tectonism, & volcanism. How did it form? First, to orient you, lets look at what is on the

surface. This lake which has cut down through the surrounding sedimentary bedrock is near

a volcanic dome that is surrounded by fallout deposits Now, let's remove the foreground

in a giant fault to expose the cross section. In general, young rock layers overlie older

rock layers. Gaps in the sequence, called unconformities are due to erosion, absence

of deposition, or faulting. We start with a layer of undeformed bedrock

on a tectonic plate that gets drown by a sediment-laden sea, both precipitating limestones and forming

sandstones near the shores. Marine or lacustrine sedimentation is the the process where particles

either precipitate out of solution or where rock, mineral and/or organic material break

down and accumulate as strata. The seas recede and the layers are subjected

them to regional compressional forces which deform the tectonic plate creating folds in

the bedrock. Over millions of years windblown sediment, as well as water- and gravity-transported

sands and gravels from eroding uplands blanket the area. Subsequent regional extension stretches

the plate until it drops along faults in basin-range fashion.

During extension, the thinning crust allows the slow rise of magma from the mantle. Lava

erupts to the surface, but is displaced during continued extension. Sediment accumulation

creates an angular unconformity over the faulted rock. Large lake covers the area and continued

sedimentation forms flat-lying bedding. Renewed extension results in more faulting. A long

period of sedimentation and erosion is accompanied by the slow rise of magma below the thinned

crust and an explosive volcanic eruption through a lake.

Quiet dome building caps the story.