Lecture #3: Rocks
(Abbott, pp. 44-46, 190-200, 229-234)
Underlined terms have internet links.
What are Rocks?
are assemblages of two or more minerals (Abbott, p. 497) or other
Some rocks mainly contain organic compounds (
Rocks are divided into three major types:
solidify from molten rocks.
They may be divided into
or volcanic) and
(igneous rocks that solidify deep within the Earth).
refers to molten rock deep within the Earth.
If the melt reaches the Earth’s surface, it’s
Intrusive (also called
) rocks are generally coarser grained (that is,
the minerals are larger) than volcanic (extrusive) rocks, since intrusive rocks cool more
slowly deep within the Earth and their minerals have more time to grow larger.
contrast, lavas may come into contact with surface water and cool rapidly (quench) to
form volcanic glass (Figure 8.19, p. 197).
Cool air will also quickly solidify lava and
produce a lot of glass. Because plutonic rocks are deep (kilometers) within the Earth, they
are insolated from cool surface water and air, and do not contain glass.
Intrusive (Plutonic) and Extrusive (Volcanic) Igneous Rocks:
Sediments (like sand, gravel, and mud) form from water (including ice) and wind
breaking down rocks at the Earth's surface.
Sediments may accumulate in rivers, lakes,
glaciers, deserts, seas, and oceans. Sediments may also consist of minerals that have
precipitated from water (such as calcium carbonate, CaCO
, or sodium chloride, NaCl).
Once sediments are buried, they may be compacted and transformed into
at temperatures of about 40-200
C and depths as great as 8 kilometers or more.
are examples of sedimentary rocks.
common around Lexington and mostly consist of the mineral
They formed from the burial of ancient sea sediments.