Review questions and answers for final exam--Geology of Oregon and the Pacific NW

1) Although the Cascadia subduction zone has produced no major earthquakes in historic times, it seems capable of generating a magnitude 9 earthquake sometime in the future.

a) What is the evidence for large earthquakes on this zone in pre-historic times? b) One explanation for the lack of seismicity is that the zone is locked? What does it mean for the zone to be locked--and why might it be locked?

answer: evidence for large earthquakes: drowned tree stumps in surf zone up and down coast, concurrent turbidite deposits up and down coast.

being locked means that the zone is stuck-- it accumulates strain energy rather than slipping (like stretching a rubber band more and more). It is locked probably because the subducting Juan de Fuca plate is so young (the spreading center is right there)--therefore, it is still warm and buoyant and so has a more difficult time sliding down the subduction zone.

2) Interpret this cross section. --see handout.

a) Label the footwall (FW) of fault ³a².
-------> the footwall lies beneath the fault--so in this case, it's on the right side.
b) What type of fault is at ³a²?
--------> normal (the footwall side went up relative to the hangingwall side.)
c) Did fault ³a² form by extension or compression?
-------> extension
d) Approximately when did fault ³a² slip?
-------> after 4.2 Ma and before 6.85 Ka.

e) Assuming that this fault is somewhere in central Oregon, what geologic event likely produced the layer at ³b²?
-----> eruption of Mt. Mazama.

3) What is magnetic declination. How is it relevant to our understanding of the Blue Mountains?
-----> magnetic declination is the angle between true and magnetic north. We can determine the orientation of ancient magnetism in rocks--and from this we can see if the rocks pointed towards the true magnetic north of the time. If it does not, then the rocks must have rotated. From paleomagnetism, we can tell that the Blue Mtns have rotated up to about 65° clockwise since the end of the Mesozoic.

4) How are Cretaceous plutons in Oregon relevant to our understanding of terrane accretion?
-----> they intrude them and so give us an age by which the terranes had to have been accreted.

5) This stratigraphic column shows the Pleistocene sediments in the Puget Sound area. see handout.

a) What does it tell us about the glacial history of the region?
-----> it records the movement of the continental ice sheet into the area. The Ft. Lawton Clay formed in lakes (first the area had a bunch of lakes). The Esperance Sand formed as outwash (as the glacier moved southwards, its outwash filled the lakes with sand and the place became covered by rivers). The Vashon Till was deposited right off the glacier (arrival of the ice).

b) Using the stratigraphic section, explain why landsliding is such a common event in Seattle.
-----> water seeps through the permeable Vashon Till and Esperance Sand and pools at the top of the impermeable Ft. Lawton Clay. That encourages the overlying material to slide off.

6) What are the two main competing hypotheses for the origin of the Columbia River Basalt? Also, name at least one problem with each interpretation.

-----> Back arc spreading and Yellowstone Hot Spot.
Problems with Back arc spreading are that the basalts erupted to the east of the true back arc. Also, the area of basalt distribution is square rather than elongate.
Problem with Yellowstone Hot Spot--the track, from Yellowstone to central Oregon, would have to make a huge bend, indicative of a major change in plate motion direction. However, there is no corroborating evidence of this change in plate motion at about 7 Ma.

7) What are the main components of an ophiolite?
Ophiolites consist of the oceanic lithosphere---so, they're made of peridotite, gabbro, sheeted dikes, pillow basalt, deep sea sediments.

8) Below is a map of Oregon, Washington and southern British Columbia--locate and answer these questions about the following features.

--for locations, consult the larger version of the map which you got in class as a handout.

Important notice--On many of the maps, the area of Columbia River Basalt looks a lot darker than it does on the legend. The CRB's area is the big squarish area that straddles the OR-WA state line and follows the Columbia River to the Ocean. Note that it is distinctly lighter gray in shading than the "Eocene basalt".

a) Columbia River Basalt (CRB) What is its approximate age?
-----> 17.5 to 6 Ma. Most of the eruptions were at about 16 Ma.

b) Accreted Eocene Basalt of the Coast Ranges (Teb). approximately where is the youngest exposure of Teb?
-----> about where the Columbia empties into the ocean.

c) Cretaceous intrusive rock (Ki). Is it younger or older than the Triassic?
-----> younger.

d) Rocks of the Cascade Volcanoes. (Cascades) What is the dominant rock type?
-----> basalt.

e) Wrangellia. (W)

f) Straight Creek fault (SCF) Draw arrows to show its sense of slip.
-----> right-lateral.

g) Coast Plutonic Complex What is its age?
-----> mostly Cretaceous.

h) A region underlain primarily by sandstone (ss)
-----> Coast Ranges.

i) the Basin and Range Province. What is the dominant fault type in this region?
-----> normal.

j) The Olympic Mountains (Oly).
-----> ya ya ya.

See you on Thursday! Good luck getting ready and afterwards, have a great summer.