Question

GEOLOGY QUESTIONS

6. The volcanic mountains of the Cascades are composed primarily of andesite. Andesite is an intermediate igneous extrusive rock. Why are the Cascades volcanoes composed of andesite rather than basalt?

7. The article describes how the accretion of the Siletzia terrane around 60 million years ago “.. managed to bring everything to a grinding halt. It literally stopped the subduction and shut off the arc!” What does the author mean when she says “shut off the arc”?

9. Most of the fossils recovered in Washington State are not older than about 55 million years. Why do we not find older fossils in this region?

Answer 1

6)Cascade moutain is located in western margin of United states of America.

Western margin of USA is active plate boundary where Pacific plate and America plate is present.

But there is a micro plate known as Juan de fuca oceanic plate is present which is subducting below continental American plate.

The broad composition of oceanic plate is basaltic and continental plate is granitic.At the position of arc ,these two contrasting magma series mixes to form intermediate composition andesitic composition. This has been shown in diagram.

7)Here shut off the arc mean mean due to accretion of siiletzia accretion, the subduction zone in north western margin of America plate has been stopped.

It shifted the subduction zone and close  of Laramide orogeny which was uplifting the Rocky mountain..

The origin of it little bit controversial.

8)Most of fossil in Washington state are not older than about 55 million years because of contemporary magmatism.

Magma bodies that cooled and solidified underground spread throughout the older terrain and volcanics covered wide area.

The magma centre shifted towards western with the subduction.

Except this above siltez crescent accretion on the western side of the state particular this time.

Usually igneous rock doesn't contain fossil because it wipe out the signature of fossil as it forms from igneous melt.