CH 5:
I.
HOW EARTH'S ROCKS WERE FORMED
Part 1: Igneous Rocks
IGNEOUS ROCKS
A.
Uniformity of Process Tl
1. Uniformitarianism
a.
Concept that present key to past
(1) Geological processes now/active in past
(2) Present physical features formed by these
processes over long time period.
(3) Prior to 1800 most thought earth's features
formed from spectacular events/catastrophes
b.
Concept developed by James Hutton (1795)
(1) Scottish geologist
(2) Father of modern geology
c.
Principles used by modern geologists to explain
origin of rocks (mineral groups bound together)
B.
Three Groups of Rocks T2
1.
IGNEOUS: from cooling/hardening of hot molten rock
2.
SEDIMENTARY: from hardening/cementing of sediment
layers containing rock fragments, chemicals, or
plant/animal remains
3.
METAMORPHIC: existing rock changed by heat/pressure
C.
Recognizing Igneous Rock T3
1.
Intrusive (Plutonic) *Pluto/god of underworld
a.
Form underground from cooled magma
b.
Seen at surface when covering rock worn away
c.
Forms distinct interlocking mineral grains
d.
Ex: Granite
2.
Extrusive (Volcanic)
a.
Forms when lava cools on earth's surface
b.
Sources: magma/volcanic dust/ash
c.
Similar to plutonic in mineral composition but
lacks distinct mineral grains
D.
Kinds of Magma T4
1.
High silica magma
a.
Forms most plutonic rock
b.
Thick/slow flowing
c.
Not much calcium, iron, magnesium
d.
Hardens to rock w/ light colored minerals
e.
Forms felsic rocks (feldspar & silica)
ex. granite
2.
Low silica magma
a.
Forms most volcanic rock
b.
Hotter, thinner, more fluid
c.
High % calcium, iron, magnesium
d.
Hardens to rock w/ mostly dark ferromagnesian
minerals
e.
Forms mafic rocks ex. Basalt
E.
Igneous Rock Textures T5/T6
1.
Depends on size, shape, arrangement of mineral
crystals
2.
Crystal size = most important factor affecting
texture
1
a.
3.
4.
5.
6.
F.
Longer the magma stays liquid, longer atoms
free to move, larger crystals become
b.
Higher % dissolved gas in magma helps crystals
grow large faster
Glassy smooth texture = obsidian
a. Hardens very quickly at surface/no time for crystal development
Coarse-grained texture = granite
a.
Cooled slowly/visible crystals
Pine grained texture = basalt
a. Hardens quickly at surface/microscopic crystals
Porphyry = rock w/ 2 distinct textures
a. Formed from 2 different rates of cooling
Families of Igneous Rock T7
1. Granite:
from felsic magma, mainly orthoclase
feldspar & quartz, light colored
2. Gabbro: from mafic magma, dark colored, contains
orthoclase feldspar & augite
3. Diorite: composition & color between granite/gabro
G. Descriptions of Common Igneous Rock T8
1. Granite: most common continental igneous rock
a.
Found in Rockies, Adirondacks (NY), Black Hills, SD
b.
Plutonic rock made of orth. feldspar, quartz
+ one or more minerals as mica or hornblend
c.
Rhyolite = lt.grey to pink/fine grained felsite
2. Obsidian: volcanic glass
a. In granite family but dark brown/black from tiny
scattered bits of dark minerals
b. Conchoidal fracture/hard/brittle
3. Pumice: "sponge w/ small holes"
a.
From felsic lava hardened while steam & gasses
still bubbling out
b.
Light enough to float
4. Basalt: Most common of gabbro family
a.
Igneous rock of ocean floor
b.
Most common on land from lava flows
(1) HA, Iceland, Columbia/Snake River Plateaus
5. Scoria - like pumice but more dense
COOLED INSIDE
COOLED OUTSIDE
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CH 5: HOW EARTH'S ROCKS WERE FORMED
Part 2: Sedimentary Rocks
I1)SEDIMENTARY ROCKS
a)Kinds of Sediments T9
i)Sediment hardens = Sed. Rock/covers 75% surface
ii)Types
(1)Clastic: from other rock fragments
(2)Chemical: from mineral grains falling out of solution
(3)Organic: from remains of plants/animals
b)How Clastic Rocks Form T10
i)From weathering of existing rock
ii)Running water, wind, glaciers transport weathered rock/particles
smoothed & rounded by running H2O
iii)Most sediments deposited by running water
iv)Sediments become cemented usually/sometimes just pressed together
(if fine) from overlying pressure
v)Freshwater natural cements give color to rock
(1)Silica (grey or white)
(2)Calcite (grey or white)
(3)Iron oxide (red, brown, rust colored)
c)Sorting of Sediments T11
i)Occurs when river drops sediment load as slows entering lake or ocean
(1)1st dropped: lg. sediments as pebbles/gravel settling on
bottom/shallow areas near shore
(2)2nd: small sands
(3)3rd: silt/clay in calm water
ii)Result = cementing together = sed. rock
(1)Pebbles/gravel become conglomerate
(2)Sands form sandstone
(3)Silts/clays form shale
d)Conglomerate (Breccia), Sandstone, Shale T12
i)Conglomerate
(1)Coarsest clastic
(2)Cemented mixture of rounded pebbles/sand
(3)Quartz common in pebbles because durable
ii)Sandstone
(1)Quartz grains common
(2)Up to 30% air space
(3)Porous/permeable
(4)Rough/gritty texture
iii)Shale
(1)Clays of tiny kaolin flakes
(2)Impermeable
(3)Smooth/soft/easily broken
iv)Breccia
(1)Similar to conglomerate but rock pieces angular indicating
sediments from wind eroded area
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e)Sedimentary Rocks of Chemical Origin T13
i)Formed when dissolved minerals fall out of solution
(1)Originate in sea, lake, swamp, underground H2O
(2)From evaporation/chemical action
ii)Most common
(1)Limestone
(a)Formed from tiny calcite grains deposited from sea/lake
water
(2)Rock salt
(a)Natural form of table salt (NaCl)
(b)Found in thick layers/mostly pure halite
(3)Rock gypsum
(a)In layers as pure veins of mineral gypsum
f)Sedimentary Rocks of Organic Origin T14
i)From remains of plants/animals
ii)Most common
(1)Coal (formed from mosses, ferns trees)
(a)Contains carbon/used as fossil fuel
(b)Types of coal: CH 6 pg 93
(2)Limestone
(a) Calcite shells from shallow ocean H2O cement into
limestone
(b)If formed nearer shore/contain more clay
(c)ID by acid test
g)Sedimentary Features: Stratification T15
i)Stratification = arrangement in visible layers
ii)Beds/layers separated by horizontal bedding planes
iii)Cross bedding occurs when sed. deposited at angle
h)Fossils in Sedimentary Rocks T16
i)Shells dissolved made rock impressions
ii)Plant impressions formed in rocks from swamp sediments
i)Ripple Marks and Mud Cracks T17
i)Ripples from action of wind/water on sand
ii)Mud cracks from wet clay drying/contracting
(1)Cracks fill w/different material & harden
(2)Found in shale
j)Nodules, Concretions, Geodes T18
i)Nodules = lumps of fine-grained silica (chert or flint) in limestone
ii)Concretions = round calcium carbonate masses in shale
iii)Geodes = hollow spheres in rock from groundwater
(1)Crystals either quartz or calcite
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CH 5: HOW EARTH'S ROCKS WERE FORMED
Part 3: Metamorphic Rocks
I.
METAMORPHIC ROCKS
A.
What Metamorphic Rocks Are T19
1.
Meta = change morph = form
2.
Formed from existing rock by
a.
Heat
b.
Pressure
c.
Chemicals
B.
Regional(Dynamic) Metamorphism T20
1.
Forms most metamorphic rock
2.
Occurs during mountain building
3.
Large areas of rock under intense heat/pressure causing change
a.
Heat source
(1) Existing heat in deeply buried rock
(2) Friction of moving rock layers
b.
Pressure source
(1) Great weight of overlying rock
(2) Squeezing pressure of moving rock masses
c.
Result of Change
(1) Grains squeezed closer together
(2) Rock more dense/less porous
(3) Minerals are reformed or formed by
heat/chemical action
d.
Rock examples
(1) Marble (metamorphosed limestone)
(2) Quartzite (metamorphosed sandstone)
C.
Metamorphism of Shale T21
1.
Shale changes w/ dynamic/regional metamorphism
a.
More dense
b.
More crystalline
c.
New minerals formed ex. mica, hornblende
d.
Foliation occurs
(1) Squeezed into parallel layers along which rock easily
splits
2.
Rocks formed from shale
a.
Slate:
1st formed/micro thin layers
b.
Phyllite: 2nd formed/shiny
c.
Schist: 3rd (most intense metamorphism)
(1) Layers seen easily
(2) Flaky/mica sparkles
3.
Rocks formed from non-shale source
a.
Schist from basalt/sandstone
b.
Gneiss from granite/conglomerate
(1) Coarsest foliation of all metas
(2) Thick/parallel bands
D.
Thermal (Contact) Metamorphism T22
1.
Occurs when hot magma forces way into overlying rock
a. Heat bakes contacted rock
2.
Effects much less rock than dynamic metamorphism
3.
Less drastic changes/no foliation
4.
Ex: Hornfels (from shale), fine-grained, dense, hard
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