Chapter 4 Rocks Notes
Classifying rocks
How do Geologists classify rocks?
a. When studying a rock sample, geologists observe the
rock’s texture,
color and determine its mineral
composition.
What is Texture?
a. Texture – the look and feel of the rock’s surface
b. Most rocks are made up of grains and other materials.
c. A rock’s grains give it its texture.
How do Geologists describe a rock’s texture?
a. Terms:
i. Coarse grains are large and easy to see.
ii. Fine grains-small, need microscope to be seen.
iii. No visible grain-glassy.
How Does Grain Size Help Identify a Rock?
a. Geologists identify rocks by coarse grains, fine
grains, and no visible
grain.
How Does Grain Shape Help Identify a Rock?
a. Different rocks have different grain shapes– some are
crystals, while
others are fragments of other
rocks.
How Does Grain Pattern Help Identify a Rock?
a. The grains in a rock form patterns; some are flat
layers and others are
swirling patterns.
What Does It Mean to Have No Visible Grain?
a. Some rocks cool so quickly they have no crystal
grain.
b. Ex. obsidian
What do geologists do if they can’t determine a rock by its
texture?
a. They look at the mineral’s composition under a
microscope.
i. A small sample of rock can show shape and size of
the crystals.
b. They use mineral tests also; –scratch test, acid
test, or a magnet
What Are the Major Groups of Rocks
a. Igneous.
“ignite-fire”
b. Sedimentary
c. Metamorphic
“made up of sediments”
“morph, to change”
How Are Igneous Rocks Formed?
a. Igneous – forms from the eruption of molten rock –
either intrusive,
below the surface or extrusive
at the surface.
How Are Sedimentary Rocks Formed?
a. Sedimentary – forms when small, solid pieces of other
rocks or the remains of plants and animals are pressed and
cemented together
How Are Metamorphic Rocks Formed?
a. Metamorphic – formed when an existing rock is changed
by heat or
pressure, or chemical reactions
b. Most of these rocks are formed underground.
Igneous Rocks
● Igneous rocks come from the cooling of melted rock material, called
magma.
● Intrusive igneous rocks form when the magma cools slowly under the
Earth’s surface. Intrusive igneous rocks have large crystals.
● Extrusive igneous rocks form when the magma cools quickly on the
Earth’s surface. Extrusive igneous rocks have very small crystals.
Characteristics of Igneous Rocks
● Most igneous rocks are very hard
● Made of crystals
● Layering is not common in igneous rocks
Sedimentary Rocks
● Sediment is produced through the process of weathering. Large rocks
are broken down very gradually and the smaller pieces are transported by wind,
water, and ice.
● Under high temperature and pressure, the sediment becomes
compacted (“smashed” together) and cemented (hardened) into sedimentary
rock.
● Usually this happens as the sediment is buried more and more deeply
under layers of new sediment.
Characteristics of Sedimentary Rocks
● Often contain layers
● Look for grains in the rock
i. Coarse grain-large crystals
ii. Fine grain-small crystals.
iii. No grain- glassy
Three types of Sedimentary Rock:
a. clastic rock- sedimentary rocks that are squeezed together.
(ex. shale, sandstone)
b. organic rock- remains of plants or animals. (ex. coal)
c. chemical rock- minerals that dissolve in solution and crystallize.
(ex.-limestone)
Metamorphic Rocks
● Sedimentary and igneous rocks can become buried deep in the Earth’s
crust.
● Metamorphic rocks form when the buried rock is changed under high
temperature and pressure.
● The rock does not melt. New minerals grow and the rock
remains solid.
Characteristics of Metamorphic Rocks
● Layering is common
● Different layers are made up of different minerals
● Often made of plates and flaky layers
Rocks from Reefs What is a Coral Reef?
 A structure of calcite skeletons built up by coral animals in warm, shallow
ocean water
 Geologists can determine that limestone deposits that began as coral reefs
can be found on continents in areas that had warm climates and shallow
ocean water.
The Rock Cycle
● The rock cycle is the combination of all the processes that act to break
down rocks, move sedimentary rocks from place to place, and make new rocks.
● The processes of the rock cycle are very gradual and happens over times
that range from thousands to millions of years or longer.