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.