Rock Hound Lab
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Transcript of Rock Hound Lab
Rock Hound Lab
Identify Major Characteristics of Different Rock Textures
Igneous rocks can be identified by the determination of the
composition and texture of the rock.
Once these two characteristics have been identified, the Igneous Rock
Identification chart is used to identify the rock name.
Igneous Rock Identification ChartCOMPOSITION
TEXTURE Felsic Intermediate Mafic Ultramafic
Pegmatitic Granite Pegmatite
Diorite Pegmatite
Gabbro Pegmatite
Phaneritic Granite Diorite Gabbro Dunite
Aphanitic Rhyolite Andesite Basalt
Porphyritic Rhyolite Andesite Basalt
Glassy Obsidian Basaltic Glass
Vesicular Pumice Scoria
Pyroclastic Volcanic Tuff
Composition• Color is often an indicator of the composition of most igneous rocks• Felsic composition - Light colors, including white, light gray, tan and
pink• Felsic compositions are rich in silica (SiO2) (sand, quartz, diatoms)• Mafic or Ultramafic composition - Dark colors, such as black and dark
brown • Mafic compositions are poor in silica, but rich in iron (Fe) and
magnesium (Mg)• Intermediate compositions - often gray or consisting of equal parts of
dark and light mineral• Beware that even though an igneous rock may have a felsic
composition (light color), the rock can contain dark colored minerals• Mafic rocks may contain light colored minerals as well• As mentioned above, the composition of most igneous rocks can be
identified using this system, formally known as the Color Index
Two Most Notable Exceptions!
• Obsidian and Dunite• Obsidian is volcanic glass which erupts as a lava flow• Most obsidian is felsic in composition, yet typically it
will have a very dark color (dark brown to black)• Dunite has an ultramafic composition yet is apple
green to yellowish green in color• Dunite is composed almost entirely of the mineral
olivine which usually contains both iron and magnesium
Color Index
Felsic Intermediate Mafic
Igneous Rock Composition Chart
Felsic Intermediate Mafic
EARTH SCIENCE LABComposition of Igneous Rocks
Texture
• The texture of an igneous rock does not refer to the roughness or smoothness of the surface• Textures are based primarily
on crystal size
Texture• Pegmatitic texture is composed of very large crystals
(larger than 2-3 cm)• Phaneritic texture is composed of crystals which are large
enough to see but smaller than pegmatitic texture, and the entire rock is composed of crystals
• Aphanitic texture is a fine grained texture but the crystals are too small to see (ie. Basalt)
• Porphyritic texture is composed of crystals of two different sizes (larger crystals embedded in finer grain matrix)
• Large crystals (phenocrysts) are visible while the smaller crystal are not (referred to as groundmass)
Phaneritic Texture • Phaneritic ("FAN-a-RIT-
ic") rocks • Have mineral grains that
are large enough to be seen with the naked eye or a hand lens
• like this granite
Equigranular Texture
• Rocks with equigranular ("EC-wi-GRAN-ular") texture
• Have mineral grains that are generally the same size
• This example is a granite
Aphanitic Texture• Aphanitic ("AY-fa-NIT-
ic") rocks • have mineral grains that
are mostly too small to be seen with the naked eye or a hand lens
• like this rhyolite
Porphyry • Porphyry ("PORE-fer-
ee") • Is a name used for any
igneous rock with conspicuous larger grains—
• phenocrysts—floating in a fine-grained groundmass
Porphyritic Texture
• Rocks with porphyritic ("POR-fi-RIT-ic") texture
• Like this andesite • Have larger mineral
grains, or phenocrysts ("FEEN-o-crists"), in a matrix of smaller grains
• Andesite is an extrusive or intrusive igneous rock that is higher in silica than basalt and lower than rhyolite or felsite
Texture
• Glassy texture is the most readily recognized. The rock is composed entirely of glass. Few, if any, crystals will be visible
• Vesicular texture is formed when lava solidifies before gases are able to escape ("bubbly" appearance)
• Pyroclastic texture is composed of volcanic fragments. These fragments or clasts [fine (ash) or coarse (lapilli) or very coarse (bombs and blocks)]
Glassy Texture • Glassy (or hyaline or
vitreous) rocks • Have no or almost no
grains at all, as in this quickly chilled pahoehoe basalt or in obsidian
• Obsidian is an extrusive rock:
• lava that cooled without forming crystals, giving it a glassy texture
• Basalt most common lava rock type
Pahoehoe and Aa BasaltPronounced – “Pa-ho-ho” "ah-ah."
Both of these basalt flows have the same composition, but while they were molten, the smooth pahoehoe lava was hotter than the jagged aa lava
Pahoehoe Basalt
Obsidian
Basalt
Obsidian Flow in Oregon
Obsidian Blocks Obsidian flows develop a blocky surface as their outer shell quickly solidifies
Big Obsidian Flow in Newberry Caldera, Oregon
Obsidian Flow Texture Obsidian may display complex folding and segregation of minerals in bands and round masses consisting of feldspar or cristobalite (high-temperature quartz).
Weathering of Obsidian Water attacks obsidian readily because none of its material is locked up in crystals, making it prone to alteration into clays and related minerals.
Obsidian Tools
Obsidian is the best material for making stone tools. The stone doesn't need to be perfect to make useful implements
Vesicular Texture • Rocks with vesicular
("ve-SIC-ular") texture
• Are full of bubbles• It always indicates a
volcanic rock, like this scoria
• Pumice is basically lava froth
• An extrusive rock frozen as its dissolved gases come out of solution
• It looks solid but often floats on water
Scoria (Volcanic Cinders)
Like pumice, is a lightweight extrusive rock, but scoria has large, distinct gas bubbles and a darker color (landscape product and running tracks)
Unlike pumice, scoria usually has broken, connected bubbles and does not float in water
Pumice
Used for abrasive grit or soil amendments
Pyroclastic Texture • Rocks with
pyroclastic ("PY-ro-CLAS-tic") texture
• Are made of pieces of volcanic material
• Created in an explosive eruption
• Like this welded tuff• Tuff is technically a
sedimentary rock formed by the accumulation of volcanic ash plus pumice or scoria
Test Your Igneous Rock Identification IQ• http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/igneous/Igsamp01.htm
• Click on the link above• You will see your 1st rock to identify. • Click on all the images provided for different views of this unknown rock• Identify the Composition by identifying the color and determine the
Texture by examining the crystal sizes that are present. • Use the information provided in class about igneous rock Composition and
Textures.• Now, click the appropriate Composition and Texture for this rock, then click
the right Rock Type.• Once this is done, click “Grade Identification” for your answer.• Write all your answers for each of the categories on the answer sheet
provided to help you identify each rock type• Do all 12 samples.
#1 -
• Composition• • Texture•
#2 -
• Composition• • Texture•
#3 -
• Composition• • Texture•
#4 -
• Composition• • Texture•
#5 -
• Composition• • Texture•
#6 –
• Composition• • Texture•
#7 -
• Composition• • Texture•
#8 -
• Composition• • Texture•
#9 - Rhyolite
• Composition• Felsic• Texture• Porphyritic
#10 -
• Composition• • Texture•
#11 -
• Composition• • Texture•
#12 –
• Composition• • Texture•
Metamorphic Rocks
These are what happens when sedimentary and igneous rocks become changed, or
metamorphosed, by conditions underground
The 4 Main Agents That Metamorphose Rocks
• Heat• Pressure• Fluids• Strain• These agents can act and interact in an infinite
variety of ways. • As a result, most of the thousands of rare minerals
known to science occur in metamorphic ("shape-changed") rocks.
Heat and pressure
• The clay minerals of sedimentary rocks are a good example
• Clays are surface minerals, which form as feldspar and mica break down in the conditions at the Earth's surface.
• With heat and pressure they slowly return to mica and feldspar
• Feldspar & Mica Clays + Heat & Pressure • Feldspar & Mica
Fluids• Important agent of metamorphism• Every rock contains some water• Sedimentary rocks hold the most• First there is the water that was trapped in the
sediment as it became rock• Second is the water that is liberated by clay
minerals as they change back to feldspar and mica• This fluid may be acidic or alkaline or full of sulfides
or carbonates or metals
Strain• Refers to any change in the shape of rocks due to
the force of stress• Movement on a fault zone is one example• In shallow rocks, shear forces simply grind and
crush the mineral grains
Foliation• Mineral layers, observed when identifying a
metamorphic rock• Increased strain, the foliation becomes more
intense, the mineral layers become thicker • The foliated rock types that form under these
conditions are called schist or gneiss, depending on their texture
• Schist is finely foliated • Whereas gneiss is organized in wide bands of
minerals
The Basic Metamorphic Rock Types
Sedimentary rock
• Shale slate phyllite mica-rich schist• The mineral quartz does not change under high
temperature and pressure, although it becomes more strongly cemented
• Sandstone quartzite• Mixture of sand and clay mudstones schists or
gneisses• Limestone marble
Metamorphic Rocks• Rocks that have undergone a change from their
original form due to • Temperature, pressure or chemical alteration• Their classification is based on the minerals that are
present • And the temperature and pressure at which these
minerals form
Texture
Texture is divided into two groups
Foliated and Unfoliated Rocks • First steps towards identifying metamorphic rocks is
to determine if it has foliation or not• Foliation means banding or alignment of the
minerals resulting from pressure• Foliated textures can be flat like a piece of slate or
folded. • Non-foliated textures have minerals that are not
aligned. Essentially, the minerals are randomly oriented.
• A good example of that is the banding present in a metamorphic rock called gneiss (pronounced nice)
Foliation• Four types of foliation• Slaty cleavage - composed of platy minerals that
are too small to see. Typically, these rocks split along parallel, planar surfaces
• Phyllitic foliation - composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye
• The larger size gives the foliation a slightly shiny appearance
Foliation• Four types of foliation• Schistose foliation - is composed of larger minerals
which are visible to the unaided eye• Platy minerals tend to dominate• Gneissic banding - is the easiest of the foliations to
recognize• It is composed of alternating bands of dark and light
minerals
Non-Foliation• Identified by their lack of planar character• Identification of non-foliated rocks is dependent on
the composition of the minerals or components in the rock
• Anthracite coal is similar to bituminous coal• Both are black in color , and is composed of carbon• Anthracite coal is generally shiny in appearance and
breaks with a conchoidal fracture (broken glass also shows this type of fracture)
Non-Foliation• Metaconglomerate is composed of pebbles and
gravel that have been flattened due to directed pressure
• Quartzite is composed of quartz sand grains• Quartz has a hardness of 7, which makes it difficult
to scratch• Marble is composed of calcite and will readily react
to a small drop of HCl
Metamorphic Rock Identification Chart
TEXTURE FOLIATION COMPOSITION TYPE PARENT ROCK ROCK NAME
Foliated
slaty mica Regional Mudstone Slate
phyllitic quartz, mica, chlorite Regional Mudstone Phyllite
schistose mica, quartz Regional Slate Schist
schistose amphibole, plagioclase Regional Basalt or Gabbro Amphibolite
gneissic banding feldspar, mica, quartz Regional Schist Gneiss
Non-Foliated
carbon Contact or Regional Bituminous Coal Anthracite Coal
quartz, rock fragments Contact or Regional Conglomerate Metaconglomerate
calcite Contact or Regional Limestone Marble
quartz Contact or Regional Sandstone Quartzite
Test Your Metamorphic Rock Identification IQ
• http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/metamorf/Meta02.htm
• Click on the link above• You will see your 1st rock to identify. • Click on all the images provided for different views of this unknown
rock• Identify the correct Texture (Foliated or Non-Foliated) by using the
information provided in class about metamorphic rocks • Now, click the appropriate Foliation, Composition, and Parent Rock
for this rock type, then click the right Rock Type.• Once this is done, click “Grade Identification” for your answer.• Write all your answers for each of the categories on the answer sheet
provided to help you identify each rock type• Do all 8 samples.
#1 - • Texture• • Foliation• • Composition• • Parent Rock•
#2 - • Texture• • Foliation• • Composition• • Parent Rock•
#3 - • Texture• • Foliation• • Composition• • Parent Rock•
#4 - • Texture• • Foliation• • Composition• • Parent Rock•
#5 - • Texture• • Foliation• • Composition• • Parent Rock•
#6 - • Texture• • Foliation• • Composition• • Parent Rock•
#7 – • Texture• • Foliation• • Composition• • Parent Rock•
#8 – • Texture• • Foliation• • Composition• • Parent Rock•
Sedimentary Rocks
Sedimentary Rocks
• Composed of sediment.• Sediment is deposited in a number of
environments of deposition, by both moving air and moving water• Identification is primarily based on
composition
Texture
• Three types of "texture" are considered – • Clastic, Chemical, Biologic
1. Clastic – • Contain clasts• Fragments or pieces of rock or minerals• The composition of clastic sedimentary rocks is divided into three
types – • Clay/silt, sand, and gravel• Clay and silt are less than 1/16 mm• Not visible to the unaided eye• Sand is clasts between 1/16 and 2 mm in size• Gravel is greater than 2 mm
Texture2. Chemical –• Mineral from which they are composed.• Quartz, halite, gypsum and calcite• Quartz has a hardness of 7 and is very difficult to scratch • Gypsum is relatively soft (Hardness =2) and can be
scratched easily with a fingernail• Halite is common table salt and is most easily identified by
taste (not recommended)• Halite has a hardness of 2.5 and cannot be scratched by a
fingernail (unpolished fingernail) • Calcite readily reacts with a small drop of HCl
Texture3. Biologic –• Form as the result of the accumulation of organic material or
biologic activity• Coal is usually obvious to most people • The dark brown to black color is the most obvious charateristic• Coquina and limestone are both composed of calcite• Coquina is composed almost entirely of shell or fossil fragments• Limestone may or may not contain fossils fragments• Both will react to HCl• Limestone containing fossils is referred to as fossiliferous
limestone
Sedimentary Rock Identification ChartTEXTURE GRAIN SIZE COMPOSITION ROCK NAME
Clastic
>2 mm rounded quartz, feldspar and rock fragments Conglomerate
>2 mm angular quartz, feldspar and rock fragments Breccia
1/16 - 2 mm quartz, feldspar Sandstone>1/16 mm feldspar, quartz Arkose
<1/16 mm quartz, clay minerals Siltstone(Mudstone, Shale)
<1/256 mm quartz, clay minerals Claystone
Chemical
silica (quartz) Chertdolomite Dolostone
calcite Limestonehalite Rock Salt
gypsum Rock Gypsum
Biologic
silica (quartz) Chert
loosely compacted organic material and plant fragments Peat
densely compacted organic material and plant fragments Bituminous Coal
calcite Limestone
calcite, micro-skeletal fragments Chalk
calcite, almost entirely shell and skeletal fragments Coquina
calcite with some shell and skeletal fragments Fossiliferous Limestone
dolomite with some shell and skeletal fragments Fossiliferous Dolostone
Test Your Sedimentary Rock Identification IQ
• http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/sediment/Sed01.htm
• Click on the link above• You will see your 1st rock to identify. • Click on all the images provided for different views of this unknown rock• Identify the Texture, Grain Size, Composition and Rock Type• Identify the correct Texture (Clastic, Chemical, or Biologic) by using the
information provided in class about sedimentary rocks • Now, click the appropriate Grain Size and Composition • Then click the right Rock Name• Once this is done, click “Grade Identification” for your answer.• Write all your answers for each of the categories on the answer sheet
provided to help you identify each rock type• Do all 12 samples.
#1 - • Texture• • Grain Size• • Composition•
#2 - • Texture• • Grain Size• • Composition•
#3 – • Texture• • Grain Size• • Composition•
#4 – • Texture• • Grain Size• • Composition•
#5 – • Texture• • Grain Size• • Composition•
#6 - • Texture• • Grain Size• • Composition•
#7 – • Texture• • Grain Size• • Composition•
#8 - • Texture• • Grain Size• • Composition•
#9 - • Texture• • Grain Size• • Composition•
#10 - • Texture• • Grain Size• • Composition•
#11 - • Texture• • Grain Size• • Composition•
#12 – • Texture• • Grain Size• • Composition•
Now that you are done……
Study your identifications for each Rock Type from each Rock Class. Know the difference and how each
class was formed. We will go over your answers tomorrow and review for the quiz on Friday
Turn in your answer sheet at the end of the period.