ROCKS and how to identify them

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Transcript ROCKS and how to identify them

Rocks
 Are made up of minerals
Ex: Granite ----- > 4 minerals
 Essential Minerals:
always occur in the rock
 Accessory minerals: sometimes occur
Classify Rocks By How they Form!
 Igneous-
Molten Rock cools and hardens
 Sedimentary – Made from sediments
 Metamorphic – Rocks Changed by heat and
pressure
High Silica content forms
light-colored rocks (FELSIC).
While lower (but
only by about 25%)
Silica content forms
darker-colored
rocks (MAFIC).
THE ROCK CYCLE
Rocks are naturally occurring combinations or coherent
aggregates of minerals, fossils or other hard materials.
They are classified by the way in which they form. The
three rock types are: igneous, sedimentary and
metamorphic.
All rocks on Earth are locked into a system of cycling and
re-cycling known as the ROCK CYCLE. (*)
IGNEOUS ROCKS
vent
lava
land surfac
e
conduit
magma
Anatomy of a Volcano
IGNEOUS ROCKS are “born of
fire”. In other words, they
were once molten and upon
cooling, the magma (molten
rock) crystallized into solid
rock. Igneous rocks may
form deep inside the Earth or
at the Earth’s surface when a
volcano erupts. (*)
IGNEOUS ROCKS : intrusive
Slow cooling deep beneath the
 Large Crystals
Earth’s surface allows crystals  Both large and small crystals
to grow to large size. These
= porphyry (some grow before
crystals are easily visible and
others begin
distinguish this group of
 Texture size or arrangement
igneous rocks as
of crystals
 To get Large: Cools below Earth’s
INTRUSIVE.
surface
Igneous - Intrusive
GRANITE is a coarse to
medium-grained rock that
forms from the cooling of
magma deep within the Earth
light color - felsic
DIORITE is very similar to
granite, but is distinguished in
the hand specimen by the
absence of visible quartz.
Generally it has a salt and pepper
appearance (about ½ black
and ½ white).
Igneous Rocks
Rapid cooling near or at
the Earth’s surface,
produces many small
crystals. This group of
igneous rocks is called
EXTRUSIVE
 Eruption at surface
 Numerous holes at the
surface

Small mineral crystals
ex: basalt
 No mineral crystals
ex: obsidian
 Top Part of Lavagases trapped inside of
hardened lava = scoria
 From lava thrown out
ex: pumice

Igneous - Extrusive
Rhyolite is a fine grained, felsic
rock that forms from the
cooling of lava above Earth’s
surface
Basalt a dark fine grained rock
formed from thin to massive
lava flows (mafic)
Pahoehoe is a type of
basalt that forms on the
surface of very fluid
lava flows
Igneous Review
Lava
 Extrusive
Through Volcanoes
Through hydrothermal vents
Magma 
Intrusive/Plutonic
- Cools and Crystalizes in Magma Chamber
CLUES TO
IGNEOUS ROCKS
COLOR
TEXTURE
COARSEGRAINED
(You can see
different
minerals)
LIGHT COLORED
Felsic
INTERMEDIATE
COLORED
PORPHYRITIC
(2 grain sizes)
FRAGMENTAL
VERY DARK
COLORED
Ultramafic
GRANITE:
DIORITE:
Can see crystals. Usually
gray
or pink. Can see
quartz - gray, glassy grains.
Can see feldspar - pink,
buff, or white.
Composed of 90Ca n s e e c ry s t a ls wi t h
Can see crystals — lots of
somewhat
more light colored flat shiny cleavage surfaces. 100% olivine
feldspar grains
than dark
colored minerals. A mix of light Usually black to greenish
PYROXENITE:
and dark but with no quartz. Salt black.
Composed of pre& pepper appearance.
dominately pyroxene
FINERHYOLITE:
ANDESITE:
GRAINED
Usually gray, pink,
Light to dark gray.
pastel.
Might
see
small
small
black crystals
(You can NOT
clear, rectangular crystals.
see crystals, for Sometimes banded.
the most part)
GLASSY
DARK COLORED
Mafic
OBSIDIAN:
PERLITE:
PUMICE:
GABBRO:
BASALT:
Normally has
PERIDOTITE:
AMPHIBOLITE:
Usually black or rust red.
Composed of preMay have some or lots of
dominately amphiboles
gas bubble holes, some
(such as hornblende)
holes may be filled. May
see small green grains.
Black, red, green, GLASS
Usually pearly gray. May contain Apache Tears.
LOTS of gas bubble holes, very lightweight, will float on water. Abrasive.
ALL CRYSTALLINE IGNEOUS VARIETIES
may exhibit porphyritic texture
TUFF: Compacted volcanic fragments generally less than 4mm diameter (ash)
VOLCANIC BRECCIA: Mixed tuff and angular large (>32mm diameter) fragments
AGGLOMERATE: Mixed ash and rounded/sub-rounded large (>32mm diameter) fragments
(*)
SEDIMENTARY ROCKS
SEDIMENTARY ROCKS are composed of particles
derived from pre-existing rocks or by the crystallization
of minerals that were held in solutions
 Weathering :
Rocks are broken into pieces by water,
wind or ice
Water – enter cracks, expands and breaks rocks down
Rain – acid rain dissolves minerals
Movement in Rivers – Collect on the bottom
•Formation – Build very slowly
layers will grow until environment changes
 Cementation – minerals acting as cement holding
sediments together
 Compaction – pieces compact due to weight
squeezing them together
 Precipitation – water evaporates and minerals are
left behind
Unusual Formation
Chemical Rocks – made up of
chemical sediments or once living
things
Organic Rocks – made up of remains
of living things
Sedimentary Features
•
•
•
•
•
Layers
Only type of rock that contains Fossils!
Ripple Marks – Air or water sand, silt, and clay
Concretion – water comes into contact with fossils and
mineral deposits
Geode – hollow rocks with mineral crystals inside
Classifying
Clastic Rocks (pieces of other rocks):
Conglomerate & Breccias:
Large sediments
Sandstones:
Small grains
Siltstone :
Very Small grains
Shale :
Very Small grains Clay pressed together
Classifying
 Non-Clastic Rocks (minerals in water or past
life):
 - Look at composition: calcite, halite,
gypsum or quartz
Limestone - Calcite and seashells
Rocks Salt - Halite
Rock Gypsum - Gypsum
Chert (flint) - Quartz
Coal – Past life
METAMORPHIC ROCKS
METAMORPHIC ROCKS have changed (meta) their form
(morphic). Under the influence of heat, pressure and fluids,
pre-existing rocks are modified in form
Original rock is called the parent rock
This is done within the solid state, i.e. without melting.
3 Types of Changes that occur include:
- Change in Chemistry of Rock
- Englargement of crystals
- Rearrangement of mineral grains
Classify
Burial Metamorphism – weight of overlying rock, causes a
reorientation of mineral grains
 Regional Metamorphism – As mountains rise, large amount of
magma is forced toward the surface , it hardens into the mountain
core
 Contact Metamorphism – Small amount of magma intrudes
overlying rock

The nearby Rock is altered by heat, solutions, and gases from the magma

*Metamorphism is the MOST intense near the core where heat
and pressure cause the formation of new minerals of existing
mineral grains
Features

Metamorphic rocks that
exhibit parallel
alignments of minerals
are called FOLIATED. In
these rocks, the minerals
all line up perpendicular
to the exerted pressure.

Metamorphic rocks
composed of minerals
that are not elongated or
flat, do not exhibit
parallel alignments and
are called NONFOLIATED.
The Rock Cycle
Thinking about relationships among
the major rock groups
Major Rock Groups
• Igneous
– Formed from a melt (molten rock)
– Plutonic (intrusive):slow cooling and crystallization
– Volcanic (extrusion): quick cooling at the surface
• Sedimentary
– Formed at the Earth’s surface
– Clastic (Mineral Fragments or grains, clays)
– Chemical (crystalline chemical/biochemical precipitates)
• Metamorphic
– Changed by pressure, temperature and fluids.
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Fig. 2.9
MAGMA
IGNEOUS
Crystallization
MAGMA
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IGNEOUS
Plutonic
Crystallization
MAGMA
26
Volcanic
IGNEOUS
Plutonic
Crystallization
MAGMA
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Weathering
Volcanic
IGNEOUS
Plutonic
Crystallization
Uplift
MAGMA
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Weathering
SEDIMENT
Volcanic
IGNEOUS
Plutonic
Crystallization
Uplift
MAGMA
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Weathering
SEDIMENT
Compaction
Cementation
Evaporation
Volcanic
SEDIMENTARY
IGNEOUS
Plutonic
Crystallization
Uplift
MAGMA
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Weathering
SEDIMENT
Compaction
Cementation
Evaporation
Volcanic
SEDIMENTARY
IGNEOUS
Plutonic
Crystallization
Uplift
MAGMA
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Weathering
SEDIMENT
Compaction
Cementation
Evaporation
Volcanic
SEDIMENTARY
IGNEOUS
Plutonic
Increased P&T
METAMORPHIC
Crystallization
Burial
Uplift
MAGMA
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SEDIMENT
Weathering
Compaction
Cementation
Volcanic
Can you see
IGNEOUS
any shortcuts?
Evaporation
SEDIMENTARY
Plutonic
Increased P&T
METAMORPHIC
Crystallization
Melting
Burial
Uplift
MAGMA
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Weathering
SEDIMENT
Compaction
Cementation
Evaporation
Volcanic
SEDIMENTARY
IGNEOUS
Plutonic
Increased P&T
METAMORPHIC
Crystallization
Melting
Burial
Uplift
MAGMA
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