Reviewing Ch. 3: The Dynamic Crust

The causes of earthquakes: Stress builds up along a zone of weakness (fault). A break occurs at a focus (a point on the fault). The first shakings are felt at the epicenter – the point on the surface of the Earth just above the focus.

Measuring quakes: Intensity = the Mercalli scale – based on reports by people who were there! Magnitude = the Richter scale – uses a seismometer and the distance to the epicenter to get a value from 1 – 10. (each number is 10x greater in magnitude than the number below!)

Earthquake Waves: P-waves: arrive FIRST, move FASTER and can go through SOLID and LIQUID. S-waves: arrive SECOND, move SLOWER and can only go through SOLIDS. You can find out how far a seismograph station is from the earthquake epicenter by measuring the TIME DIFFERENCE in the arrival of both the P and S-waves.

Finding Epicenter Distance: Subtract P and S-wave arrival times. Use the edge of a piece of paper and p. 11 in the ESRTs. Mark off the time difference on the edge of your paper using the “travel time” scale on the y-axis. Slide the paper up the graph until the separation between the P and S lines matches the time span.

The Origin Time: The farther you are from the epicenter, the longer it takes the 1st P wave to arrive AND the longer the wait until the first S-wave arrives. You can find out when the quake actually began by subtracting the travel time for the P-wave at your distance from the epicenter (must find distance first!) FROM the arrival time of your first P-wave.

Earth’s Layers: Crust: Less dense than the rest. Continental = least dense of all! Ocean = a little denser. Mantle: Denser than the crust. Earthquake waves move faster here because it’s denser! Core: Separated into “outer” and “inner.” The inner core is the densest! Probably made of iron and nickel! (Like meteorites!) All this is on p. 10 in the ESRTs.

Shadow Zones: Since the P and S-waves bend as they go through material of different densities AND because the S-waves can’t go through liquids (Outer Core!) there is a “shadow zone” where no waves can be detected. See p. 62 in rev. bk.

Quakes and Volcanoes: Quakes, volcanoes and mountains all tend to be found near each other – in specific zones or belts. See p. 5 in the ESRTs for a look at the tectonic plates.

Evidence for Continental Drift: Outlines of continents Similar rocks, fossils and mountain ranges on widely separated continents. Ocean floor evidence – at the rifts (ridges) – like the Mid-Atlantic Ridge. New ocean floor at the opening, old (and cool) material far away. Magnetic reversals locked into rock. (see p. 67)

Types of Boundaries: Convergent (come together) – often cause trenches (ocean-continent) or high mountains (cont. to cont.) Divergent (move apart) – often cause ridges or rifts. Transform (slide past) – at the San Andreas fault and often along many ocean fault zone. SEE P. 5 in the ESRTs!!!!

What drives the plate movement? Convection currents in the mantle make heat rise, separate (leading to rifts) and cold areas sink (leading to trenches). Other interesting things… Hot spots are in random locations in the mantle. These lead to volcanic features that pop up on the crust and continue to pop up as the crust moves over the “Hot Spot.” (like Hawaii!)