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.