Download Classification of Rocks

Geol 301, Spring 2007
Inquiry into the Rock Cycle
Part 1. The stories are in the rocks and the rocks are recycled
1. Read or listen to the story, Stone Wall Secrets. What stories do the rocks in
grandpa’s wall contain?
2. Examine Sample set I. These samples represent materials (sediment and rocks)
described in the story. Describe the three samples.
a. Sample A Description:
b. Sample B Description:
c. Sample C Description:
d. How are these samples the same and how are they different?
e. Look at the attached Rock Cycle diagram that accompanies the story.
Where do Samples A, B, and C fit in this rock cycle?
f. According to this rock cycle and the story, what processes transformed the
granite into quartz sand and then into quartz sandstone?
3. Add samples D and E to your collection. Describe these samples.
a. Sample D Description:
b. Sample E Description:
c. How are Samples D and A similar and how are they different?
d. How are samples E and C similar and how are they different?
Note that the Earth process of “metamorphism” can transform
rock A  rock D, and
rock C  rock E.
4. Examine Sample Set II.
a. How are samples F and G similar and how are they different?
b. What relationship might there be between these two samples?
5. Examine Sample Set III.
a. How are samples H and I similar and how are they different?
b. What relationship might there be between these two samples?
6. Examine Sample Set IV.
a. How are samples J and K similar and how are they different?
b. What relationship might there be between these two samples?
7. Examine Sample Set V. These are both minerals that you described and identified
in a previous activity. What minerals are the?
a. Mineral L =
Chemical formula is (Na,Ca)(Si,Al)4O8
b. Mineral M =
Chemical formula is Al2Si2O5(OH)4
c. How are samples L and M similar and how are they different?
Although it isn’t obvious by their physical appearance, the Earth process of
“weathering” will transform mineral L  mineral M.
Part 2. Get to know YOUR rock
What are its properties or characteristics? Use descriptors that make sense to you. (What
do you observe? How did you observe it?)
1. My rock is/has:
Part 3: Building the Lithosphere
Goal: to learn how to distinguish among igneous, metamorphic, and sedimentary rock
groups, and identify common rocks within each group.
Introduction
(adapted with permission from Abbott, R.N., 2002, http://www.appstate.edu/~abbottrn/rck-id/)
A rock is a naturally occurring aggregate of minerals, and certain non-mineral materials
such as fossils and glass. Just as minerals are the building blocks of rocks, rocks in turn
are the natural building blocks of the Earth's LITHOSPHERE (crust and mantle down to
a depth of about 100 km), ASTHENOSPHERE (although this layer, in the depth range
from about 100 to 250 km, is partially molten), MESOSPHERE (mantle in the depth
range from about 250 to 2900 km), and even part of the CORE (while the outer core is
molten, the inner core is solid). Most rocks now exposed at the surface of the Earth
formed in or on continental or oceanic crust. Many such rocks, formed beneath the
surface and now exposed at the surface, were delivered to the surface from great depths
in the crust and in rare cases from the underlying mantle. There are two general ways
that rocks come to be exposed at the surface:
1. Formation at the surface (e.g., crystallization of lava, precipitation of calcite or
dolomite from sea water)
2. Formation below the surface, followed by tectonic uplift and removal of the
overlying material by erosion
For the following discussion refer to the attached generalize Rock Cycle diagram.
There are three major classes of rocks, IGNEOUS, SEDIMENTARY, and
METAMORPHIC, with the following attributes:
IGNEOUS ROCKS form by crystallization from molten or partially material, called
MAGMA. Magma comes mainly from two places where it is formed, (1) in the
asthenosphere and (2) in the base of the crust above subducting lithosphere at a
convergent plate boundary. There are two subclasses of igneous rock, VOLCANIC
(sometime called EXTRUSIVE), and PLUTONIC (sometimes called INTRUSIVE).
VOLCANIC ROCKS form at the Earth's surface. They cool and crystallized
from magma which has spilled out onto the surface at a volcano. At the surface,
the magma is more familiarly known as LAVA.
PLUTONIC ROCKS form from magma that cools and crystallizes beneath the
Earth's surface. In a sense, this is the portion of the magma that never makes it to
the surface. For the plutonic rock to become exposed at the surface, it must be
tectonically uplifted and the overlying material must be removed by erosion.
SEDIMENTARY ROCKS form from material that has accumulated on the Earth's
surface. The general term for the process of accumulation is DEPOSITION. The
material consists of the products of weathering and erosion, and other materials available
at the surface of the Earth, such as organic material. The process by which this otherwise
unconsolidated material becomes solidified into rock is variously referred to
LITHIFICATION (literally turned into rock), DIAGENESIS or CEMENTATION. Like
volcanic rocks, some sedimentary rocks are "lithified" right at the surface, for instance by
direct precipitation from sea water. Other sedimentary rocks, like plutonic igneous rocks,
are "lithified" below the surface, when they are buried under the weight of overlying
sediment. And like the plutonic rocks, sedimentary rocks which were lithified below the
surface only become exposed at the surface by tectonic uplift and erosion of the overlying
material.
METAMORPHIC ROCKS form when a sedimentary or igneous rock is exposed to high
pressure, high temperature, or both, deep below the surface of the Earth. The process,
METAMORPHISM, produces fundamental changes in the mineralogy and texture of the
rock. The original rock, prior to metamorphism, is referred to as the PROTOLITH. The
protolith can be either an igneous rock or a sedimentary rock, as just indicated. The
protolith could also be a previously metamorphosed rock. Ultimately however, if you go
far enough back into the history of a metamorphic rock you would find that the first
protolith was either a sedimentary or igneous rock. Because all metamorphic rocks form
below the surface, for them to become exposed at the surface, they must undergo tectonic
uplift and removal of the overlying material by erosion.
In this part of the exercise you will examine some common igneous, sedimentary and
metamorphic rocks, and learn to classify them.
Classification of Rocks
The classification of rocks is based on two criteria, TEXTURE and COMPOSITION.
The texture has to do with the sizes and shapes of mineral grains and other constituents in
a rock, and how these sizes and shapes relate to each other. Such factors are controlled
by the process that formed the rock. Because igneous, sedimentary, and metamorphic
processes are distinct, so too the resulting textures are distinct. Thus there are distinct
igneous textures, distinct sedimentary texture, and distinct metamorphic textures. For the
purposes of this exercise and routine classification, the kinds of minerals and their
proportions, or MINERALOGY, are taken as the natural expression of composition.
Fortunately for you, just as the three classes of rocks each have distinct textures, so too
do they have distinct mineralogies. Details of TEXTURE and COMPOSITION are
highlighted on the individual classification charts for igneous, sedimentary and
metamorphic rocks.
Following are rock identification charts for igneous, metamorphic and sedimentary rocks.
Use these to help you identify the set of common rocks provided to you for this lab
activity.
Understanding Rocks
Igneous
Basalt
Granite
Obsidian
Gabbro
Pumice
Scoria
Rhyolite (porphyry)
Dunite or Peridotite
[Tuff]
Metamorphic
Marble
Slate
Schist
Gneiss
Quartzite
Anthracite Coal
Greenstone
Sedimentary
Siltstone
Dolostone
Bituminous Coal
Micrite (fine-grained limestone)
Fossiliferous Limestone
Shale
Sandstone
Conglomerate
Chalk
Oolitic Limestone
Phosphate nodule
[Gyprock]
Rock
#
Rock Type
1
Igneous
2
Igneous
3
Igneous
4
Igneous
5
Igneous
6
Igneous
7
Igneous
8
Igneous
9
Metamorphic
10
Metamorphic
11
Metamorphic
12
Metamorphic
13
Metamorphic
14
Metamorphic
15
Metamorphic
Texture
Composition
Other
Distinguishing
Characteristics?
Rock Name
Rock
#
Rock Type
16
Sedimentary
17
Sedimentary
18
Sedimentary
19
Sedimentary
20
Sedimentary
21
Sedimentary
22
Sedimentary
23
Sedimentary
24
Sedimentary
25
Sedimentary
26
Sedimentary
Texture
Composition
Other
Distinguishing
Characteristics?
Rock Name
Answer the following questions to further help you become more familiar with
characteristics of particular rocks:
1. Is rock I-1 intrusive or extrusive?
How do you know?
2. Is rock I-3 intrusive or extrusive?
How do you know?
3. How can you distinguish between dolostone and micrite?
4. What is the protolith of marble?
5. What is the protolith of quartzite?
6. How can you distinguish among marble, quartzite, and quartz (clean) sandstone?
7. How can you distinguish between slate and shale?
8. How can you distinguish among coal, basalt, and obsidian?
Food for thought questions……
9. How does the biosphere (living things) contribute to the formation of some
sedimentary rocks?
10. Which rock do you think best represents the composition of the mantle?
11. Which rock(s) do you think form at mid oceanic ridges?
Part 4. Getting to know YOUR rock….again
1. Without looking back at your original description in Part 1, describe your rock
again.
My rock is/has:
2. Where do you think your rock fits in the rock cycle? Is it igneous, sedimentary, or
metamorphic? Why?
Check with me to find out if you are correct.
3. By following the approach laid out in Part 2, you should now be able to give your
rock a name.
a. My rock is named:
b. By what process does your rock form?
c. By what processes can your rock be destroyed (or to be more positive,
reborn as something else)?