Download Chapter 6 Whole Notes

Name __________________________________________ Row_________ Period _________
Chapter 6 - Rocks
Lesson 6.1 -Three Major Types of Rock
On the earth's surface, there are three types of rocks, metamorphic, sedimentary, and igneous.
Igneous Rock
Igneous rock is rock formed by the hardening of molten material that originates deep within the earth.
The word "igneous" comes from the Greek word for fire; some people call igneous rock a fire ball rock.
Lava rock beach where Waipio Valley meets the Pacific Ocean
Sedimentary Rock
The forces of weather break rocks into small pieces that are carried away and deposited elsewhere.
These small pieces are often deposited in shallow seas or lakes as sediments.
As the layers of deposits pile up, pressure from the weight of the sediments above, perhaps over millions of
years, turns the lower layers into solid rock.
Sand may turn into sandstone; silt and clay become shale. Such rock, made of sediment, is called
sedimentary rock.
Metamorphic Rock
Metamorphic rock is rock that has been altered by heat or by heat and pressure.
Metamorphic means to change of form.
Heat and pressure can change the forms of many things, for example, a glassmaker uses heat to change a
certain kind of sand into glass.
Metamorphic rocks are sedimentary or igneous rocks that have been modified or changed in form, that is,
the size, shape and arrangement of the minerals in rocks, by heat or pressure.
There are two major agents to metamorphism.
Through contact as magma intrudes or forces its way into existing rock.
Or through Regional metamorphism and the mechanisms associated with mountain building.
The Rock Cycle
The rock cycle refers to the series of processes in which rock forms, changes from one form to another, is
destroyed, and forms again by geological processes.
All rocks do not have to go through the complete cycle.
Properties of Rock
All rocks have physical and chemical properties that are determined by how and where the rock formed.
The rate at which rock weathers and breaks apart are determined by the chemical stability of the minerals in
the rock.
Bowens Reaction Series
Early 1900's, N. L. Bowen and others began experimental studies into the order of crystallization of the
common silicate minerals from a magma.
Bowen determined that specific minerals form at specific temperatures as a magma cools.
The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being
calcium-rich to more sodium-rich.
The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and
biotite mica.
As magma cools, certain minerals tend to crystallize first.
In this graph magma high temperature and low silica produces olivine, but if that same magma was allowed
to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the
series (in this case pyroxene).
At lower temperatures, the branches merge and we obtain the minerals common to the felsic rocks- feldspar,
mica, and quartz.
Section 6.2 Physical Stability of Rocks
Rocks have natural zones of weakness that are determined by how and where the rocks form.
When rock that formed under intense pressure is uplifted to Earth’s surface, decreased pressure allows the
joints or fractures to open.
Same is true for bake goods – for example a muffin.
Once these weaknesses are exposed to air and water, the processes of chemical and mechanical
weathering begin.
Igneous Rock
Igneous rocks begin as hot, fluid material that erupted at the Earth's surface as lava, or as magma at
shallow depths, or in deep bodies (plutons).
3 Types of Igneous Rocks
Rock that formed from lava on the surface of Earth is called extrusive igneous rock.
Rock that formed from shallow magma beneath Earth’s surface is called intrusive igneous rock.
Rock that formed deep beneath Earth is called plutonic.
You can tell the three types of igneous rocks apart by their texture, starting with the size of the mineral
grains.
Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains.
Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains.
Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles.
Igneous Rock
Because they solidified from a fluid state, igneous rocks tend to have a uniform fabric of mineral grains
packed together tightly.
Think of the texture of a piece of bread as a similar example.
Igneous Rock Types- Basalt and Granite
In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. – (Like chocolate chips
in a cookie)
Other extrusive rocks have distinctive textures like glassy obsidian which formed when lava hardens quickly.
Igneous rocks are classified by the minerals they contain.
The two best-known igneous rock types are basalt and granite, which differ in composition.
Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions.
Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock.
So basalt is mafic and either extrusive or intrusive.
Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion.
It is rich in feldspar and quartz (silica) and hence is called a felsic rock.
So granite is felsic and plutonic.
Where Igneous Rock Forms
Igneous rocks form in three main places:
(1) where lithospheric plates pull apart at mid-ocean ridges.
(2) where plates come together at subduction zones.
(3) where continental crust is pushed together.
Where Igneous Rocks are Found
The oceanic crust is made of basaltic rocks.
Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along
the edges of continents.
Continental magmas tend to be less basaltic and more granitic.
The continents are the exclusive home of granitic rocks.
Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and
eventually reach granite.
In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher
than the oceanic crust on top of the Earth's mantle.
Lesson 6.3- Types of Igneous Intrusive and Extrusive Rocks
Intrusions form underground.
Intrusions form when magma intrudes, or enters, into other rock masses and then cools deep inside Earth’s
crust.
Batholiths
Batholiths (Like Half Dome –in Yosemite) are intrusive formations that spread over at least 60 square miles
when they are exposed on Earth’s surface.
Another example is the Sierra Nevada Mountains.
More than 100 million years ago, granite formed deep underground California.
The range started to uplift 4 million years ago, and erosion by glaciers exposed the granite and formed the
light-colored mountains and cliffs that make up the range.
Laccoliths
A Laccolith (including Devil’s Tower in Wyoming) is an intrusive formation that forms when magma has
been injected between two layers of sedimentary rock and pushes the overlying rock layers into a dome.
Like a pimple on our face
Devils Tower- Wyoming
This is not a volcano but an igneous intrusion. Magma welled up into the surrounding sedimentary
rock. There it cooled and hardened. The sedimentary rock has since eroded away to show the tower.
Sills
When magma flows between layers of rock and hardens to form a body of rock that is parallel to the layers
of rock that surround it, a sill forms.
Geological Dike
When magma flows cut across the existing layers of rock perpendicular to the layers of rock that surround it
is called a geological dike.
Difference Between and Geological Dike and a Sill
The difference between a sill and a dike is that dikes cut across layers rather than lying parallel to the rock
layers.
Pluton
A pluton is a body of intrusive igneous rock that formed deep beneath Earth’s surface.
In practice, "pluton" usually refers to a distinctive mass of igneous rock like: batholiths, dikes, sills,
laccoliths, and other igneous bodies.
Extrusive Igneous Rock Structures
Igneous rock masses that form on Earth’s surface are called extrusions.
Volcanic cones and volcanic necks are common examples of extrusive igneous structures.
Lava flows, lava plateaus, and tuff layers are other common extrusions.
Tuff layers are a layer of solidified volcanic ash.
Lesson 6.4- Formation of Sedimentary Rocks
Sedimentary rocks are formed by the accumulation of sediments.
Most sedimentary rock is made up of combinations of loose fragments of rock, minerals, and organic
materials.
Two main processes convert loose sediment into sedimentary rock
(1) Compaction &
(2) Cementation
Compaction
Compaction occurs when a build of sediment pushes most of the air and water out and particles of sediment
closer together.
Like dog piling on a friend.
Cementation
Cementation is the next step in the recipe for making sedimentary rocks.
Loose sediments are glued together by natural glues like calcite and silica.
Classification of Sedimentary Rocks
Geologists classify sedimentary rocks by the processes by which the rocks form and by the composition of
the rocks.
There are three main classes of sedimentary rocks—chemical, organic, and clastic.
Chemical Sedimentary Rocks
3 Major types of chemical rocks (1) evaporates (2) chemical limestone (3) and tufa.
As the name implies, chemical sedimentary rocks form due to chemical reactions.
Atoms that are naturally attracted to each other form ionic and covalent bonds as they crystallize into solid
mineral crystals through precipitation.
Chemical Sedimentary Rocks
Precipitation can occur within an ocean, a lake, a cave, or where a hot spring flows out onto Earth's
surface.
Minerals formed due to precipitation can be quite useful; for example:
Halite is the salt that is used on french fries,
Gypsum is the main ingredient in plaster.
As minerals precipitate from water, they tend to grow attached to previously formed crystals.
The resulting rock is composed of intergrown mineral crystals, usually all of the same composition.
Organic Sedimentary Rocks
Organic sedimentary rock forms from the remains of plants or animals.
Coal and some limestone’s are examples of organic rocks.
Limestone
Organic limestone forms when marine organisms, such as clams and oysters remove the chemical
components of the minerals calcite from sea water.
The organisms make their shells from these minerals, and when the organisms die, their shells settle to the
bottom of the ocean.
Over time they accumulate, and compact to form limestone.
This process is also a part of the long-term carbon cycle.
Fossils
Almost all fossils are discovered in sedimentary rock.
Fossils provide clues to past geologic events, climates and the evolution of living things over time.
Lesson 6.5- Clastic Sedimentary Rock
Clastic sedimentary rock is rock that forms when fragments of pre-existing rocks are compacted or
cemented together.
Clastic sedimentary rocks are classified by the size of the sediments they contain.
Conglomerate Sedimentary Rock
Rock that contains large, rounded pieces is called conglomerate.
Breccia Sedimentary Rock
Rock that contains large, angular pieces is called breccia.
Sandstone Sedimentary Rock
Rock that is composed of sand-sized grains is called sandstone.
Shale Sedimentary Rock
Rock that is composed of clay-sized particles is called shale.
The Four Main Agents of Transportation
Sediments are transported by four main agents:
(1) water,
(2) ice,
(3) wind,
(4) the effects of gravity.
Physical Characteristics of Sediments
In general, both the distance the sediment is moved and the agent that moves the sediment determines the
physical characteristics of that sediment.
Sorting
The tendency for currents of air or water to separate sediments according to size is called sorting.
In well-sorted sediments, all of the grains are roughly the same size and shape.
Poorly sorted sediment consists of grains that are many different sizes (empty space).
The sorting of a sediment is determined by the speed of the agent that is moving the sediment.
Angularity
When particles first break from the source rock, they tend to be angular and uneven.
Particles that have moved long distances from the source tend to be more rounded and smooth.
Poorly Sorted Sediments
The way a rock becomes sorted can also tell us a lot about the agent that moved it.
If the agent is fast (river) or strong (glacier) it will produce poorly sorted sediment
Stratification
Layering of sedimentary rock is called stratification.
Stratified layers, also called beds, vary in thickness and composition and occurs when the conditions of
sediment deposition change.
Cross-Beds
Cross-beds, or slanting layers within rock strata, commonly form in sand dunes or river beds.
Graded Bedding
Graded bedding is a feature in which various sizes and kinds of materials are deposited in one layer, with
the largest grains at the bottom and smallest grains at the top.
Lesson 6.6- Formation of Metamorphic Rocks
Metamorphism the process in which one type of rock changes into metamorphic rock because of chemical
processes or changes in temperature and pressure.
During metamorphism, heat, pressure, and hot fluids cause some minerals to change into other minerals.
Minerals within the rock may also change in size or shape, or they may separate into parallel bands that
give the rock a layered appearance.
Hot fluids may circulate through the rock and change the mineral composition of the rock by dissolving
some materials and by adding others.
Composition of Metamorphic Rock
The type of rock that forms because of metamorphism can indicate the conditions under which the original
rock changed.
The composition of the rock, the amount of pressure, and the presence or absence of certain fluids will
cause different combinations of minerals to form.
2 Types of Metamorphic Rock
Two types of metamorphism occur in Earth’s crust—
(1) Contact metamorphism &
(2) Regional metamorphism.
Contact Metamorphism
Contact metamorphism means the rock has changed in texture, structure, or chemical composition due to
contact with magma.
Regional Metamorphism
Regional metamorphism means the rock has changed texture, structure, or chemical composition due to
temperature and pressure changes over a large area, generally due to tectonic forces.
Foliated Rock
Foliation the metamorphic rock texture in which minerals grains are arranged in bands or stripes.
Extreme pressure may cause the mineral crystals in the rock to realign or re-grow to form parallel bands.
Non-Foliated Rock
Non-foliated rocks are metamorphic rocks that do not have minerals grains arranged in planes or bands.
Many non-foliated metamorphic rocks contain grains of only one mineral or contain very small amounts of
other minerals, therefore the rock does not form bands of different minerals.
Other non-foliated metamorphic rocks contain grains that are round or square.
These grains are unlikely to change shape or position when exposed to directed pressure.
Non-foliated metamorphic rocks include the common rocks marble and quartzite.