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Rocks of South Dakota
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Rock Groups
History of a rock (group) depends upon one main
thing

• How it formed

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Hot Rocks(from magma or lava)

• Harney Peak is formed from Precambrian granite
  that cooled 1.7 billion years ago.
• It formed 8 12 miles underground, pushing up
  7,500 feet of sedimentary rock that were
  gradually eroded to expose this granite.

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Hanging out with Granite-Man
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What are these hot rocks called?What determines
crystal size?

• Intrusive Rocks
• These form from magma deep underground
• Granite
• Gabbro
• They may form dikes and sills, laccoliths and
  batholiths in between existing rock layers.

• Crystal size
• This varies due to differences in rates of
  cooling
• Large crystals slow cooling of magma or lava
• Small crystals rapid cooling of magma or lava

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• Bear Butte is a laccolith located NE of Sturgis,
  SD. It, along with Devils Tower in Wyoming, is
  considered to be part of the Black Hills uplift (
  between 58 and 50 million years ago), and is made
  of magma that never reached the surface to erupt.
  

• After the magma cooled and solidified, erosion
  stripped away the overlying layers of rock.

• Bear Butte rises 1,253 feet (382 m) above the
  surrounding plain and its elevation above sea
  level is 4,422 feet (1,348 m).

• It is considered to be a sacred site by the
  Cheyenne and Lakota Sioux, and is maintained as
  part of the state park system.

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Erosion Over Time
This erosion process at Devils Tower is similar
to what happened to Bear Butte and to the many
dikes which today are exposed at Sylvan Lake and
Needles Highway.
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Beautiful Sylvan Lake
Needles Highway
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Extrusive Rocks

• These form from lava that has been pushed out of
  the ground (extruded). These types of rocks cool
  rapidly so crystal size is small or
  insignificant.
• Basalt fine grained dark rock
• Pumice light colored, airy, spongy rock
• Obsidian dark colored, glassy rock

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Sedimentary Rocks
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These rock layers formed from sediments that were
deposited in large bodies of water.
South Dakota was covered many times by inland
seas over hundreds of millions of years. This was
long before the Black Hills uplift occurred.

• Sandstone formed from beach sands deposited along
  the shorelines of these seas. Colors are pink,
  red, tan, or brown.

• Shales formed from mud and clay deposited in
  quiet ,still water. Colors can be red, pink,
  gray, black and even purple.

• Limestone rock layers formed in fairly calm
  waters as a result of chemical deposits and/or
  shells of sea animals. Colors are usually light
  gray, pink or tan.

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Opeche shale in Custer State Park.
Limestone rock of the Madison formation forms
tall cliffs.
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Metamorphic Rock Changed

• These schists are highly folded and the main
  crystals we see are of mica.
• What could cause such changes to solid rock like
  sandstone, shale, granite and limestone?

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PRESSURE from overlying rock layers or from plate
tectonics.
HEAT from magma or lava that bakes and changes
the surrounding rock layers.
This is a granite intrusion that has been exposed
in a roadcut. On both sides of it are the gray,
flakey schists that were changed by the heat of
the molten magma.
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