This is presentation one of a two presentation series I have developed for the College Park Scholars, Life Sciences program in Alaska. To learn about Alaska during the last ice age, follow this link.


What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.

Geologists usually think of rocks in a second important way, however. Please memorize this and recite it like a mantra:

* A rock is a record of the environment in which it formed. *

Rock types:

Additional information

Plate Tectonics

In the 19th century, people expected that the topography and composition of the ocean's floor should resemble that on land. The first practical test of that hypothesis occurred in 1872 when the British government sponsored the first interdisciplinary research expidition to expore the world's oceans - the four-year voyage of the H. M. S. Challenger. The deep oceans defied expecataions:

Clearly the geology of the oceans was unlike that of the continents. WTF?

Answering that question took about eighty years. Many people contributed to the answer, but two major figures were:

At the beginning of the sixties, he led a group of colleagues who finally put the pieces together: Wegener had been right to say that the continents moved, but for the wrong reasons. Hess's view is the foundation of the theory of Plate Tectonics, which has become the unifying theory of modern Geology. Between 1960 and 1970, the academic community was won over to it. Here is its essence:

Layers of the upper Earth:

Plate Tectonics in Alaska

Only two large plates, the North American and the Pacific, are responsible for the action in Alaska:

Plate motion has one other consequence: In the past, small bits of continental crust (like today's New Zealand or Seychelles Islands) have been rafted northward on the Pacific Plate. They are too light to subduct. Instead, they have steadily been added onto Alaska's southern margin. (Indeed, some day Southern California will come to rest there.)

Why volcanoes?

One way to get rock to melt is to lower its melting point by adding water or some other volatile substance. The Pacific plate has been sitting under the Pacific ocean for hundreds of millions of years before it dives into the trench. Thus, it is very wet. When it reaches a critical depth, the water it carries percolates into surround hot mantle rock, causing it to melt. The magma rises and you get volcanoes.

Moreover, you get a particular kind of volcano: One that erupts a viscous magma with a relatively high silica content (Rhyolitic to andesitic, if you must know) and with lots of water vapor in solution. Thus, it tends to erupt explosively;

More like this:

than like this:
What force wears them down?


In Alaska, the same agents of erosion that operate in the lower 48 are at work, but there is another we need to account for:


Definition of a glacier:

Ice mechanics: The ice crystals that people interact with typically are brittle. When confining pressure is great, however, they deform ductilely, like rocks in the asthenosphere. The weight of 40 m. of overlying ice is usually enough to cause ice to deform ductilely. As a result, accumulations of ice thicker than this tend to flow downhill under their own weight.

Big concept: Landmarks like the toe of the glacier may move uphill or downhill depending on climate conditions, but any given parcel of ice always moves downslope.

Valley Glaciers: Rivers of ice flowing down hill follow the same path that a stream would follow, in a drainage that is roughly same scale as a stream drainage.

Valley glacier profile:

Crevasse: When glacier turns a corner, or flows over a topographic irregularity, the brittle upper 40 m of ice breaks in deep extensional gashes called a crevasses.

Processes of Glacial erosion:

Erosional features:

Erosional land forms that form from valley glaciers: These are what we think of as the typical features of regions like the Alps and high mountains of western North America. They may be occupied by active glaciers or may be left over when the glacier melts.

Glacial sedimentation: Glaciers carry copious amounts of unsorted sediment. When the ice melts, the sediment may be: