Rock Types and the Rock Cycle

There are three basic rock types. This classification is based on how they are formed:

Igneous
Sedimentary
Metamorphic

Igneous Rocks

Formed directly from cooling magma (mainly), but may also be grown from aqueous solutions at or near a magma chamber
- pegmatite has large crystals because, in water, the mineral ions are able to quickly find and crystallize to similar types
Composition of magma varies and this effects the composition of the rock
- High silica content = very viscous magma (honey) = granite
- Low silica content = low viscosity magma (melted butter) = basalt
Rate of Cooling effects size of the crystals formed
- Slow cooling in Intrusive (plutonic) rocks (big crystals) = granite
- Faster cooling in Extrusive rocks (smaller crystals) = rhyolite
- Very fast cooling = no crystals (glass) = obsidian

Magma also has gasses in it (usually at great pressure)
- 1- 5 % of total weight
- They provide the pressure to help the magma force itself to the surface
  - mostly water vapor
  - carbon dioxide
  - some nitrogen
  - some sulfur
- These gasses were the source of the earth's early atmosphere
- Gasses explode violently out of viscous magma
  - Mount St. Helens in 1980 (Washington State)
- Gasses quickly bubble (gently) out of magma with low viscosity
  - Kilauea (Hawaii)
- When gasses escape magma, it is called lava
Examples are:
- Granite (intrusive ... big crystals ... cooled very slowly)
- Rhyolite (extrusive ... smaller crystals ... cooled quicker)
- Obsidian (glassy texture .... cooled very quickly)
  - all these rocks (above) have the same chemical composition but formed under different conditions.
- Basalt (oceanic crust and moon rocks)
It is possible to "date" igneous rocks using radiometric techniques (radioactivity).

Sedimentary Rock

Formed when loose sediments get buried (and compacted) and then lithified (turned to rock) by minerals (usually precipitating out of water) cementing the particles together.
Usually formed in a marine environment (bog, river bed, lake, sea, ocean)
Identified by strata (layers)

Strata in the Morrison Formation
Only rock type containing fossils which gives us clues about earlier life forms.
Some sedimentary rocks have economic value - coal (as a fossil fuel) and limestone (building industry)
Although only 5% of the crust, it represents 75% of all rocks we can see at the surface. This tells us that this rock type forms at or very close to the earth's surface.
- If these rocks were formed under water, what changed (on the earth) so we see them now all around us (on dry land) ????

Common Examples:
- Sandstone from sand
- Shale (most common sedimentary rock) from silt/clay (mud)
- Conglomerate from gravel
- Coal from fern trees
- Limestone from marine animals (coral, snails, clams, etc.)
  - Dolomite is a sedimentary rock and may be considered an altered form of limestone (but slightly harder and denser), the carbonate fraction being, calcium magnesium carbonate [CaMg(CO₃)₂]. Dolomite formations are widespread and are notable in the region of the Alps called the Dolomites, where the rock was first studied. The south - eastern bedrock of Wisconsin is mainly Silurian dolomite (laid down 417 - 443 million years ago in shallow seas... when plants first emerged on land and fishes dominated the seas).
- Salt (halite) from evaporation
Type of particles (in sedimentary rock) give clues to the environment in which it formed
- Shale = stagnant water (bog)
- Conglomerate = rushing water (river bed)
- Sandstone = beach
Difficult to "date" sedimentary rocks using radiometric techniques

Click here to see all kinds of sedimentary rocks.

Metamorphic Rock

A rock that is converted (by heat, pressure or chemicals) to another type of rock.
Usually the rock is subjected to conditions much different from which it originally formed.
Parent rock may be any type.
Examples are:
- Granite to Gneiss
- Shale to Slate
- Limestone to Marble
- Sandstone to Quartzite
- Graphite (Coal) to Diamond
Difficult to "date" metamorphic rocks using radiometric techniques.

Click here to see some images of metamorphic rocks.

The Rock Cycle

Study the diagram below. You will see that nothing lasts forever on this earth (not even rocks). The rock cycle shows us that given enough time, rocks form, they break up, they reform, they get transformed. This is hard for us to imagine because a human lifetime is almost nothing to a rock.

This chart shows us that nothing stays the same
Rocks can change from one type to another in many different ways
This is a very slow cycle (taking millions or billions of years)
The earth must be very old!