Chapter 6 ROCKS
I Rock and the Rock Cycle
- rock – the material that makes up the solid part of Earth
- can be composed of one or more minerals
- can be made of solid organic material
- geologists study the forces and processes that form and
change rock
A. Three Major Types of Rock
1. igneous rock
a. igneous – Latin term means “from fire”
b. forms when magma (molten rock) cools and
hardens
c. lava – magma (molten rock) on Earth’s surface
2. sedimentary rock
a. agents of erosion
1a. wind
2a. water
3a. gravity
b. sediments – rocks, mineral crystals and organic
matter that have been broken down into
fragments
c. sedimentary rock is formed when sediments are
compressed or cemented and harden
3. metamorphic rock
a. metamorphic means “changed form”
b. forms from tremendous pressure, extreme heat
and chemical processes
B. The Rock Cycle
1. rock cycle – the series of processes in which rock
forms, changes from one type to another, is destroyed
and forms again by geological processes
2. three major types of rock can be changed into
another of the three types
3. cyclic change of rocks during the rock cycle
a. igneous rock is exposed to Earth’s surface
b. igneous rock is broken down into sediments
by a series of processes
c. sediments from igneous rock is compacted or
cemented together
d. sediments become sedimentary rock
e. sedimentary rock is exposed to intense heat and
pressure
f. sedimentary rock may be transformed into
metamorphic rock
g. under certain temperature and pressure
conditions, metamorphic rock can melt
h. the molten rock can cool and harden to form
igneous rock
4. a particular body of rock does not always pass
through each stage of the rock cycle (Fig. 2, page 126)
C. Properties of Rock
- all rock has physical and chemical properties
- properties are determined by how and where
the rocks formed
- physical characteristics of rock reflect the chemical
composition
- the way that minerals and rocks form is related to the
stability of the rock
1. Bowen’s Reaction Series
a. minerals crystallize at different rates as
magma cools
b. as certain minerals crystallize, the magma’s
composition changes
- Why?
- as minerals crystallize, they remove
specific elements from the magma
c. minerals generally form in the same order
- Why?
- melting/freezing points of a mineral is
the same
d. minerals form in two ways
1d. a gradual, continuous formation of
minerals that have similar chemical
compositions
2d. a sudden change in mineral types
3d. pattern of mineral formation depends on
the magma
2. Chemical Stability of Minerals
a. rate of mineral breakdown is dependent on
chemical stability
b. chemical stability – a measure of the tendency
of a chemical compound to maintain its original
chemical composition rather than break down to
form a different chemical
c. dependent on the strength of the chemical bonds
d. minerals that are most resistant to weathering are
the ones with the highest number of bonds
between the elements silicon and oxygen
3. Physical Stability of Rocks
a. rocks have natural zones of weakness
1a. sedimentary rock tends to break between
layers
2a. metamorphic rock tend to break in layers
where minerals align during metamorphism
3a. igneous rock
aa. joints – evenly spaced zones of weakness
in igneous rock that form as the rock
cools and contracts
bb. igneous rock will split at joints
b. weakness may also form when rocks are under
intense pressure
1b. when rocks are exposed to decreased
pressure, joints and fractures open
2b. these areas of weakness are exposed and the
processes of chemical and physical
weathering begin