Download Unit 2 Chapter 6 - McGann

Unit 2
Chapter 6
Rocks
Section 1 Rocks and the Rock Cycle
Three Major Types of Rocks
1. Igneous - Formed from the cooling and hardening
(solidifying) of magma and lava from within the earth.
2. Sedimentary Formed by the hardening of layers of
sediment
Types of sediments
1. rock fragments
2. plant and animal remains
3. chemicals (water)
3. Metamorphic - Rocks that are changed into new kinds of
rocks
http://geology.com/rocks/
Rock Cycle
This means that any rock can become either:
•
Igneous
•
Sedimentary
•
Metamorphic
depending on how it is influenced by the environment.
Properties of Rocks
Bowens Reaction
Canadian geologist in 1928 proposed a hypothesis on the formation of
minerals.
As the magma cools different minerals can form.
Chemical Stability of Minerals
Physical Stability of Rocks
Section 2 Igneous Rocks
Formation of Magma
Magma
Melted rock under the surface
Lava
Melted rock on top of surface
Textures of Igneous Rocks
1. Extrusive - formed outside the earth's
crust from lava.
Extrusive exited from the earth
2. Intrusive - formed within the crust from the
magma.
Intrusive inside the earth
TEXTURE -
depends on the size of the rock crystals. The rate of
cooling determines crystal size.
a. Intrusive
slow cooling - large mineral grains
(plutonic after the Roman God of the underground Pluto)
ex. granite - course grained
b. Extrusive
cools faster - small mineral grains (volcanic)
ex. basalt - fine grained
c. intermediate cools
no crystals form, smooth as glass
ex. Obsidian
Vesicular
Gas pockets form in rock when the magma cools quickly
PORPHYRY - (Por Fi Ree) - rocks with two textures
a. One mineral solidifies - forms large crystals
b. The other mineral solidifies later - there is less room - therefore the crystals
are smaller.
Intrusive Igneous Rocks
Batholiths and Stocks
Batholiths
Large igneous intrusions at least 100kms.
Forms mountain bases like Sierra Nevada in Cal.
Stock
Smaller than 100kms
Laccoliths
When magma pushed up between rock layers, if forms a dome
Black Hill of South Dakota
Sills and Dikes
Sills are parallel layers, small area between layers
Dikes cut across rock layers
Extrusive Igneous Rocks
When lava hardens on the surface into a
rock .
Composition of Igneous Rocks
Felsic
High silica acid type of magma.
NOT much Ca, Fe or Mg only light colored Si.
Mostly intrusive magma - slow thick - does not reach the surface
easily.
Forms granite, quartz, orthoclase feldspar.
Mafic
Low Si concentrations
High in Ca, Fe and Mg
Much hotter - easier to flow to the surface, extrusive
Forms basalt and gabbro
Intermediate
Made up of mostly plagioclase feldspar, hornblende, pyroxene, and
biotite mica.
Diorite and andesite
Section 3 Sedimentary Rocks
Formation of Sedimentary Rocks
Compaction
When sediments get squeezed down, the pore space is reduced.
Cementation
When sediments become glued together with another mineral
These cements enter the pore spaces between sediments. They glue
them together to make a clastic sediment rock
Types of cement:
1. Silica - from weathered quartz - grey or white
2. Lime - from weathered calcite - grey or white
3. Iron - from weathered Fe containing rock - red or brown
4. Pressure can act as a cement in clay and silt.
Chemical Sedimentary Rock
Forming Chemically Derived Sedimentary Rocks
Minerals must leave water and form a layer on the bottom either by evaporation or
chemical actions
Limestone tiny grains of calcite deposited in the seas or lakes. Grey or tan in
color. Smooth and dense.
Rock Salt table salt - made of mineral Halite. Forms when salt water dries up,
the salt is left behind. Found in thick layers around the world.
Gypsum
Found in layers around the world. Formed from mineral gypsum
Organic
Formed from remains of once living organisms (plants and animals)
Coal- plant remains, Fossil Limestone and Chert
Clastic Sedimentary rocks
Fragments must come from pre-existing rocks
Fragments come from weathering and erosion
agents: water - rain, rivers, wind, glaciers
Fragment sizes -
boulders, cobbles, pebbles, gravels, sand, silt and clay
Fragments need to be cemented together to make rocks.
Conglomerate - very course mixture of rounded pebbles and sand of various different
sediment types (breccia is angular fragments).
Sand Stone - grains of quartz cemented together. Very porous with a lot of small holes or
openings. Permeable, able to absorb water.
Shale - tiny flakes of kaolin (clay). Held together by pressure, not cement. Very tiny
holes, cannot absorb water. Smooth and easily broken.
Characteristics of
Clastic Sediments
The characteristics of the sediments are determined by
the way they are transported and deposited.
4 ways sediments are transported
Water, Ice, Wind, Gravity
Sorting
River Deposits A river's motion causes it to carry sediment. When the river slows it cannot carry its
sediments. Sediments fall to the bottom
https://www.classzone.com/books/earth_science/terc/content/visualizations/es0604/es0604page01.cfm?chapter_no=vis
ualization
Angularity
Not rounded sediments. Broken and sharper edges
Sedimentary Rock Features
1. Stratification -visible layers (beds) within rocks or rock structures. This layering is caused by changes in the
rock materials.
ex. a layer of Sand
a layer of Clay
a layer of Silt
2. Cross-beds and Graded Beds
Cross bedding - bedding layers formed near or on sand dunes or river delta's - not flat but slanted
Graded Beds - different types of materials deposited within a layer
3. Ripple Marks
Caused when wind or water left a mark on the sandy surface and hardened.
4. Mud Cracks
Occur when clay and silt dry, they contract and spaces form. These spaces can get buried and become
rock.
5. Fossils and Concretions
Fossils
Are remains, impressions or other evidence of existing plants and animals preserved in rocks.
*Only Found in Sedimentary Rocks!!!!
Animal dies - bone does not decay, it gets buried. Rock eventually decays but a hole is left. The hole fills in
with minerals, the minerals harden, a fossil is formed.
Concretion nodules or "lumps" of fine grained silica.
Chert - name of silica
Flint - dark grey or black chert
Geode Limestone with hollow spheres that are lined with quartz or calcite crystals.
Limestone middle dissolves in water, then the water
deposits quartz or calcite.
Section 4
METAMORPHIC ROCKS -
Are made from existing rocks by the action
of:
1. heat - NO MELTING
2. pressure
3. chemical action
Formation of Metamorphic Rock
Contact Metamorphism
When hot magma moves into rocks, heating and
changing occurs in a small area. The Rock
Does Not Melt!
A chemical change occurs by rearranging the
minerals forming a new rock.
Regional or Dynamic metamorphism
This is the major source of metamorphic rock. It
occurs in large areas, often associated with
mountain building.
Classification of Metamorphic Rock
Properties of metamorphic rock depend on the parent rock or the minerals in the original rock. The
parent rock’s characteristics and traits can be seen in certain metamorphic rocks.
Foliated Rocks
mineral are arranged in planes or bands
Heat and pressure causes them to re-align in bands
Include
Slate, Schist, Gneiss
Nonfoliated Rocks
They do not have visible bands. Looks distorted or deformed
Distortion or deformation:
1. Low temperatures and high pressure
2. The sedimentary rock gets laid down in horizontal layers
3. They get buried deep within the earth and are subjected to low heat and pressure; like where
there is a fault line.
4.The grains of rock get squeezed together and the heat re-arranges minerals –
a new different rock is formed.
Quartzite , Marble