Angela's Tools · Chapter 25 · Volcanic Eruptions and Earthquakes Worksheet 4 of 10

California's Restless Edge: Modeling Where Magma and Earthquakes Come From

Learning target: I can use diagrams of plate settings to explain why some places have volcanoes and earthquakes while others have only earthquakes, and draw a labeled model of a subduction zone.

Diagrams and modelsAbout 45 minIndividual MS-ESS3-2 · practiceMS-ESS2-3 · supports visual modelingindependent practicewriting
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Directions

Study Figure 1 first. It describes five places along California, from the north coast to the Salton Sea and inland to the eastern Sierra. Figures 2 and 3 show two processes as step-by-step models. Use all three figures to answer Part A. In Part B you will draw your own labeled model, then answer questions about it in Part C. Draw in pencil so that you can revise.

You will need: This worksheet, Pencil, Eraser, Colored pencils (optional)

Part A: Read the models

California's plate settings, north to south California's plate settings, north to south Cascadia subduction zone (north of Cape Mendocino) The small Gorda and Juan de Fuca plates sink eastward beneath the North American Plate. Water released by the sinking plate helps mantle rock melt, feeding volcanoes such as Mount Shasta and Lassen Peak. Earthquakes happen on the shallow locked boundary and deeper inside the sinking plate. The locked boundary last broke in a magnitude 9 earthquake in January 1700. Mendocino Triple Junction (Cape Mendocino) Three plates meet: the Pacific, the North American and the Gorda. Earthquakes are frequent, making this one of the most earthquake-active areas in California. San Andreas Fault system (Cape Mendocino to the Salton Sea) The Pacific Plate slides northwest past the North American Plate. No plate sinks, so little magma forms and there are no volcanoes along most of the fault. Earthquakes are shallow, mostly within the top 15 to 20 km of the crust. Salton Trough (southern end) The boundary changes into short segments where the crust is pulled apart, linked by transform faults, like the spreading centers farther south in the Gulf of California. The crust is thin and hot: there are small young volcanic domes, geothermal power plants and swarms of small earthquakes. Long Valley Caldera (eastern Sierra, inland) East of the Sierra Nevada, Earth's crust is being stretched. Long Valley Caldera near Mammoth Lakes formed in a giant eruption about 760,000 years ago and still has earthquake swarms and episodes of ground uplift.
Figure 1. Five plate settings in and near California. Descriptions are simplified; real plate boundaries are zones of many faults.
From sinking plate to volcano From sinking plate to volcano Ocean plate sinks at the trench Sinking plate heats up and releases water roughly 100 km down Water lets the hot mantle rock above melt at a lower temperature Some of that rock melts into magma Magma rises because it is less dense than the rock around it A chain of volcanoes forms inland from the trench pressure and heat rise water seeps upward melting point drops below the rock's temperature magma collects magma reaches the surface
Figure 2. How magma forms above a sinking plate. The mantle is mostly solid rock; it melts only where something changes its melting conditions.
Stick, build, slip Stick, build, slip years pass stress keeps building sudden slip shaking ends the cycle repeats Plates keep moving, but friction locks the fault Rock beside the fault bends and stores energy Stress grows larger than the friction Rock slips at the focus and seismic waves spread out Fault surfaces catch again
Figure 3. The stick-slip cycle on a fault. One trip around the loop can take decades to centuries; the slip itself lasts seconds to minutes.
1.According to Figure 1, which setting produces many earthquakes but almost no magma?2 pts
2.Use Figures 1 and 2 to explain why northern California has volcanoes like Mount Shasta, but the land along the San Andreas Fault does not.3 pts
3.In Figure 2, the mantle rock above the sinking plate melts even though it does not get hotter. Explain how that can happen.2 pts
4.In Figure 3, which step is the earthquake that people feel? What is stored during the long quiet part of the cycle, and why does the cycle start over?2 pts

Part B: Draw your own model

✎Draw a side view (cross-section) of the Cascadia subduction zone, with the Pacific Ocean on the left and inland northern California on the right. Label: the ocean plate, the North American Plate, the trench, an arrow showing which way the ocean plate moves, the mantle, the place where water is released and magma forms, magma rising, and a volcano. Mark at least six earthquake foci with an X: some on the shallow locked boundary near the coast and some deeper inside the sinking plate farther inland.

Part C: Use your model

5.Complete the table using your drawing.4 pts
Where on your drawing (near the coast or inland, shallow or deep)Why it happens there
Shallowest earthquakes
Deepest earthquakes
Magma forming
Volcano
6.Seismologists plot quake foci along a line from the Cascadia coast inland. The foci get deeper the farther inland they are. What does this pattern trace?2 pts
7.Which California setting in Figure 1 can produce the largest earthquakes? Use your model and Figure 1 to explain why.3 pts
8.The Hawaiian volcanoes sit more than 3,000 km from the nearest plate boundary. Which part of your model cannot explain them, and what does explain them?2 pts
9.Name two things your drawing leaves out or shows inaccurately, and say why a scientist would still find the model useful.2 pts

A drawing checklist

Start with two long slabs: a thin ocean plate on the left that bends down under a thicker North American Plate on the right. The trench is the low point where the bend begins. Put the volcano well inland, above where the sinking plate is deep. Draw magma as a thin path rising from just above the sinking plate. Place X marks along the top of the sinking plate and inside it: shallow near the trench, deeper toward the right.

A boundary that is moving north

The Mendocino Triple Junction is slowly moving north along the coast as the Pacific Plate slides past. Scientists think that as it passed, no sinking plate was left beneath parts of central California, one reason subduction volcanoes stopped forming there while the San Andreas grew longer. Draw two quick sketches of the California coast, one long ago and one today, showing where subduction and the transform boundary were. Explain how this helps account for the old volcanic rock found in parts of central California.

Angela's Tools · Chapter 25 · Volcanic Eruptions and EarthquakesWorksheet 4 of 10