Download Exercise 1 Rock Review

Exercise 1
Rock Review
The purpose of this lab exercise is to help refresh your memory of igneous,
metamorphic and sedimentary rock types.
Rocks are the physical record of past events in Earth’s history. Therefore,
with an understanding of how different kinds of rocks were formed, it is
possible to reconstruct conditions and processes operating at various times
in the geologic past. The ultimate goal of geology is to resonstruct those
conditions and processes over the entire course of Earth’s history.
BUT
FIRST, A WORD ON UNIFORMITARIANISM
The Principle of Uniformitarianism holds that there are laws of nature that
have not changed in the course of time. In other words, we assume that
phenomena such as gravity and magnetism operated in the past pretty much
as they do today. The idea embodied in uniformitarianism is sometimes
expressed in the catchphrase “the present is the key to the past.”
Figure 1—Ripples formed in loose sand on a modern beach (left) and similar ripples preserved on the surface of a block
of sandstone. Ripple-forming processes are assumed to be essentially the same in the geologic past as they are today.
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Uniformitarianism is vitally important in geology because it allows us to infer
that rock-forming processes have remained more or less unchanged over
time. For example, today we can directly observe the formation of current
ripples on a sandy river bed. We can measure the size of the sand grains, the
velocity of the river current, and the size and shape of the sand ripples.
Given these observations, we can reasonably interpret the conditions and
processes that might have operated to produce similar ripples in an ancient
river bed, now preserved as sandstone (Fig. 1).
IGNEOUS
ROCKS
Recall that igneous rocks are those that crystallized from molten magma,
and they are classified according to texture and composition. Texture
refers to the size of individual mineral crystals, whereas composition refers
to kinds of minerals present in the rock.
Finely crystalline igneous rocks cooled rapidly at or near Earth’s surface, so
they are termed extrusive. Coarsely crystalline ones cooled more slowly at
depth, so they are termed intrusive. Mafic igneous rocks are composed of
magnesium- and iron-rich minerals, are generally dark colored and dense, and
they have relatively high melting temperatures. Felsic rocks are rich in
feldspars and silica, they are lighter colored and less dense, and they have
relatively lower melting temperatures (Table 1).
Table 1—Examples of common igneous rocks classified according to texture and composition.
TEXTURE
Mafic
InterCOMPOSITION mediate
Felsic
Finely crystalline
(extrusive)
Coarsely crystalline
(intrusive)
basalt
gabbro
andesite
diorite
rhyolite
granite
1–2
Igneous rock examples:
Complete the igneous rock worksheet (below) for each of the four examples.
List the minerals you can identify in each sample, identify the texture, and
give the correct rock name.
Igneous rock worksheet
Sample
No.
Mineral composition
Texture
Rock name
1
2
3
4
METAMORPHIC
ROCKS
Metamorphic rocks are those that have been altered in the solid state by
temperature and/or pressure. Metamorphism changes the original texture
and mineralogy of a parent rock to produce a new rock with unique
appearance and composition. Many metamorphic rocks are characterized by
foliated texture: i.e., the presence of banding or wavy layering caused by
elongation and alignment of mineral grains. Common examples of foliated
metamorphic rocks are schist and gneiss. Nonfoliated metamorphic rocks
are massive and lack distinct banding. Common examples are quartzite and
marble. Table 2 lists some important metamorphic rocks along with their
parent rocks and characteristics.
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Table 2—Characteristics and parents of common metamorphic rocks.
Metamorphic
rock
Parent rock
Characteristics
Limestone or dolomite
Marble
Quartzite
Slate
Schist
Gneiss
Nonfoliated, coarsely crystalline,
usually light colored, fizzes in
weak HCl
Quartz-rich sandstone Nonfoliated, massive, extremely
hard, quartz grains bound very
tightly and may be partially
recrystallized
Shale
Weakly foliated, very fine
grained, resembles shale except
harder and with distinct cleavage
planes, sometimes appears and
feels “waxy”
Virtually any igneous or Foliated, mineral grains elongated
sedimentary rock
and aligned, garnet and mica are
common minerals
Virtually any igneous or Strongly foliated, with prominent
sedimentary rock
bands of light and dark minerals,
usually coarsely crystalline
Metamorphic rock examples:
Use Table 2 to help you identify the following four examples.
Metamorphic rock worksheet
Sample No.
Rock name
5
6
7
8
1–4
SEDIMENTARY
ROCKS
Sedimentary rocks are the most common rocks exposed at the Earth’s
surface, and in general, they also form at Earth’s surface. There are two
main categories of sedimentary rocks, clastic and chemical/biochemical.
Clastic sedimentary rocks are made up of mineral grains or small rock
fragments that have been weathered and eroded from preexisting rocks and
then transported to a site of deposition. With burial, the loose sediment
becomes compacted and naturally cemented to form rock. The grains that
make up clastic sedimentary rocks may range in size from boulders to
microscopic clay flecks. Clastic sediments may be deposited in a variety of
continental and marine environments, including deserts, rivers, lakes,
swamps, tidal flats, and shallow to deep seas. Clastic sedimentary rocks are
classified according to grain size, mineralogy and texture, as summarized in
Table 3.
Table 3—Classification of clastic sedimentary rocks.
Major
category
Rock name
Grain size
Mineralogy, texture and other
characteristics
< 1/256 mm Any of a variety of clay minerals
Mudrocks
(clay)
(kaolinite, illite, smectite, chlorite),
(clay and
with platey structure (“fissility”)
silt
Mudstone
< 1/256 mm Same as above, but with blocky
sized)
(clay)
structure (no “fissility”)
Siltstone
> 1/256 mm Mostly quartz with minor feldspars
but < 1/16 and other accessory minerals
mm (silt)
Arenites Sandstone
> 1/16 mm Mostly quartz with minor feldspars
(sand
but < 2mm and other accessory minerals
sized)
Arkose
> 1/16 mm Mostly quartz but with significant
but < 2mm feldspars (~25% or more)
Rudites Conglomerate
> 2mm
Rounded rock particles of variable
(gravel
composition
sized)
Breccia
> 2mm
Angular rock particles of variable
composition
Shale
1–5
Chemical/biochemical sedimentary rocks are those that form by organic or
inorganic precipitation of minerals from water. They are classified mainly on
the basis of composition. Examples are limestone, gypsum, rock salt (halite),
and chert.
Most limestones are made up of organically precipitated calcium carbonate
(CaCO3). This calcium carbonate originates as skeletal material in aquatic
organisms (e.g., sea shells). Upon death of the organisms and decay of soft
tissue, the empty shells simply become sedimentary particles susceptible to
abrasion, transportation and deposition. Many limestones contain variable
amounts of lime mud, microscopic crystals of calcium carbonate. Although
some lime mud may be inorganically precipitated, most probably derives from
the disarticulation of weakly skeletonized marine algae.
Gypsum and halite are examples of chemical sedimentary rocks that form
from inorganic precipitation. They are so-called evaporite minerals, because
when a standing body of water evaporates it becomes more and more saline
until it reaches a point of saturation at which minerals precipitate as solid
crystals. Experiments have shown that gypsum begins to precipitate when
70% of an original volume of normal sea water has evaporated. Halite begins
to precipitate only after 90% of the original volume has evaporated.
Chert is a very fine-grained sedimentary rock composed of microcrystalline
silica (SiO2). Bedded chert forms on the seafloor when the empty shells of
siliceous marine organisms accumulate and eventually become lithified (after
a sequence of fairly complex chemical reactions). Nodular chert is normally a
replacement mineral: i.e., it is formed when SiO2 replaces CaCO3 or some
other mineral, on a molecule-by-molecule basis, within a preexisting rock.
Sedimentary rock examples:
Use Table 3 and the discussion of chemical/biiochemical sedimentary rock to
help you identify the following six examples.
1–6
Sedimentary rock worksheet
Sample No.
Diagnostic features
Rock name
9
10
11
12
13
14
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