what are the 3 basic types of rocks
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What are the 3 basic types of rocks?
Just as any person can be put into one of two main categories of human being, allrocks can be put into one of three fundamentally different types of rocks. They are
as follows:
Igneous Rocks
Igneous rocks are crystalline solids which form directly from the cooling ofmagma. This is an exothermic process (it loses heat) and involves aphase
changefrom the liquid to the solid state. The earth is made of igneous rock - atleast at the surface where our planet is exposed to the coldness of space. Igneous
rocks are given names based upon two things: composition (what they are made of)
and texture (how big the crystals are)
How do composition and texture relate to igneous rocks?
Igneous rocks are crystalline solids which cool from magma: theliquid phaseofsolid rock. Magmas occur at depth in the crust, and are said to exist in "magma
chambers," a rather loose term indicating an area where the temperature is great
enough to melt the rock, and the pressure is low enough to allow the material toexpand and exist in the liquid state. Many different types of igneous rocks can be
produced. The key factors to use in determining which rock you have are the rock's
texture and composition.
Texture
Texture relates to how large the individual mineral grains are in the final, solidrock. In most cases, the resulting grain size depends on how quickly the magma
cooled. In general, the slower the cooling, the larger the crystals in the final rock.Because of this, we assume that coarse grained igneous rocks are "intrusive," inthat they cooled at depth in the crust where they were insulated by layers of rock
and sediment. Fine grained rocks are called "extrusive" and are generally produced
through volcanic eruptions.
Grain size can vary greatly, from extremely coarse grained rocks with crystals the
size of your fist, down to glassy material which cooled so quickly that there are no
mineral grains at all. Coarse grain varieties (with mineral grains large enough tosee without a magnifying glass) are called phaneritic. Granite and gabbro are
examples of phaneritic igneous rocks. Fine grained rocks, where the individual
grains are too small to see, are called aphanitic. Basalt is an example. The mostcommon glassy rock is obsidian. Obviously, there are innumerable intermediate
stages to confuse the issue.
Composition
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The other factor is composition: the elements in the magma directly affect whichminerals are formed when the magma cools. Again, we will describe the extremes,
but there are countless intermediate compositions. (Composition relates tothemafic and felsicterms discussed in another question. If these terms are
confusing, pleaserefer to that discussionbefore continuing.)
The composition of igneous magmas is directly related to where the magma is
formed. Magmas associated withcrustal spreadingare generally mafic, andproduce basalt if the magma erupts at the surface, or gabbro if the magma never
makes it out of the magma chamber. It is important to remember that basalt and
gabbro are two different rocks based purely on textural differences - they arecompositionally the same.
Intermediate and felsic magmas are associated withcrustal compression and
subduction. In these areas, mafic seafloor basalt and continental sediments aresubducted back into the crust, where they re-melt. This allows
thedifferentiationprocess to continue, and the resulting magma is enriched in the
lighter elements. Intermediate magmas produce diorite (intrusive) and andesite(extrusive). Felsic magmas, the final purified result of the differentiation process,
lead to the formation of granite (intrusive) or rhyolite (extrusive).
What do the terms mafic and felsic mean?
These are both made up words used to indicate the chemical composition
ofsilicate minerals, magmas, andigneous rocks.
Mafic is used for silicate minerals, magmas, and rocks which are relatively high inthe heavier elements. The term is derived from using the MA from magnesium and
the FIC from the Latin word for iron, but mafic magmas also are relativelyenriched in calcium and sodium. Mafic minerals are usually dark in color and have
relatively highspecific gravities(greater than 3.0).Common rock-forming mafic
mineralsinclude olivine, pyroxene, amphibole, biotite mica, and theplagioclasefeldspars. Mafic magmas are usually produced at spreading centers,
and represent material which is newlydifferentiatedfrom the uppermantle.Common mafic rocks includebasaltand gabbro. (Please note that some geologists
with questionable motives switch the order of the magnesium and iron and come
up with the term "femag." This term is not to be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)
Felsic, on the other hand, is used for silicate minerals, magmas, and rocks whichhave a lower percentage of the heavier elements, and are correspondingly enriched
in the lighter elements, such assilicon and oxygen, aluminum, and potassium. The
term comes from FEL for feldspar (in this case the potassium-rich variety) and
SIC, which indicates the higher percentage of silica. Felsic minerals are usuallylight in color and have specific gravities less than 3.0. Common felsic minerals
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include quartz, muscovite mica, and the orthoclasefeldspars. The most commonfelsic rock isgranite, which represents the purified end product of the earth's
internal differentiation process.
CRUSTAL SPREADING
There are really only two processes: one that forms the physical earth, and anotherthat beats up the surface and tears it apart through weathering and erosion. The
formational process is called tectonics, and is manifested to those of us living on
earth by earthquakes, volcanos, and mountain building in general.
The earth is really just a sphere of liquid rock (magma) which has cooled to
thesolid statewhere exposed to the coldness of space. We call this cold and rigidouter shell thecrust, and it is actually rather thin in comparison to the overall
diameter of our planet. Because of the heat and pressure beneath the surface, thiscrust is constantly being subjected to stresses which break it up.
The earth's crustal sections are called plates, and they vary from small tocontinental in size. Immense forces cause these rigid plates to slowly move about
the surface, where they are constantly running into each other. Tectonic activity is
common at these plate boundaries, of which there arethree basic types:
Spreading centers occur where two plates are
moving away from each other, and deep cracks areopened through the crust. This lengthening of the
crust allows magma from the upper mantle to rise to
the surface and cool, commonly formingbasalt. Anexcellent example is the Mid-Atlantic Ridge. The
crust at these "zones of divergence" is thin and has a high heat flow, so volcanic
activity is persistent and earthquakes are relatively small. Clickherefor moreinformation on zones of extensional tectonics.
Subduction zones are associated with regions
where two plates are moving towards each other,and the crust of the earth is shortened. An exampleis where the western edge of South America meets
the Pacific Ocean. In this case, the collision is
between acontinental plateand an oceanic plate,and a subduction zone forms where the heavier
oceanic basalt is forced beneath the lighter continental materials along a deep
trench. This involves lots of rock being taken back down into the earth, where itcan melt. This leads to a very active volcanic environment. The crust is much
thicker here, and so earthquakes are also stronger. For these and a variety of other
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reasons, some of the most intense earthquake and volcanic activity is associatedwith these zones of compression
Elements & minerals common to various magmas
Ultramafic magmas
Olivine - Mg2SiO4 to Fe2SiO4
Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6
Mafic (basaltic) magmas
Olivine - Mg2SiO4 to Fe2SiO4
Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6
Plagioclase - CaAlSi3O8 to NaAlSi3O8
Intermediate magmas
Plagioclase - CaAlSi3O8 to NaAlSi3O8
Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
Muscovite/Biotite - KAl2(Si3Al)O10(OH)2
Quartz - SiO2
Igneous Rock Classification
Texture vs. composition
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Felsic Intermediate Mafic Ultramafic
Aphanitic
fine grainRhyolite Andesite Basalt
Conditions needed to
produce ultramafic
flows do not exist innature at this time.
Intermediate Dacite Diabase
Phaneritic
coarse grainGranite Diorite Gabbro Peridotite
Glassy Obsidian
Frothy Pumice Scoria
It is important to note that there are many, many intermediate steps between these
main divisions. Geology is a science full of "shades of gray," and the naming ofigneous rocks is certainly no exception
hat are the most important types of rock in the crust?
Excluding the rocks between my ears, I'd have to say that basalt and granite have
the honor of being the most important rocks in the crust.
Basalt and granite actually have quite a bit in common. Both are igneous rocks,which means that they cooled from a magma (the earth gets very hot just below the
surface, and there is lots ofliquidrock available). Both are made up of mineralsfrom the silicate group, so both have large amounts ofsilicon and oxygen. Bothwill hurt if you drop a big piece on your toe. But there are several important
differences, too. These differences help define and explain how the earth works.
Granite is great stuff! Not only is it my personal favorite, it is without a doubt themost common rock type on the continental land masses. Yosemite Valley in the
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Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks. Butgranitic "basement rock" can be found just about everywhere east of the Rockies if
you're willing to dig through the dirt andsedimentary rocksat the surface. Graniteis intrusive, which means that the magma was trapped deep in the crust, and
probably tooka very long timeto cool down enough to crystallize into solid rock.This allows the minerals which form plenty of time to grow, and results in
acoarse-textured rockin which individual mineral grains are easily visible.
Granite is the ultimate silicate rock. As discussed elsewhere ingreater detail, on
average oxygen and silicon account for 75% of the earth's crust. The remaining
25% is split among several other elements, with aluminum and potassiumcontributing the most to the formation of the continental granitic rocks. Relatively
small amounts of iron and magnesium occur, but since they have generally
higherdensitiesit's not surprising that there isn't very much in the granite. Due to
the process ofdifferentiation, most of the heavier elements are moving towardsthecoreof the earth, allowing the silicon and oxygen to accumulate on the surface.And accumulate it has. Enough granitic "scum" has differentiated to the surface to
cover 25% to 30% of the earth with the good stuff. We call this purified
materialfelsicbecause of the relatively high percentage of silica and oxygen.
Basalt is extrusive. The magma from which it cools breaks through the crust of the
earth and erupts on the surface. We call these types of events volcanic eruptions,
and there are several main types. Thevolcanoes that make basaltare very common,and tend to form long and persistent zones of rifting in nearly all of the ocean
basins. We now believe that these undersea volcanic areas representhugespreading ridgeswhere the earth's crust is separating. It's a lot like a cut onyour arm, which will bleed until a scab forms. Basaltic magma is like the blood ofthe earth - it's what comes out when the earth's skin is cut the whole way through.
As an eruption ends, the basalt "scab" heals the wound in the crust, and the earthadds some new seafloor crust. Because the magma comes out of the earth (and
often into water) it cools very quickly, and the minerals have very little opportunityto grow. Basalt is commonly veryfine grained, and it is nearly impossible to see
individual minerals without magnification.
Basalt is considered amaficsilicate rock. Among other characteristics, mafic
minerals and rocks are generally dark in color and high inspecific gravity. This is
in large part due to the amount of iron, magnesium, and several other relativelyheavy elements which "contaminate" the silica and oxygen. But this heavy stuff
really isn't happy near the surface, and will take any opportunity it can to head
fordeeper levels. The trick is to heat the basalt back up again so it can melt andgive the iron another shot at the core. It wants to be there, and heat is the key
which unlocks the door.
As it turns out, most of the ocean floor is basalt, and most of the continents aregranite. Basaltic crust is dark and thin and heavy, while granite is light and
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accumulates into continent-sized rafts which bob about like corks in this "sea ofbasalt."When a continent runs into a piece of seafloor, it's much like a Mac truck
running into a Volkswagon. Not very pretty, but at least there's a clear winner. Andthe seafloor basalt ends up in pretty much the same position as does the VW -
under the truck (or continent, as the case may be). This may seem like a drag forthe basalt, but remember that it isn't all that happy on the surface anyway, and this
gives it the heat it needs to re-melt and try to complete the differentiation process
which was so rudely interrupted at the spreading ridge. If successful and allowed tocontinue, what's left behind is a "purified" magma with most of the iron,magnesium, and other heavy elements removed. When it cools, guess what forms?
And thecontinental land massjust got a wee bit larger.
Felsic (granitic) magmas
Potash Feldspar - KAlSi3O8
Quartz - SiO2
Muscovite/Biotite - KAl2(Si3Al)O10(OH)2
Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
What is Differentiation?
It is my humble opinion that differentiation is one of the primary driving forces ofour planet, so pay attention. To understand the process we have to begin by
agreeing on several assumptions:
1. Different earth materials have differentdensities.
2. Given a chance, all materials will sort themselves by density, with the heaviermaterial sinking and the lighter material rising.
3. Theinterior of the earthis not a "solid" as we understand the term, but rather in
a semi-plastic state which allows ions to migrate (more or less) at will.
4. The earth's interior is zoned by density, with the heaviest material at the center(the core), and the lightest stuff floating about at the surface (the crust).
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Chemical: many of these form when standing water evaporates, leaving dissolvedminerals behind. These are very common in arid lands, where seasonal "playa
lakes" occur in closed depressions. Thick deposits of salt and gypsum can form dueto repeated flooding and evaporation overlong periods of time.
Organic: any accumulation of sedimentary debris caused by organic processes.Many animals use calcium for shells, bones, and teeth. These bits of calcium can
pile up on the seafloor and accumulate into a thick enough layer to form an"organic" sedimentary rock.
Introduction
We've studiedigneous rocks& themineralsof which they are composed
Basement rocks
Most are covered by a thin veneer of debris
Consolidated into a "rock" through slow-acting processes
Usually involving pressure and fluid penetration
Relatively simple to understand
Relatively near-surface processes
As opposed toigneous & metamorphics, which usually occur at depth
Secondary (or derived) rocks
Several main categories
Clastic sedimentary rocks - The classic sedimentary rock
Accumulations of debris derived from the disintegration of pre-existing rocks
DIGRESS TO: Terrigenous sediments
Chemical sedimentary rocks - Chemical precipitates
Usually as the result of the evaporation of water
Ex. Salt (NaCl); Gypsum (CaSO4 2H2O)
Organic sedimentary rocks
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All hydrocarbons
Coal, peat, oil, etc.
The distinction between these three categories can get pretty fuzzy at times
Ex. Limestone, chert
Hard rocks vs. Soft rocks
Origin of Sedimentary Materials
DIGRESS TO: Physical vs. Chemical weathering
Click here for additional information onwater,weathering, anderosion(RCC)
Clickherefor additional information on surface processes (GPHS)
Clasts - derived from physical (and chemical) weathering processes
Smaller solid particles
Derived directly from the source area
Reflect lithology of the source area
Wide range of sizes, from silt to boulders
Chemical processes can result in the relative enrichment of more resistant (or inert)
minerals
Ex.quartzvs.feldspar
Clay minerals
I'm not a clay kind of guy
Extremely complex mineralogy
My understanding is minimal
Easy to get confused by the term
The term "clay" refers to both a size and a mineral family
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A clast can be clay size without being clay
DIGRESS TO: "clay the size" vs. "clay the mineral"
Clay formation forms small, sheet-like minerals (look like the micas)
Lots of different clay minerals
Which mineral is formed reflects primary lithology and environment
Can change to a different mineral if moved to a different environment
Downslope? Downstream?
Near-surface, low temperature environments
Hot and humid works best
Water is a universal solvent (HOH)
Tends to work parallel toBowen's Reaction Series
The higher temperature minerals are more susceptible to chemical weathering
Therefore, especially hard on the mafics and feldspar
To repeat what was mentioned above
Chemical processes can result in the relative enrichment of more resistant (or inert)
minerals
Ex.quartzvs.feldspar
Describe "decomposed granite"
Ions
Chemical weathering also results in "ions" which are "held in solution"
The solution is usually water
Remember:Wateris a universal solvent (HOH) and will play merry hell withanything "over the course of geologic time!"
SeeStrickler's 3rd and 4th Laws of GeoFantasy
Some elements will dissolve and be held in solution
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Ex. salt, sugar
DIGRESS TO: Solution (ions) vs. Suspension (clays)
Both make fundamentally different types of sed. rocks
Common ions include: Ca+2, Na+, CO3-2, Cl-
DIGRESS TO: What do the superscripts mean?
Atomic structure & the role of the electron
These ions are responsible for the "mineral taste" in some water
Therefore, we can tell that iron and sulfur must also be common
If the amount of ions increases relative to the amount of water, minerals can
precipitate
Ex. salt (Na+ + Cl- -> NaCl)
Saturation is the key
An undersaturated solution can become oversaturated in 2 ways
Increase the dissolved ions
Decrease the solvent (water)
This is more common (probably)
Can be initiated by the evaporation process
Organisms can also extract the ions directly from the water
Use them to build shell material
Ex.: Ca+2 + CO3-2 -> CaCO3
Can result in extensive deposition of calcium or silica sediments
Environments of Deposition
(Monroe; fig. 7-3, pg. 202)
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Water plays an important role in most aspects of sedimentary rocks
Fromweatheringanderosiontotransportation and deposition
DIGRESS TO:V=Q/A
Deposition occurs in a wide variety of locations
Basically, any low spot is a potential depositional environment
On bothregional and local level- expand
Three major divisions -Continentaldeposition,marinedeposition,
andtransitional(inter-tidal)
Infinite possible combinations of environments and materials
Results in infinite possible sedimentary rocks
Fortunately, most fall into one of several common environments
And as we already know from our study of igneous rocks, most of the rocks startwith asimilar chemistry
It can still be tough to recognize the depositional environment
DIGRESS TO: This is the ultimate goal of the study of sedimentary rocks
The names are important, but only insofar as they provide clues to how they got
there
The interpretation of earth's history is the purpose of any geological examination
In any event, this will usually take lots of field work
And the examination of lots of different rocks
As well as copious amounts of lubricant to make sense of the data!
Multiple Working Hypotheses
Need to keepan open mind
Several working together with different ideas can be good
As can a "Devil's Advocate" to keep the group from getting cocky.
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It's far too easy to only see those units and/or features which support the currentlyfavorable model
Important factors include:
Sorting - key to interpreting the depositional environment
"The degree in similarity in particle size in a sediment"
Important in the clastic sediments
Particle size
Important in the clastic sediments
Particle composition
Important in chemical and organic sediments
Sedimentary Rocks
General Statements
We've studied igneous rocks & the minerals of which they are composed
Basement rocks
Most are covered by a thin veneer of debris
Consolidated into a "rock" through slow-acting processes
Usually involving pressure and fluid penetration
Relatively simple to understand
Relatively near-surface processes
As opposed to igneous & metamorphics, which usually occur at depth
Secondary (or derived) rocks
Several main categories
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Clastic sedimentary rocks - The classic sedimentary rock
We will concentrate on this type
Chemical sedimentary rocks - Chemical precipitates
Usually as the result of the evaporation of water - Ex. Salt (NaCl)
Organic sedimentary rocks
Limestone, all hydrocarbons - Coal, peat, oil, etc.
Origin of Sedimentary Materials
DIGRESS TO: Physical vs. Chemical weathering
Clasts - derived from physical (and chemical) weathering processes
Smaller solid particles
Wide range of sizes, from silt to boulders
Clay minerals
Easy to get confused by the term
The term "clay" refers to both a size and a mineral family
A clast can be clay size without being clay
Clay formation forms small, sheet-like minerals (look like the micas)
Near-surface, low temperature environments
Hot and humid works best - chemical weathering!!
Note: chemical weathering also results in "ions" which are "held in solution"
Can result in chemical sedimentary rocks
Organisms can also extract the ions directly from the water
Use them to build shell material - Ex.: Ca+2 + CO3-2 -> CaCO3
Can result in deposition of organic sediments
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Environments of Deposition
Water plays an important role in most aspects of sedimentary rocks
From weathering and erosion to transportation and deposition
DIGRESS TO: Q=AV
Deposition occurs in a wide variety of locations
Basically, any low spot is a potential depositional environment
Two major divisions - Continental and marine
Also there are inter-tidal (transitional) environmnets
Important factors include:
Sorting - The degree in similarity in particle size in a sediment
Important in the clastic sediments
Particle size
Important in the clastic sediments
Particle composition
Important in chemical and organic sediments
Continental Deposition
Sediments trapped on land
Rivers and streams
Riverbed - size directly related to energy of the stream
Can be poorly sorted (all different sizes) or well sorted (all the same size)
Floodplain - Flat surfaces adjacent to a river
Represents sediments deposited during flooding
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Usually well sorted
Glaciers
Non-turbulent flow (unlike rivers)
Can and will carry all sizes of material
Commonly poorly sorted, but not always!
Lakebeds
By nature a temporary feature
A sure trap for sediments (because Q=AV)
Evaporites - common to arid regions with seasonal lakes (playas)
Ex.: Bonneville Salt Flats
Alluvial Fans
Generally arid and semi-arid climates
Deltas
Essentially an underwater alluvial fan
Eolian Deposition
Wind can also play a role in the erosion, transportation, and deposition of
sediments
Can affect wide areas
Not confined to a defined channel like a river is
Always well sorted (unless contaminated by other processes)
Small stuff only - no boulders!
Sand dunes
Marine Deposition
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The seafloor is the final resting place for the majority of weathered rock materials
Please refer toStrickler's 3rd Law of GeoFantasy
Remember - "The earth breaks what it makes and puts it in the ocean"
Factors affecting deposition include:
Distance from shore
Related to energy
Depth of the water
These result in 3 broad zones of deposition
Relatively good sorting within each zone
In general, the shore and shelf contain the majority of "terrigenous" sediments
Gravel ---> Sand ---> Silt ---> Clay ---> Carbonate Ooze
The Shore Zone
The shore acts like a channel and restricts the "flow" of the ocean
High energy zone
Coarse sand and gravel are deposited here
Smaller material stays in suspension/solution and moves offshore
The Continental Shelf
Much broader then the shore zone
Most terrigenous sediments end up here (sooner or later)
Mostly silt & clay
Locally coarser material related to times of higher energy
Carbonate deposits also common
Inorganic and organic deposits of CaCO3 - Limestone
Common to "shallow, warm water"
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The Abyss - much of this ends up being subducted
Mostly very fine grain sediments
Water depth important in which is deposited
Calcareous to siliceous to terrestrial clay ooze
As depth increases and/or temperature decreases
Features of Sedimentary Rocks
Stratification - the most common and distinctive
Most sedimentary rocks are composed of particles which settle through water (or
air)
Generally quiet water deposition results in nearly horizontal layers
Differences through time result in visible layering
Variation in clast size
Variation in clast composition/mineralization
Special enhancements to visible layering
Graded Bedding
Cross Bedding
Size and Roundness of the clasts
Usually reflects transport distance and/or time in transit
Long distance = smaller and rounder clasts
Color
Most igneous rocks are some shade of gray
Sedimentary rocks can be quite colorful
Different pigments can fill the void spaces between the clasts
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Iron - very common
Results in shades of red, brown, pink, or yellow
Dark to black color commonly the result of organic material
EXAMPLE: Black shale
Fossils - the classic sedimentary feature
Evidence of once-living organisms
Characteristic of many sedimentary rocks
Not igneous or metamorphic
Most relate to remains of "hard body parts" (bones, shells, teeth)
But any evidence is considered a fossil
Soft body molds
Footprints
Coprolites
Some amazing parts have been preserved
Jellyfish, compound eye parts, dragonfly wings
Clues to depositional environments
EXAMPLE: Clam fossils pretty much indicate marine deposition, etc.
Used to establish the Relative Time Scale
Conversion into Rock
Lithification - "the process of converting soft, unconsolidated sediments into hard
rock"
Two major factors contribute to the lithification process
Remember: we are usually starting with a loose pile of debris, which is saturatedwith water
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Compaction
Weight of overlying sediments results in compaction
Reduction in pore space
Interstitial fluids (water) may be removed
Cementation - "The most significant process"
"The deposition from solution of a soluble substance"
Fills the interstitial pore spaces
Cements the grains together
Three common types of cement
Calcium- Probably the most common
Easily dissolved in groundwater
H20 + CO2 = H2CO3 (Carbonic Acid)
Will dissolve calcium and put it into solution
Silica - less soluble than calcite
Will form a much harder and stronger cement
Iron Oxide (Fe2O3)
Facies Changes
"Lateral change in the basic properties of a sedimentary horizon"
DIGRESS TO: Time-Stratigraphic Horizons
EXAMPLE: Conglomerate into sandstone into siltstone into shale
Reflect local variations in the depositional environment
DIAGRAM: on board
Transgression / Regression
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Unconformities
The sedimentary record is not complete
Long term gaps in the sedimentary record indicate periods of non-depositionand/or erosion
We actually can see only a small part of the earth's history in sedimentary rocks
The gaps clearly represent more time than do the beds themselves
Three major types of unconformities
Angular Unconformity
Easiest to recognize - describe
Non-parallel beds above and below
Represents: deposition, uplift, deformation, erosion, subsidence, and newdeposition
Disconformity
Parallel beds above and below
Can be real tough to recognize
Nonconformity
Sedimentary beds overlying igneous or metamorphic rocks
Represent immense time periods
Classification (types of sedimentary rocks)
As we said, there are 3 general categories
Clastic/fragmental; Chemical precipitates; and Organic
Distinction between different types often fuzzy in reality
Clastic Sedimentary Rocks - true secondary rocks
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Derived from the breakdown of pre-existing rock at the surface of the crust
Most sedimentary rocks are clastics
Quick review:
Surface weathering produces small clasts
Physical and chemical processes
As soon as a clast (at whatever size) is broken from bedrock, it is involved in the
erosion and transport process
Gravity is the ultimate driving force here
Clasts moved downslope to creek/river systems
Carried downstream to a suitable depositional environment
Weathering can continue during transport
Both physical and chemical
Its reasonable to assume that physical weathering dominates in the headwaters athigher elevations
Chemical weathering takes on a more active role at lower elevations
Smaller clast size results in greater surface area for chemical attack
Classification generally based on the size of the clasts
Conglomerate - cemented gravel
Usually poorly sorted, calcium or silica cement
Sandstone - Sand-sized clasts
Often interbedded with shale or conglomerate (facies changes)
Indicate near shore marine - your basic beach
Calcium or silica cement
Which one is present determines hardness
Friable - breaks up easily due to weak cement
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Compositional differences
Classic sandstone is generally quartz - final weathered product
Graywacke - "dirty sandstone"
Generally dark in color
Quartz, feldspar, mafics, lithic fragments all present
Indicates very short distance of transport
Silt & clay sized clasts
Lots of names based on size of clasts
Siltstone, claystone, mudstone
Shale works as a general descriptive name for most of them
Usually impossible to determine composition of clasts due to small clast size
Chemical sedimentary rocks
Evaporites
Result from the evaporation of water
Halite (salt), Gypsum (sheetrock)
Carbonates
Limestone - calcite (CaCO3)
Travertine
Hot springs deposits
Organic sedimentary rocks
Hydrocarbons
Coal - lithified plant and animal remains
Compacted swamps, etc.
Convert to coal in an anaerobic environment
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Calcium based rocks
Limestone the most common
Most limestone is organic as opposed to chemical in origin
Foraminifera
Microscopic plants & animals extract CaCO3 from seawater and use it to buildshells
These will settle to the seafloor and accumulate into Limestone deposits
Larger organisms also extract CaCO3 for shells which can accumulate on seafloor
Coquina - lithified shell debris
Can be reworked in the sea currents - broken and moved around
Are these then clastic sedimentary deposits?
Reefs
Made largely of corals and carbonate secreting algae
Like shallow, warm waters which are agitated by wave action
High in nutrients (for food)
Environment essentially free of terrigenous sediments
Can result in extremely pure limestone deposits
Commonly 30 of the equator
Silica based rocks
Chert - "general name used to cover many types of dense, hard, non-clastic,microcrystalline siliceous rocks"
Flint - dark color from included organic remains
Uniform texture - conchoidal fracture
Jasper - reddish flint
Sinter - hot springs (like travertine)
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Thick beds of chert are found throughout the geologic record
Some may result from direct chemical precipitation
White smokers at spreading axes
Most are thought to be organic (like the carbonates)
Microscopic plants & animals extract silica from seawater and use it to build shells
These will settle to the seafloor and accumulate into chert deposits
Metamorphic Rocks
The metamorphics get their name from "meta" (change) and "morph" (form). Anyrock can become a metamorphic rock. All that is required is for the rock to be
moved into an environment in which the minerals which make up the rock become
unstable and out of equilibrium with the new environmental conditions. In mostcases, this involves burial which leads to a rise in temperature and pressure. Themetamorphic changes in the minerals always move in a direction designed to
restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and
marble.
Introduction
As usual in geology, take big words apart
meta = change
morph = form
ick = tough to study
Talking about a change inmineralogyhere
Considered an "iso-chemical" process
Essentially, nothing is added or lost at the elemental level
Except for a subtle to profound loss of water
Existing elements recombine into new minerals
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Mineralogy ALWAYS changes in an attempt to restore equilibrium
One of the only times in geology when you can use the word "always"
Even toss the1st Law of GeoFantasy?
Start with any rock
Subjected to different environment conditions
Commonly due to burial, or subsidence of the crust due totectonics
Heat and pressure usually involved
Difficult process to study
Generally occurs atdepth in the crust
Impossible to observe directly
Similar in this way tointrusive igneous rocks
But generally far more complex
But not too deep - usually "less than 20 kilometers"
Higher temperatures at depth lead to complete re-melting and the formation of
magma
As always, this is a highly variable depth
Subject tolocalirregularities
Metamorphism is also considered to be a "solid-state" process
All of this happens at temperatures below the melting point of the rocks!
There are several factors which directly affect the process
Rock chemistry
Contained fluids
Heat
Pressure
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Time
There are infinite variations of these factors
Results in a very complex suite of rocks!
The study of metamorphic rocks can only take place after uplift, weathering, anderosion
And long after the actual metamorphic processes have ended
Can be real tough to determine the metamorphic history of a rock
Including what it was originally!
The metamorphics are without a doubt the toughest to understand
We'll takea very broad lookat them and just discuss the main categories
Factors involved in the metamorphic process
Rock chemistry
Metamorphism is an iso-chemical process
Therefore, what you start with is extremely important
The chemistry of the parent rock largely determines the composition of theresulting metamorphic rock
This should be a real no-brainer
Cook eggs and you get an omelette, not meatloaf
Unless you add a bunch of new stuff
But this is an iso-chemical process, so not much is added or lost
Limestone alters to marble, not quartzite!
Contained fluids
Generally water and carbon dioxide
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Similar to how volatiles affect magmas
REVIEW:mafic to felsic
The high volatile minerals tend to react early
Release their volatile components
Two things happen:
The loose volatiles tend to act as a catalyst
The best metamorphics are commonly derived fromsedimentary rocks
The resulting rock is generally decreased in the volatile components
Heat
Considered "the principle factor in the metamorphic process"
If metamorphism requires that the elemental ions migrate and recombine...
Ions diffuse easier at higher temperatures
Therefore higher temperatures tend to increase both the speed and efficiency of themetamorphic process
The increased heat directly affects the "strength" of the rock
And locally affects theBrittle-Ductile Transition Zone(REVIEW)
The resulting metamorphic rocks can be highly contorted, folded, and otherwise
deformed plastically
As a general rule: the higher the metamorphic grade the greater the plasticdeformation
DIGRESS TO: Metamorphic grade
Obviously, there are all possible ranges of heat (metamorphic grade)
From "just barely warm" to "just below the melting point"
But, what is the melting point?
REVIEW:Bowen's Reaction Series
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The metamorphic process affects the low temperature (felsic) minerals first
This results in some VERY interesting effects at the higher grades (see below)
Pressure
Heat and pressure are definitely related
Pressure leads to increased heat
In general, the increased pressure associated with the metamorphic process results
in a rock with tighter packing at the atomic level
Therefore, generally higherdensitythan the parent rock
There are several sources of pressure...
Pore-fluid pressure
Release of volatiles supplies some pressure to the overall system
Litho-static pressure (REVIEW) (Monroe; fig. 8-7, pg. 241)
The load weight of overlying rock
Equal pressure in all directions
Results in non-foliated rocks (DEFINE)
Marble, quartzite common non-foliated varieties
Directed pressure (REVIEW)
Acts in a specific direction
Generally related totectonics
Results in foliated rocks (DEFINE)
New mineral grains grow with their long axis oriented normal to the
stress (Monroe; fig. 8-10, pg. 244)
EXAMPLE: Drop a deck of cards; gravity is the directed stress
Most common metamorphic rocks fall into this category
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Time
Some of the higher grade rocks clearly required aVERY long timeto form
We can duplicate all the other factors in the lab, but not this one
This is the fatal flaw in most studies of earth processes
Clickherefor a discussion of geologic time and metamorphic rocks
Metamorphic environments and rocks
There are several major categories
Basically related to the size of the system
And the relative importance of heat and pressure
Localmetamorphic terrains
Relatively small and isolated occurances of limited extent
Regionalmetamorphic terrains
Large, fully developed, and complex environments
Metamorphic terrains of limited extent
Contact metamorphism
Usually associated with increased heat
Without a corresponding increase in pressure
Litho-static or limited directed stress
Therefore commonly non-foliated
Common along the margins of small plutons (dikes, sills, etc.)
Localized heating of country rock as magma cools
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Results in a thin "halo" of metamorphism
Also called a metamorphic aureole (Monroe; fig. 8-5, pg. 240)
Usually very thin (millimeters to a few centimeters)
Chill margin vs. baked zone(DESCRIBE)
Clickherefor a discussion of cooling history and texture
Can be larger in special cases
Hornfels: derived from shale
Dense, fine-grained, non-foliated
Skarn: derived from limestone
Skarns can be VERY important to economic geology
Calcium carbonate is highly reactive
Will extract many different elements from the cooling magma
Can result in very high grade mineral occurrences
But usually disappointingly small
Remember, they form in a contact metamorphic environment
Hydrothermal metamorphism (EXPLAIN: hydro + thermal)
Heat and chemically active solutions
Usually related to residual fluids escaping from afelsicmagma chamber
Does not have to be felsic, but is probably most common
Cataclastic metamorphism
Localized near-surface fault zones (redundant?)
Rock is tectonically broken and shattered
Increases surface area
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Leads to increased fluid penetration and hydrothermal metamorphism
Can also occur locally at greater depths
The added heat and pressure can accentuate the metamorphic processes
Mylonite: Greek for "mill" (Monroe; fig. 8-8, pg. 242)
Nearly complete pulverization of the rock
Leads to partial to complete recrystallization
Very tightly inter-grown minerals
Extremely hard and durable rock
Regional metamorphism: an overview
Clickherefor online mineral and rock ID charts
Can result in bodies of great extent
Most (but not all) are the result of directed stress environments
Also called "dynamo-thermal" metamorphic rocks
Associated with continental mountain building processes
Combined withgranite, these form the cores of the continental land masses
Calledcratons
Shields where exposed
Platforms where obscured by sedimentary layers
Heat, pressure, and volatiles are all important
Usually results in prominent foliation (but not always)
And very complex mineral assemblages related to local variations in rockchemistry and metamorphic grade (more later)
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THE ROCKS ---
Clickherefor online mineral and rock ID charts
Non-foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)
Heat and litho-static pressure predominate
Results in a recrystallization of existing material
These factors are everywhere beneath the surface
Therefore, takinga very broad view, all rocks can be considered non-foliated
metamorphics to some degree
There are several common non-foliated rocks
Quartzite: derived from sandstone (Monroe; fig. 8-17, pg. 247)
Very hard and durable
Looks like sandstone
But, the rock will break through the quartz grains, not around them
Hornfels: derived from shale (usually)
Also very hard, dense, and durable
Marble: derived from limestone (Monroe; fig. 8-16, pg. 247; and "Marble,"pg.234)
In most cases, the parent limestone had impurities
Add color and pattern to the marble
Can be dense and compact, but softer than quartzite or hornfels
It's made from CaCO3 like calcite and limestone
Good for carving, building stone, facing stone
Josephine County Courthouse
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All three represent common marine sedimentary facies which are probablymetamorphosed by the weight of overlying debris
Foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)
Clickherefor online mineral and rock ID charts
Result of increasing heat and directed pressure
Increasing metamorphic grade generally results in a coarsening of texture
As well as a concentration of felsic and mafic constituents
Increasing grade also results in a progression specific minerals (Monroe; fig. 8-18,pg. 248)
Obviously dependent upon original rock chemistry
Called a metamorphic facies (Monroe; fig. 8-20, pg. 250)(Monroe; fig. 8-21, pg.248)
Examples: staurolite facies, actinolite facies, greenschist facies
The same elements recombine to form different minerals at different temperature
and pressure environments
Each facies indicates temperature, pressure, and fluid conditions at the time of themetamorphism
Platy minerals: mica, chlorite, graphite
Common at lower metamorphic grade
Orientation results in "foliation"
Elongate minerals: hornblende, staurolite, pyroxene
Common at higher metamorphic grade
Orientation results in "lineation"
The resulting progression of metamorphic rocks is fairly specific
With infinite gradations and variations!
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Let's start with deep-water marine sediments and follow the process
Add heat and pressure between (and within) each step
Metamorphics are the ultimate "shades of gray" situation in geology
These are only the broadest of category names
The variations are endless
Shale
A commonsedimentary rock
Very fine grain
Toss in a little sandstone and limestone and you've got your basicmarine
sedimentary assemblage
Slate
Little or no significant visible change (Monroe; fig. 8-11, pg. 244)
Still microscopic grains
But the mineralogy has begun to change
Usually to mica, graphite, or chlorite
Low temperature minerals with one perfect cleavage
A very hard and durable rock
Commonly used as pool table tops, roofs, and chalkboards
Phyllite
Begin to see mineral grains
Commonly lots of mica - gives rock a shiney look
Can be up to 50% muscovite
But can also be graphite or chlorite
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Schist
A very broad category (Monroe; fig. 8-12, pg. 245)
Significant change in mineralogy, texture, and visible foliation
Well developed foliation of micaceous minerals (usually greater than 50%)
Also called schistosity
The characteristic wavy or undulating rock cleavage common to schist
May not parallel original bedding
Most primary textures and features are lost
Other minerals begin to form based on composition of original rock and new
environmental conditions
Use additional minerals as modifier of name
EX: mica schist, quartz schist, hornblende schist, quartz-mica-hornblende schist,etc.
Gneiss
High grade metamorphic rock(Monroe; fig. 8-13, pg. 245)
Color banding of light and dark minerals
DIGRESS TO: layers vs. lenses
Lineation: orientation of prismatic minerals
Hornblende, actinolite, tourmaline, staurolite
Migmatite
Almost there! (Monroe; fig. 8-15, pg. 246)
Partial melting and recrystallization of felsic minerals
REVIEW: Reverse order ofBowen's Reaction Series
Results in a rock with layers of felsic igneous rock and very high grade maficgneiss
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To summarize...
Clickherefor online mineral and rock ID charts
Increasing grade very common in sedimentary sequences
Layers of sediment pile up deeper and deeper
Leads tolithificationof the lower layers
As additional layers of sediments are added on top, the lowest portions begin to
metamorphose
Followed to its logical conclusion...
Imagine an unbroken transition from unconsolidated sediments to sedimentaryrock through increasing metamorphic grade to...
Migmatites and the Formation of Granitic Magmas
Migmatite - a very high temperature metamorphic rock
Because ofBowen's, thefelsicconstituents have reached theirmelting point
But themaficsstill have a way to go
So we end up with a highly contorted, mixed igneous and metamorphic rock
Called "roof pendants" because they usually grade into felsic intrusives at greater
depth
Several excellent examples
Kaweah River - Sierra Nevada foothills
Near south entrance to Sequoia National Park
Convict Lake area - eastern Sierra Nevada
Ashland pluton - Siskiyou Mountains, Oregon and California
Add more heat and the whole thing melts - aphase change
When I was in this class, most granitic magmas were "emplaced from below"
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Usually through "forceful injection"
Kind of an ominous thought
...and where did they come from?
Direct differentiation from the upper mantle is hard to believe
In almost every case, magmas we see coming out of deep rifts in the crust aremafic
Your basicoceanic spreading ridge
They begin to purify into felsic materials where they are re-worked along the
continental margins
Subduction zonesand the cores of volcanic arcs
Therefore, the ultimate source of most granitic magmas must be a metamorphicprocess
Clickherefor additional information on the formation of granitic magmas
Clickherefor additional thoughts on the directdifferentiationof granitic magmas
from theupper mantle
The realms of dynamo-thermal metamorphism
No clear-cut answers, but lots of circumstantial evidence
Commonly in elongate bodies
10's to 100's of miles wide
100's to 1000's of miles long
Associated with deep- seated plutonic rocks
Batholiths like the Sierra Nevada
Form the axes of many of the world's mountain ranges
Sierra Nevada, Alps, Rocky Mountains, etc.
Intermediate to high temperatures
http://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.html -
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Intermediate to high directed pressure
Clearly long and well developed crustal tectonic environments
Time spans measured in 100's of millions of years
Moderate to great depth - but still in the crust
All this adds up to subduction complexes as the most logical location
These metamorphic suites most likely form the cores of thesubduction zones
Metamorphic Rocks
Overview of the process
Start with any rock
Subjected to different environment conditions
Commonly due to burial or subsidence of the crust due to tectonics
Heat and pressure usually involved
Generally at depth in the crust
Mineralogy ALWAYS changes in an attempt to restore equilibrium
Solid-State process - explain the reality of what this means
Iso-chemical process - explain the reality of what this means
Contact vs. Regional metamorphism
Litho-static vs. directed stress environments
Foliated vs. non-foliated rocks
The metamorphic process
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Foliated metamorphic rocks
Usually associated with regional metamorphism
Result of heat and directed pressure
Therefore will generally exhibit a distinct layering
There is a fairly specific progression through the main metamorphic sequence
For example, starting with Shale - a common sedimentary rock
Very fine grain
Add HEAT and PRESSURE and it metamorphoses to...
Slate - little or no significant visible change
Still microscopic grains
Mineralogy begins to change
Usually to mica
Add more HEAT and PRESSURE and it metamorphoses to...
Phyllite - begin to see mineral grains
Commonly lots of mica - gives rock a shinny look
Add more HEAT and PRESSURE and it metamorphoses to...
Schist - significant change
Foliation of micaceous minerals (muscovite and/or biotite)
Other minerals begin to form based on composition of original rock and newconditions
Use additional minerals as modifier of name
EX: Hornblende schist, quartz schist, etc.
Add more HEAT and PRESSURE and it metamorphoses to...
Gneiss - high grade metamorphic rock
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Color banding of light and dark minerals
Add more HEAT and PRESSURE and it metamorphoses to...
Migmatite - Partial melting of felsic minerals
RememberBowen's Reaction Series?
The felsic minerals will melt at lower temperatures
Results in a rock with layers of "granite" and high grade mafic gneiss
Add enough HEAT and PRESSURE and it ultimately melts to form magma...
Granite or ??? - a new igneous rock after complete melting
Non-Foliated metamorphic rocks
Usually associated with contact metamorphism
Result of heat and litho-static pressure
Therefore will generally not exhibit a distinct layering
Marble - metamorphosed limestone
Relatively soft and will pass the fizz test
Generally coarsely crystalline
But can also be fine grained
Can be quite beautiful - many colors
Commonly used in carving
Carrera Marble - Italy
Very vine grain and pure
Quartzite - metamorphosed sandstone
Generally very hard and resistant
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Andesite
Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. The specimen shown isabout two inches (five centimeters) across.
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Basalt
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Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters)across.
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Diorite
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz. The specimen shown above is abouttwo inches (five centimeters) across.
http://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/diorite.shtmlhttp://geology.com/rocks/basa
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