Do the qustion in the Word, follow the video. The PDF is the supplemental things
Geology 1 laboratory
This lab will introduce you to the wonderful world of igneous rocks!! Please use the supplied Ã¢â‚¬Å“Igneous
Rocks Supplemental InformationÃ¢â‚¬Â sheet and webpage/videos linked below to answer the questions in
this lab. Have fun!!!
Informational/Lab Webpage: http://profharwood.x10host.com/GEOL101/Labs/Igneous/index.htm
Igneous Rock Identification Video: https://www.youtube.com/watch?v=cjyF-te4lQI
Supplemental Information Sheet: Supplied on Canvas!!
1. Please define the following terms in your own words:
a. Igneous Rock:
b. Magma: _________________________________________________________________
c. Lava: ___________________________________________________________________
d. Igneous Rock Texture:
e. Igneous Rock Composition:
2. Please define the following principle igneous rock types in your own words:
a. Extrusive (i.e. Ã¢â‚¬Å“volcanicÃ¢â‚¬Â) Igneous Rocks:
b. Intrusive (i.e. Ã¢â‚¬Å“plutonicÃ¢â‚¬Â) Igneous Rocks:
3. What is the main difference between extrusive and intrusive igneous rocks that can be used to
tell these rock types apart (HINT #1: ItÃ¢â‚¬â„¢s not color!! Hint #2: Pictures of extrusive/intrusive rocks
are provided below):
4. IGNEOUS ROCK TEXTURES: Please complete the table below by: 1) placing the following igneous
rock TEXTURES next to their matching descriptions/definitions, 2) placing a picture of a rock
exhibiting this rock texture in the column marked Ã¢â‚¬Å“PICTURE COLUMNÃ¢â‚¬Â, and 3) specifying
whether each rock texture is an intrusive/plutonic or an extrusive/volcanic rock feature, OR
BOTH. The first is done for you:
Mineral grains are large enough to
be seen without the assistance of a
microscope and are of similar sizes.
Formed when magma cools very
rapidly resulting in the formation of a
Volcanic rocks containing lots of
Ã¢â‚¬Å“holesÃ¢â‚¬Â or vugs resulting from the
release of gases while lava cools.
Mineral grains are too small to be
seen without the assistance of a
Texture describing igneous rocks
made of very large crystals (i.e.
minerals >1cm in diameter).
Minerals are both large and small.
Small minerals may be so small that
they cannot be seen without a
microscope, thus having a much
Rocks formed when ash/rock/pumice
fragments lock together during a
5. Please complete the following table by: 1) placing the following igneous rock COMPOSITIONS
next to their matching descriptions/definitions, 2) specifying the intrusive rock with that
composition 3) specifying the extrusive rock with that composition. The first is done for you:
Rock containing more
than 50% iron-rich,
dark, heavy minerals
like olivine, pyroxene,
amphibole and biotite.
A mixture of ~30-50%
mafic materials and
More than 80% ironrich, dark, heavy
minerals like olivine
6. Please visit the interactive Lab Webpage linked here:
At the bottom of the page are 12 Ã¢â‚¬Å“Samples to IdentifyÃ¢â‚¬Â. Please visit the pages for each of the 12
samples and supply your responses in the table below. Please copy and paste the rock pictures provided
on the webpage for each sample in the final column of the table. Sample #1 is done for you:
***Note*** Ã¢â‚¬â€œ To complete this exercise, you will need to select the proper texture, composition,
and rock name (i.e. a total of 3 boxes will be selected for each sample). To see if you have the correct
texture, composition, and rock name, click the Ã¢â‚¬Å“Grade IdentificationÃ¢â‚¬Â button at the bottom of the page
and feel free to modify your responses for each of these items until you get the right answers!! Provide
those correct answers in the table below!!
Geology 1 laboratory
Igneous rocks are those that crystallize, or solidify, from molten rock. Intrusive igneous rocks (plutonic rocks) can
be distinguished from other rock types because they are formed of interlocking crystals that have grown in place.
1. Textures of plutonic rocks:
Magma (underground molten rock) that cools and crystallizes slowly beneath the Earth’s surface is called intrusive
igneous rock or plutonic rock. Because intrusive rocks cool and crystallize very slowly, the crystals have time to
grow to large sizes, so they are all large enough to see without the aid of a microscope. Such rocks are said to
have a phaneritic texture. If the magma at first cools extremely slowly the first crystals to form grow quite large.
Then sometimes the partly crystallized magma will begin to cool a little bit faster, so that the rest of the crystals are
somewhat smaller. The resulting texture of large crystals (phenocrysts) surrounded by smaller ones (groundmass)
is called a porphyritic texture.
Pegmatitic texture describes plutonic rocks that made mostly of crystals that are larger than 1 cm in diameter.
2. Textures of volcanic rocks:
When magma erupts onto the Earth surface where it cools rapidly, the resulting rocks are called extrusive igneous
rocks or volcanic rocks. Extrusive igneous rocks cool so rapidly that often no crystals are visible in the rock.
Volcanic rocks may have several different textures.
Glassy textures results when magma cools so rapidly that atoms cannot organize to form the ordered atomic
structures of minerals and thus a glass is formed. The rock is a random jumble of atoms with no crystalline
structure. A common volcanic rock that has a glassy texture is known as Obsidian. The obsidian usually exhibits
a curved fracture when broken, this is called a conchoidal fracture.
Aphanitic textures result when magma cools quickly, but a little more slowly than for a glassy texture. If this type
of rock is viewed without a microscope, it appears to have no crystalline texture. However when a microscope is
used the rock is clearly composed of microscopic sized interlocking crystals.
Rocks with an aphanitic texture are named on the basis of their composition, which can be determined by the rocks
color. For example: a light-colored aphanitic rock is normally a Rhyolite, whereas a black colored aphanitic rock is
a Basalt and rocks that have an intermediate composition are usually grayish in color and are Andesites.
Porphyritic textures are those in which there are some larger crystals in a much finer grained ground mass. This
texture can form in either volcanic or plutonic rocks. In volcanic rocks, it forms when magma begins to crystallize
slowly underground (forming large crystals), and then is erupted onto Earth’s surface where the remaining liquid
cools very rapidly to form much finer grained crystals or glass. The larger crystals in a rock with a porphyritic
texture are called phenocrysts.
Vesicular texture is a term that describes volcanic rocks that have holes in them. The holes are called vesicles
and form as the result of gas bubbles in the magma. Rocks that have a moderate amount of vesicles are named
according to their composition, such as vesicular basalt. Rocks that are extremely vesicular are called pumice if
they are rhyolitic (light-colored), or scoria if they are basaltic (dark-colored).
Pyroclastic texture describes rocks that formed from the accumulation of volcanic ash, crystals, pumice and rock
fragments that were explosively ejected from a volcano. Rocks that have a pyroclastic texture are often called tuff.
If the volcanic ash is very hot when it falls to the ground, the particles may be fused together to form a rock called a
Classification of igneous rocks:
Igneous rocks are classified based on their texture and composition. The texture of an igneous rock tells you
whether it is plutonic or volcanic. Plutonic rocks have a phaneritic texture; volcanic rocks may have glassy,
aphanitic, vesicular or pyroclastic textures.
The descriptions below list the plutonic names first, then their extrusive or volcanic equivalent.
This rock normally contains abundant Quartz, Potassium Feldspar, Plagioclase Feldspar, and may contain some
combination of Muscovite, Biotite, Hornblende, and Pyroxene. If the crystals of this rock are very coarse, and are
mostly more than one centimeter in diameter than we call this a Pegmatitic Granite. If magma with a granitic
composition is erupted on the surface of the Earth we call it a Rhyolite. Rhyolite typically has either a glassy texture
with no mineral grains present, or a porphyritic texture with a few mineral grains present. Rocks often are pinkish.
WTP S2018 v1.8
Rocks with this composition typically contain Plagioclase Feldspar and Hornblende, as well as Pyroxene and
Biotite. This type of rock has less than 10 percent Quartz and often none at all. Diorite and Andesite are known as
intermediate rocks because they are between the light-colored Granite/Rhyolite and the very dark-colored
Gabbro/Basalt. The Diorite commonly looks like salt-and-pepper, while the volcanic equivalent Andesite typically
looks gray or tan. Like Rhyolite, Andesite typically has either a glassy texture with no minerals present, or a
porphyritic texture with a few mineral grains present.
Rocks with this composition always contain Plagioclase Feldspar and Pyroxene, and may contain Hornblende and
Olivine. These rocks are typically black to dark green in appearance and may feel very heavy.
Rocks with this composition are derived from the mantle and are composed mostly of Olivine, with a little Pyroxene
and occasionally some metals such as Iron Pyrite. Komatiite is the extrusive equivalent of peridotite but is
If we look just at the dark mineral component of igneous rocks. the following rules hold true:
Granite and Rhyolite are composed of 2 to 20% mafic (dark colored) minerals.
Diorite and Andesite are composed of 20 to 30% mafic minerals.
Gabbro and Basalt are composed of 30 to 70% mafic minerals.
Peridotite has more than SO% mafic minerals.
Hints for identifying minerals in igneous rocks:
1. Quartz has no cleavage and usually appears as a gray or colorless blob, it may be shiny or dull.
2. Plagioclase feldspar is typically white, and makes rectangles. Straight lines called striations may also be visible.
3. Potassium feldspar (k-spar) is typically pink or buff colored and makes rectangles. Wavy lines called exsolution
lamellae are usually visible.
4. Pyroxene is dark green to black and typically makes square shapes with a very dull luster.
5. Hornblende (amphibole) typically makes crystals that are longer than they are wide and are a very shiny black.
6. Biotite may be black or brown or even gold in color, and often appears as rounded grains with a flaky surface.
7. Muscovite may be clear or silver and also appears as rounded grains with a flaky surface.
S. Olivine is typically light green in color, may be shiny or dull, and is often a mass of small grains. Olivine may also
9. Garnets are typically reddish-brown in color and appear as rounded grains.
10. Iron pyrite is gold in color and may be any shape. It also has a metallic luster.
Observing Igneous Rocks:
1. Textures — review:
Phaneritic: entire rock made of interlocking crystals large enough to see with the naked eye. The rock is plutonic.
Pegmatitic: rock made of crystals greater than 2 cm in size. The rock is plutonic.
Porphyritic: rock made of crystals of 2 very different sizes. The rock may be volcanic, or plutonic.
Aphanitic: rock contains crystals to small to see with the naked eye. The rock is volcanic.
rock contains no crystals. The rock is volcanic.
Vesicular: rock has holes formed by expanding gasses (looks like bubbles). The rock is volcanic.
Pyroclastic: rock is formed of volcanic ash and rock fragments.
2. Composition — review:
Felsic: greater than 70% light silica-rich minerals like quartz, feldspar and muscovite.
Intermediate: a mixture with 30 – 50% mafic and 50 – 70% felsic mineral grains.
Mafic: greater than 50% iron-rich, dark, heavy minerals like olivine, pyroxene, amphibole and Biotite.
Ultramafic: greater than SO% iron-rich, dark, heavy minerals like olivine and pyroxene, may have metals too.
WfP S2018 v1.8
Nammg Igneous Rock s:
“full of holes” May proceed any of
The names of Aphanitic rocks.
e.g. vesicular basalt, etc.
Pumice – very vesicular frothy glass
Scoria – vesicular frothy basalt, may look similar to but is
heavier than pumice
Obsidian – solid glass, usually black
Tuff – powdery rock made of lithified volcanic ash
Volcanic Breccia – A Tuff that contains pieces larger
than 2 millimeters
*REMEMBER, THERE ARE ALWAYS EXCEPTIONS TO THE RULES!
Darkness and specHie gravity increase
Igneous rock mineral composition chart
Charts for estimating percentage composition of rocks and sediments, after R. D. Terry and G.
V. Chilingar, Journal of Sedimentary Petrology, v. 25, pp. 229 – 234, 1955.
WTP S2018 v1.8
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