Idaho Batholiths
While western Washington and Oregon are famous for their large, snow-capped volcanoes, Idaho's mountains are much less distinct, in part because they were formed by different geologic processes. So how did Idaho's mountains form?
Review the subduction zone animation from the previous page. In the animation, composite volcanoes form when magma rises to the surface and produces a series of eruptions. But not all "bubbles" of magma make it to the surface. What does the animation say happens to the magma that doesn't make it the surface? What does it form?
Another animation shows this process in more detail. Log in to BBLearn, go to Module 2, selecte the Study Area, and then Interactive Animations. Watch the animation called 'Igneous Features' and answer the following questions:
1) What are three examples of intrusive igneous features seen on the landscape after volcanic activity has ended and magma has cooled?
2) What intrusive igneous feature forms the core of the Sierra Nevada mountain range in California?
Review the subduction zone animation from the previous page. In the animation, composite volcanoes form when magma rises to the surface and produces a series of eruptions. But not all "bubbles" of magma make it to the surface. What does the animation say happens to the magma that doesn't make it the surface? What does it form?
Another animation shows this process in more detail. Log in to BBLearn, go to Module 2, selecte the Study Area, and then Interactive Animations. Watch the animation called 'Igneous Features' and answer the following questions:
1) What are three examples of intrusive igneous features seen on the landscape after volcanic activity has ended and magma has cooled?
2) What intrusive igneous feature forms the core of the Sierra Nevada mountain range in California?
The picture above was taken on the Banks-Lowman Road along the Payette River northeast of Boise (see map at left). Both the erosive power of the Payette River and the construction of the road along the side of the very steep canyon exposed miles of the underlying geology along the road.
Notice that while most of the rocks is light-colored, there is a dark, linear feature running vertically through the right side of the image. Based on what you learned in the animation, what do you think this dark feature is?
Notice that while most of the rocks is light-colored, there is a dark, linear feature running vertically through the right side of the image. Based on what you learned in the animation, what do you think this dark feature is?
While the Sierra Nevada mountain range in California is the more famous national example of a batholith, this is primarily because the granite that the batholith is comprised of is very exposed, especially at places like Yosemite National Park. In contrast, the mountains of Idaho are mostly covered with vegetation, particularly thick forests, making it difficult to see the underlying intrusive igneous features except where there are road cuts or other surface disturbances.
The Idaho batholith forms much of the spine of Idaho's mountains, particularly in the most northern and central parts of the state. The most southern mountainous areas, such as those that border Nevada and Utah, are part of the Basin and Range province -- which we'll cover in a later module. The Idaho batholith plutons were formed (or "intruded") approximately 50-100 million years ago, making them much older than the Cascade mountains (the oldest volcanoes there are only about 2 million years old).
One of the best places in Idaho to see the granitic plutons of this batholith is near McCall, where the 'sandy' beaches of Payette Lake are actually billions of eroded granitic crystals!
One of the best places in Idaho to see the granitic plutons of this batholith is near McCall, where the 'sandy' beaches of Payette Lake are actually billions of eroded granitic crystals!
Log into BBLearn, go to Module 2, and select the Mastering Assignments icon. Complete "Assignment 2.2: Igneous Features."