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
II Igneous Rock
igneous rock – rock that forms when magma cools and solidifies
most can be identified as crystalline (made of crystals)
chemical composition and texture determine the identity of the igneous rock
A. Formation of Magma
1. three factors that affect melting
a. temperature – determined by the chemical composition of the minerals in the rock
b. pressure – when excess pressure is removed from rock that is at a temperature above the melting point
c. presence of fluids – addition of fluids generally decreases the melting point of certain minerals
2. Partial Melting
a. different minerals have different melting points
b. minerals that have lower melting points are the first to melt
c. partial melting – process by which different minerals in rock melt at different temperatures
3. Fractional Crystallization
a. cooling process (freezing) is the reverse of the process of partial melting
b. minerals that have the highest freezing points crystallize first
c. fractional crystallization – the crystallization and removal of different minerals from the cooling magma
1c. minerals that form first tend to settle at the bottom of the magma chamber or stick to the ceiling and walls of the magma chamber
2c. crystals that form early are commonly the largest
- Why? - they have the longest to grow
B. Textures of Igneous Rock
- classified according to where magma cools and hardens
- intrusive igneous rock – rock formed from the cooling and solidification of magma beneath Earth’s surface
- extrusive igneous rock – rock the forms from the cooling and solidification of lava at Earth’s surface
- texture of igneous rock is determined by the size of the crystals in the rock
1. Coarse-Grained Texture
a. intrusive igneous rocks commonly have large mineral crystals
b. slow loss of heat allows the minerals to form large, well-developed grains
c. coarse-grained texture – igneous rock that are composed of large mineral grains (ex. granite)
2. Fine-Grained Texture
a. many extrusive igneous rocks are composed of small mineral grains that cannot be seen by the unaided eye
b. magma (lava) cools rapidly
c. large crystals are unable to form
d. fine-grained texture – igneous rocks that are composed of small crystals (ex. basalt and rhyolite)
3. Other Igneous Rock Textures
a. porphyritic texture – igneous rock that has a mixture of large and small crystals
b. glass texture – highly viscous magma cools quickly that contains a very small percentage of dissolved gases (obsidian)
c. vesicular texture
1c. igneous rock that contains vesicles
2c. vesicles – holes that are formed in igneous rock from highly viscous magma that has a high percentage of dissolved gases that
become trapped as bubbles in rock
C. Composition of Igneous Rock
- the mineral composition is determined by the chemical composition of the lava/magma
1. Felsic Rock
a. felsic – describes magma or igneous rock that is rich in feldspars and silica and is generally light in color
b. commonly also contains plagioclase feldspar, biotite mica and muscovite mica
c. examples include granite, rhyolite, obsidian and pumice
2. Mafic Rock
a. mafic – magma or igneous rock that is rich in magnesium and iron and that is generally darker in color
b. main mineral components are plagioclase feldspar and pyroxene minerals
c. ferromagnesian minerals – minerals that are rich in iron (Fe) and magnesium (Mg)
d. examples include basalt and gabbro
3. Intermediate Rocks
a. made up of minerals plagioclase feldspar, hornblende, pyroxene and biotite mica
b. contain lower portions of silica than felsic
c. contain higher portions of silica than mafic
d. examples include diorite and andesite
D. Intrusive Igneous Rock Structures
- intrusions – rock masses that form underground
- forms when magma intrudes or enters into other rock masses and then cools deep inside Earth’s crust
1. Batholiths and Stocks
a. batholith
1a. largest of all intrusions
2a. spread over 100 km2
3a. means “deep rock”
4a. form the cores of many mountain ranges, such as the Sierra Nevadas
b. stock – similar to batholiths but cover areas less than 100 km2
2. Laccoliths
a. pushes overlying rocks into a dome
b. means “lake of rock”
c. commonly occur in groups
d. ex. Black Hills of South Dakota
3. Sills and Dikes
a. sill
1a. magma flows between layers of rock and hardens
2a. lie parallel to the layers of rock that surrounds it
3a. vary in thickness from a few centimeters to hundreds of meters
b. dike
1b. forms when magma follows existing vertical fractures or creates new ones and solidifies
2b. cut across rock layers rather than lying parallel
3b. common in areas of volcanic activity
E. Extrusive Igneous Rock Structures
1. rock masses that form on Earth’s surface
2. volcano – a vent or fissure in Earth’s surface through which magma and gases are expelled
3. volcanic neck – solidified central vent of a volcano
4. lava flow – flat masses of rock
5. lava plateau – series of lava flows that cover a vast area with thick rock
6. tuff – volcanic ash deposits that can be hundred of meters thick and cover areas of several hundred km
III SEDIMENTARY ROCK
- sediment – loose fragments of rock, minerals and organic material
- most sedimentary rock is made up of combinations of different types of sediment
- characteristics are determined by the source of the sediment, the way the sediment was moved and the conditions under which the sediment was deposited
A. Formation of Sedimentary Rocks
1. source of the sediment determines the sediment’s composition
2. Two main processes convert sediments into sedimentary rock
a. compaction – the process in which the volume and porosity of the sediment is decreased by the weight of the overlying sediments as a result of
burial beneath other sediments
b. cementation – the process in which minerals precipitate into pore spaces between sediment grains and bind sediments together to form rock
3. Classifying Sedimentary Rock
a. process by which the rock forms
b. composition of the rock
4. Three main classes of sedimentary rock
a. chemical
b. organic
c. clastic
b. these classes contain their own classification based on shape, size and composition
B. Chemical Sedimentary Rock
1. sedimentary rock that forms when minerals precipitate from a solution or settle from a suspension
2. evaporation
a. minerals dissolved in water remain when the water evaporates
b. concentration of minerals in the water remaining becomes high enough to cause minerals to precipitate out
c. evaporites – rocks that form from the minerals left behind
3. examples include gypsum and halite
C. Organic Sedimentary Rock
1. sedimentary rock the forms from the remains of plants and animals
2. organic limestone
a. marine organisms remove chemicals from sea water
b. shells are made from calcite and aragonite
c. shells eventually become limestone
3. chalk is an example
D. Clastic Sedimentary Rock
1. sedimentary rock that forms when fragments of preexisting rocks are compacted or cemented
2. classified by size of sediments
a. conglomerate – rock composed of rounded fragments that range in size from fine mud to boulders
b. breccia – rock composed of fragments that are angular and have sharp corners
c. sandstone – sedimentary rock that is composed of sand-size grains
d. shale – sedimentary rock that is composed of clay-size particles
E. Characteristic of Clastic Sediments
- physical characteristics are determined by the way the sediments were transported
- four major agents of transportation
- water
- ice
- wind
- gravity
- speed of transport affects size and shape of sediments
- distance traveled affects size and shape of sediments
1. Sorting
a. tendency for currents of air or water to separate sediments according to size
b. well-sorted – all grains are roughly the same size
c. poorly-sorted – grains consists of many different sizes
d. sorting is a result of changes in the speed of the agent that is moving the sediment
1d. large grains deposited first
2d. fine grains stay suspended longer
3d. fine grains are deposited farther from shore or on top of coarser sediments
2. Angularity
a. particles collide with each other and other objects as they are transported
b. collisions can cause a change in size and shape
c. the first break tend to be angular and uneven
d. the longer the distance traveled, the more rounded and smooth the sediment becomes
e. the farther the sediment travels, the smoother the sediment becomes
F. Sedimentary Rock Features
- deposition environment – setting in which the sediment is deposited
- rivers, deltas, beaches and oceans
1. Stratification
a. layering of sedimentary rock
b. occurs when the conditions of deposition changes
1b. change in sediment type
2b. change in depositional environment
c. beds
1c. stratified layers
2c. vary in thickness
aa. length of time sediment is deposited
bb. how much sediment is deposited
d. massive beds
1d. beds that have no internal structures
2d. similar sediment is deposited for long periods
3d. large amounts of sediments are deposited at one time
2. Cross-Beds and Graded Bedding
a. cross-beds
1a. slanting layers of sedimentary rock
2a. generally form in sand dunes or rivers
b. graded bedding
1b. various sizes and kinds of materials are deposited within one layer
2b. occurs when different sizes and shapes settle at different levels
3b. largest grains at bottom
4b. smallest grains at top
5b. reverse grading – smallest grains on bottom and largest grains on top
3. ripple marks
a. caused by the action of wind or water on sand
b. sediment was once part of a beach or river bed
4. mud cracks
a. form when muddy deposits dry and shrink
b. flood plain or dry lake bed are common places
5. fossils and concretions
a. fossils – the remains or traces of ancient plants
and animals
b. concretions
1b. lumps of rock that have a composition that is different from the main rock body
2b. form when minerals precipitate from fluids and build up around a nucleus
3b. geode – minerals that crystallize inside cavities to from a special type of rock
IV. METAMORPHIC ROCK
- metamorphism – the process in which one type of rock changes into metamorphic rock because of chemical processes or changes in temperature and pressure
A. Formation of Metamorphic Rock
1. heat, pressure and hot fluids cause some minerals to change into other minerals
2. minerals may change in size and shape
3. minerals may separate into parallel bands
4. type of rock may indicate conditions the rock formed under
5. Contact Metamorphism
a. a change in the structure or chemical composition of a rock due to contact with magma
b. only a small volume of rock that surrounds the magma is changed
6. Regional Metamorphism
a. a change in the structure or chemical composition of a rock due to changes in temperature and pressure over a large area, generally as a result
of tectonic forces
b. tremendous heat and pressure at the edge of rocks during tectonic activity
c. heat and pressure cause chemical changes in the rock
d. most metamorphic rock is formed this way
B. Classification of Metamorphic Rock
1. foliated rocks
a. foliation – the metamorphic rock texture in which mineral grains are arranged in planes and bands
b. slate, schist and gneiss
2. Non-foliated Rocks
a. non-foliated – the metamorphic rock texture in which mineral grains are not arranged in planes or bands
b. quartzite, marble, granite