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  • This hypothetical cross section represents a hundred million of years of sedimentation,

  • 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.

This hypothetical cross section represents a hundred million of years of sedimentation,

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