries A divergent boundary (rift) is found at the mid-ocean ridges where upwelling material creates new crust that moves away from the ridge in both directions.

What Moves the Plates? Convection currents within Earth enable heat to escape from the Earth’s interior. These currents create and expand the ocean bottoms, and they carry the continents as “rafts” of lighter rock.

Hot Spots In several places on earth, hot plumes of magma pierce the crust. As a crustal plate moves over this source of magma, volcanoes form at the hot spot. This movement of a plate leads to the formation of a chain of volcanoes of differing ages. One example is the Hawaiian Islands.