Rocks and Minerals
Rocks are composed of minerals. Minerals are naturally occurring chemicals that can be
represented by a chemical formula. Minerals are to rocks what flour and sugar are to a cake. Rocks
generally contain more than one mineral although some rocks contain only one mineral. Granite is
an example of a rock. I cannot write a chemical formula for granite as it is a mixture of several
minerals including feldspar and quartz.
Examples of Minerals:
Feldspars (Si3O8)- white, gray or pink in color; forms elongated crystals in igneous rocks
Quartz (SiO2)- colorless when pure, white or glassy appearance. Impurities may cause the
color to be pink, yellow or violet. Most sand is composed mostly of grains of quartz.
Quartz is found in granite and many other rocks.
Calcite (CaCO3)- colorless or white, glassy, reacts with weak acids. Common in
sedimentary rocks.
Pyrite (FeS2)- “fools gold”, forms cubic crystals
Additional Resources
http://www.windows.ucar.edu/tour/link=/earth/geology/min_intro.html
Three Types of Rocks
1. Igneous Rocks
Igneous rocks form from molten rocks (magma or lava). (Molten rock under the surface of
the Earth is called magma. If it reaches the surface as in a volcanic eruption it is called lava.)
Another way to think about this is that for igneous rocks to form, rock has to melt then cool. No
fossils are found in igneous rocks as they would be destroyed when the original rock melts.
Clues regarding the origin of igneous rocks:
Color- dark color suggests the rock formed from magma deeper within the Earth and it
probably contains more Fe, Mg…. (heavier elements) An example of a dark colored igneous rock
is basalt. A light color suggests it formed from magma closer to the surface and contains more Si,
O and other lighter elements. An example is granite.
Crystal Size- Crystals take time to grow. Rocks with large crystals cooled slowly, deeper
within the Earth surrounded by more insulating rock. Rocks with small crystals cooled more
rapidly.
Intrusive Igneous Rocks- these igneous rocks form when magma cools and solidifies
within the Earth. Because they solidify underground, they are only visible if erosion removes the
layers of soil and rock originally lying over it. When rocks cool slowly, larger crystals have time
to form. If the rocks cool quickly crystals will be smaller. Granite is the most common intrusive
igneous rock.
Extrusive Igneous Rocks- forms when lava cools on the surface of the Earth. Extrusive
rocks cool quickly so the crystals are small. Sometimes the crystals are too small to be seen without
a microscope. Obsidian cools so quickly the rocks are as smooth as glass. Basalt is the most
common extrusive igneous rock.
2. Sedimentary rock
Sedimentary rock forms when layers of sediment harden. The sediments may consist of
clay, sand or gravel, plant or animal remains, or chemicals that settle out of solution on the bottom
of lakes or the ocean. Weathering breaks down existing rocks into smaller pieces. The fragments
are transported by water and wind through the process of erosion. As water or wind deposits this
material in a new location it may eventually form sedimentary rock.
Examples of sedimentary rock include sandstone, shale, limestone and coal. Coal is not
formed from rock fragments but from the remains of plants that died millions of years ago and were
trapped under layers of water and sediments. Heat, pressure and low oxygen levels cause the
vegetation to change over millions of years into coal. The sites of coal deposits were once ancient
swamps.
Fossils may be found in sedimentary rocks.
3. Metamorphic Rocks
Metamorphic rocks form when heat, pressure or chemicals change existing rocks.
Metamorphic rocks often resemble the rocks from which they form. Sandstone is a sedimentary
rock which is converted into quartzite, a metamorphic rock. Limestone (a sedimentary rock) is
converted into marble (a metamorphic rock). Shale is a sedimentary rock that is converted by heat
and pressure into slate, a metamorphic rock. Slate can then be converted by more heat and pressure
into schist, then into gneiss (pronounced “nice”).
Other Resources
http://www.windows.ucar.edu/tour/link=/earth/geology/rocks_intro.html
Review Questions
1.
If you were searching for fossils, which of the three types of rocks would you check?
2.
Which type of rock forms from existing rocks that are changed by heat or pressure?
3.
Which type of rock forms from rock that has melted?
4.
Which type of rock forms from particles or pieces of rock that are cemented together?
5.
What is the difference between rocks and minerals?
6.
Classify each of the following as either a rock or a mineral.
_____________feldspar
_____________quartz
_____________granite
_____________sandstone
_____________pyrite
____________basalt
___________NaCl
7.
What is the difference between an extrusive igneous rock and an intrusive igneous rock?
8.
If you find an igneous rock with large crystals, what does that tell you about how quickly it
cooled?
What might cause it to cool at this rate?
Classify each of the following as igneous, sedimentary or metamorphic rocks.
9.
granite ________________
10.
slate___________________
11.
marble_________________
12.
sandstone_______________
13.
basalt____________________
14.
What does the color of an igneous rock tell you?