Earth to Class

Key ideas --Earthquakes, Volcanoes, and Plate Tectonics

Earthquakes

Earthquakes occur when sections of the crust, and sometimes mantle, suddenly shift past each other.

Most earthquakes are associated with movements at the boundaries of tectonic plates. Some are not associated with any known plate boundaries.

The exact location where movement occurs is the focus. The spot on the surface above the epicenter is the epicenter.

There are 3 types of faults:
      Normal faults: Rocks move past each other and retain their origin relationship.
     Reverse faults: One rock layer moves over a layer that was originally above it.
      Strike-slip faults: Rock layers slide horizontally past each another. These occur at transform plate boundaries. The most familiar example is California's San Andrea Fault.

Earthquakes create seismic waves that spread in all directions through the Earth.
     P-waves— Primary or compressional waves travel fastest, and can travel through liquids and solids.
     S-waves— Secondary waves or tensional waves have side to side movement, are slower, and can not travel through liquids.
     L-waves— Surface waves moving with an up-and-down movement are last wave to arrive, but produce most of the damage to buildings and other structures.

Earthquakes are detected with seismometers. The record is called a seismograph.

To locate an earthquake epicenter, you need the difference in P- and S-wave arrival times from three seismic stations, and then draw circles around the stations representing the distance from the point of origin. The epicenter is where the three circles intersect.

P- and S-wave travel time curves are found on ESRT p. 10.

Volcanoes

Volcanoes occur when molten rock rises and erupts on the surface.

Magma is molten rock beneath the surface, and lava is molten rock at the surface.

There are three basic types of volcanoes: Cinder cones, made of ash and particles; shield cones, with a relatively wide base compared with its height (including the Hawaiian Islands); and composite cones, with alternating layers of ash and lava.

Most volcanoes occur at tectonic plate boundaries, although some occur at hot spots within a plate.

Volcanoes and earthquakes occur in the same zones. The most famous is the Circumpacific Ring of Fire.

Plate Tectonics

Plate Tectonics is the theory that Earth's crust consists of relatively thin and hard lithospheric plates moving on top of softer material in the asthenosphere of the upper mantle.

The plate tectonics theory grew out of the Continental Drift theory proposed by Alfred Wegener. He tried to explain that the continents were once together and drifted apart through the oceans. Evidence he cited includes the fact that the shapes of South America and Africa seem to fit together like a jigsaw puzzle. He also pointed out that South America and Africa have identical fossils, rock beds, glacial records, mineral deposits, mountain ranges. Wegener was not believed because he could not explain what might drive such movements.

Convection currents in the mantle are the commonly cited explanation now for what causes tectonic plates to move. (However, the real cause is more complicated, although still density-driven.

Mid-ocean ridges are areas where new crust is being created as tectonic plates move apart at a divergent plate boundary or spreading center.

Solidified basalt moving away from the mid-ocean ridge form the oceanic crust. Compared with continental crust, this crust is thinner, denser, and generally younger.

Subduction occurs where an oceanic tectonic plate sinks under a continental plate into the Earth’s mantle. Common features at such convergent plate boundaries are deep-sea trenches and volcano arcs. Japan and the Philippines are examples of such island arc-trench systems.

In locations within the ocean plates, horizontal movements produce transform plate boundaries.

The rate of seafloor spreading can be measured using magnetic striping patterns preserved in the ocean basalts. It has been found that the youngest crust in the oceans is near the mid-ocean ridges, and becomes oldest nearer the continents. These patterns results from reversals of Earth’s magnetic poles through geologic history.

The oldest rocks in the ocean date back to the Permian Period about 270 million years ago. But this means that all ocean is much younger than much of the continents. The reason is simple: Although new ocean materials are forming at the mid-ocean ridges, the oceans are being destroyed as they sink at subductions. Volcanic mountains associated with such trenches, such as the Andes, become the basis for expanded continents.

Additional Concepts

Faults much be younger (have occurred later) than the rocks in which they appear.

Rock layers that are undisturbed have oldest layers beneath younger layers, but faulting can reverse this sequence.

Intrusions of igneous rock are younger than the rocks into which they have intruded.