Isostacy: the crust is essentially floating on the mantle, less dense than mantle. Think of
the crust floating on the mantle the same way blocks of wood or ice cubes float on the
water. Isostatic equilibrium.
The more crust sticks above the surface, the more it sticks down below. We have
indirect evidence that mountain chains have roots that extend down into the mantle.
Use gravitational anomalies to tell what rocks there are. Rocks that cause gravitational
anomaly, heavier. Basalt has higher density than average continental crust like granite.
Massive. Basalt erupting into rift zone, lava can erupt. If mantle beneath mountain range
is displaced by low-density crust. Balanced by low-density crust that replaces higher-
Uplifted mountain range, maximum elevation
If things are eroded evenly off the top, will uplift
But things don’t erode evenly. Streams, glacial valleys can make peaks even higher.
Examples of mountain uplift: Sevier Orogeny (ca. 140-50 Ma)
Laramide Orogeny (ca .75-45 Ma)
Basin and Range Extension (ca. 20-0 Ma)
FDR FUN (footwall moved down reverse fault, footwall moved up normal fault)
When cross beds form around ripples, (left to right), sand rolls up, avalanches down
other side. Top of most ripples erode away, cross beds in front of ripple. more and more
horizontal, top of crossbeds is truncated, more perpendicular
Monocline – only one fold
Faster plate tectonics, different styles of deformation
Basin and Range orogeny – majority of structures are normal faults.
Graben: Valley surrounded by footwalls.
Sevier orogeny: Oldest orogeny that happened in North America, ca. 140-50 Ma
Laramide orogeny, ca. 75-45 Ma
Thrust sheet (tobacco Root Mtns, Montana)
Anticline, reverse faults, synclines, all show compression.
Principle of superposition only works if anticlines don’t turn over (older rock is on
Monocline is from Greybull, WY
Layers of mountains
Stacking thin sheets of sedimentary rock on themselves.
Difference between Sevier and Laramide mountains:
Sevier: Thrust faults stacked on each other
Laramide: Reverse faults, pushes up rock, thick skin of sedimentary rock on surface
granite of Washington/Oregon during Sevier orogeny happened in Idaho during the
Basin/Range orogeny: Much younger than the Sevier and Laramide orogenies.
Evaporite mineral: Halite, Gypsum.
Stages of continental rifting
Death Valley – Basin and Range Orogeny
Alluvial fan size difference in Death Valley – why is there a difference?
Every time you build fans on the east side of the valley, they truncate along the
Clastic sedimentary / Chemical sedimentary