Coast Ranges in Oregon and Washington
While Mt. Rainier poses considerable hazards to the inhabitants of Puget Sound, quite the opposite is true of another National Park mountain range less than 100 miles away. Sitting on a ferry crossing the Puget Sound on a clear, sunny day, you might never guess that the subduction zones miles beneath you has produced two very different kinds of mountains so close together. To the east the white hulk of Mt. Rainier looms over Tacoma. But to the west, the Olympic Mountains provide an equally stunning (especially at sunset) view and pose no volcanic hazard to the region whatsoever. How can this be?
To better understand how the Olympic mountains (the center of Olympic National Park) formed, we need to go back and review how subduction works. Review the Interactive Animation on Subduction Zones and the section in your textbook if you need to, then try to answer the following questions:
1) Why does basaltic ocean crust subduct under more andesitic or composite continental crust, but not the other way around?
2) What do you think happens to island arc volcanoes when they move far enough away from the active subduction zone? Do they still have magma rising underneath them?
To answer the second question and understand how the Olympic Mountains formed, Log in to BBLearn, go to Module 2, and then to Mastering Assignments. Complete "Assignment 2.3: Accreted Terrain." When you have completed the assignment, leave the Terrain Formation animation open and use it to help you think about the formation of the Olympics and answer the questions below?
1) Why does basaltic ocean crust subduct under more andesitic or composite continental crust, but not the other way around?
2) What do you think happens to island arc volcanoes when they move far enough away from the active subduction zone? Do they still have magma rising underneath them?
To answer the second question and understand how the Olympic Mountains formed, Log in to BBLearn, go to Module 2, and then to Mastering Assignments. Complete "Assignment 2.3: Accreted Terrain." When you have completed the assignment, leave the Terrain Formation animation open and use it to help you think about the formation of the Olympics and answer the questions below?
1) Based on what you saw in the animation, how do you think the Olympic Mountains formed?
2) What kind of plate boundary (what two types of crust and what type of collision or divergence) produced the original material for the Olympics?
3) Do you think you would be more likely to find basalt or andesite rocks in the Olympics? Why? (HINT: think about the original formation/parent material)
4) Based on the sequence of events that must have occurred, do you think the Olympics are older or younger than the Cascade volcanoes? Why?
If you are stumped, read this very short online article on the geologic development of Olympic National Park.
2) What kind of plate boundary (what two types of crust and what type of collision or divergence) produced the original material for the Olympics?
3) Do you think you would be more likely to find basalt or andesite rocks in the Olympics? Why? (HINT: think about the original formation/parent material)
4) Based on the sequence of events that must have occurred, do you think the Olympics are older or younger than the Cascade volcanoes? Why?
If you are stumped, read this very short online article on the geologic development of Olympic National Park.