GY 112 Lecture Notes
D. Haywick (2006)
1
GY 112 Lecture Notes
Rock Review
Lecture Goals:
A) Recap of rock types
B) Recap of the rock cycle
C) Sedimentary rocks: their role in earth history
Textbook reference: Levin 7th edition (2003), Chapter 2; Levin 8th edition (2006), Chapter 4
(For those of you who have already completed GY 111, you will find your lecture and lab
notes of use for this part of GY 112)
A) Recap of rock types
To cut to the chase, there are 3 main divisions of rocks, but in GY 112, the sedimentary
rocks are by far the most important. The reason? They are commonly fossiliferous
(meaning that they host fossils). First off, here are the 3 main rock divisions:
Igneous (formed from molten rock)
Sedimentary (formed from particulate material)
Metamorphic (alteration of parent rocks through heat, pressure and fluids)
Each of these rock groups
have subdivisions. As I said
earlier, it is the sedimentary
rocks that are the most
important group in GY 112.
Let’s start with them:
Sedimentary rocks are
classified according to two
different parameters: 1)
Grain size (see chart to left)
and composition. The grain
size refers to the size of the
particles that make up the rocks. These are the “things” that were initially deposited
before the sediment became “lithified”, which just means conversion into “rock”. There
are 4 major size divisions of sediment that you need to remember: 1) gravel (the largest
division; grains ≥ 2.0 mm), 2) sand (grains <2.00 mm but ≥ 0.63 mm), 3) silt (grains
<0.63 mm but ≥ 0.004 mm) and clay (grains ≤ 0.004 mm). Obviously, you can’t see the
really small silt and clay grains. Sedimentologists (those really cool and wonderful
scientists like Dr. Haywick that study sedimentary rocks), frequently have to use
microscopes and scanning electron microscopes to study the smallest sedimentary
particles.
GY 112 Lecture Notes
D. Haywick (2006)
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As far as composition is concerned, sedimentary rocks are classified into four subdivisions:
1) Clastic (also known as siliciclastic); rocks contained particles rich in silica-rich
minerals such as quartz, feldspar and clay minerals. Examples include quartz arenite1
sandstone, arkose sandstone, greywacke sandstone, shale, claystone, siltstone and
mudstone.
2) Bioclastic; rocks containing remains of once living creatures (aka beasties). Examples
include limestone (carbonate shells and skeletal elements), chalk (microscopic
carbonate shells and body parts) and biogenic chert (microscopic silica body parts).
3) Chemical; rocks containing chemically precipitated minerals. Examples include the
evaporite minerals halite, gypsum, dolostone and anhydrite, as well as travertine
(cave precipitates composed of calcite) and oolitic limestone.
4) Organic; rocks containing the remains of plants (i.e., the coals). Examples include
peat, lignite, bituminous coal and anthracite.
Please note that many of these sedimentary rocks may also be fossiliferous. For example,
a siltstone containing shells can be named a fossiliferous siltstone. A fossil-bearing quartz
arenite sandstone can be named a fossiliferous quartz arenite.
You should also be familiar with the following sedimentary figure. Apart from grain size,
it also summarizes grain shape and sorting, concepts that we will consider in the lab.
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Note: You need to know the names of all of the rocks that are highlighted red.
GY 112 Lecture Notes
D. Haywick (2006)
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This is a summary chart of the sedimentary rocks from the GY 111 lab manual. It is
included here to provide completeness to this lecture. Turn to the last page for a list of the
sedimentary rocks that you will see in GY 112.
GY 112 Lecture Notes
D. Haywick (2006)
The other two rock divisions are included here for completeness of your notes, but you
aren’t going to see many of them in GY 112 (only 3):
Igneous rocks; two sub-divisions.
1) Extrusive (also known as volcanic) if they are erupted at the Earth’s surface from a
volcano. Examples include basalt (mafic); andesite (intermediate) and rhyolite
(felsic).
2) Intrusive (also known as plutonic) if they cool within the interior of the Earth.
Examples include dunite and peridotite (ultramafic), gabbro (mafic), diorite
(intermediate) and granite (felsic).
Metamorphic rocks; three sub-divisions.
1) Foliated; rocks containing a distinct arrangement of minerals (including rock
cleavage). Examples include slate, phyllite, schist, gneiss and amphibolite.
2) Non-foliated; rocks lacking foliation. Examples include marble, quartzite and
hornfels.
3) Cataclastic; rocks pretty much busted up through faulting. Examples include
mylonites.
B) The rock cycle
One of the most
important things that
you will learn in
introductory geology
class (e.g., GY 111)
is the rock cycle.
This is the
fundamental concept
that relates all of the
rock divisions into
one tight little
package. Each rock
type can be
converted into any
other rock type
through different
physical processes. For example, igneous rocks are converted into sedimentary rocks
through weathering followed by lithification. Sedimentary rocks can be converted into
metamorphic rocks through heat and pressure. Metamorphic rocks can be converted into
igneous rocks through melting. You get the idea. The best way of explaining the rock
cycle is through a diagram like the one pictured to the right.
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GY 112 Lecture Notes
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C) Sedimentary rocks; keys to Earth history
As I indicated previously, I am a sedimentologist which means that I study sedimentary
rocks. I do this because I like them. I am terribly biased in favor of them (as any of my
students will attest to). One of the reasons why I like sedimentary rocks is their
usefulness in interpreting Earth history. They are useful because they frequently preserve
all sorts of information regarding the depositional environment of their formation (the
so called paleoenvironment). They preserve information about current speeds, current
directions, water/air temperatures, climate, and the age and tectonic setting of the
materials being deposited. Geologists use the term paleocurrent when they are referring
to these ancient depositional currents, or paleoclimate if they are describing the climate
millions of years ago. They also employ terms like paleotectonic setting if they are
dealing with the ancient tectonic environment of the rocks being studied. How do
sedimentary rocks do all this?
Paleocurrents are determined on the basis of sedimentary structures. When currents flow
(air or water), sand particles are moved along the Earth’s surface as bed load. The sand
frequently develops ripples and other types of sedimentary structures like crossstratification. The direction of the currents responsible for the cross-stratification can be
easily measured in outcrops. This gives you the paleocurrent direction. The paleocurrent
speed can be estimated on the basis of the sediment size and the type of sedimentary
structures. This is discussed further in GY 344, one of the core courses to which we
subject our geology majors.
Paleoclimate is determined primarily through examination of rock suites (groups of
similarly-aged rocks collected from a specific area). For example, rocks formed in a
humid area will preserve a record of wet conditions. They may contain abundant organic
material (coals), or they may record constant water current activity (cross-bedded
sandstone). In contrast, arid regions tend to produce red bed type rocks (red shales,
hematite stained sandstone), or in some cases, evaporite minerals such as banded gypsum
or halite.
Paleotectonic settings are also determined on the basis of rock suites. A sequence of
rocks that contains both volcanic rocks and sedimentary rocks probably came from a
tectonically active area. Now I know I said that we weren’t going to look at many
igneous rocks in this class, but you will have to remember a couple in order to resolve
paleotectonic environments. Divergent plate boundaries tend to produce basalts so if you
GY 112 Lecture Notes
D. Haywick (2006)
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encounter basalt (a finely crystalline
generally black rock; see image to
the left from the Drexel University
web site), you suspect a divergent
plate paleotectonic setting. In
contrast, a convergent place
paleotectonic setting would produce
granite, a coarsely crystalline,
generally pink rock (pictured to the
right), as well as sedimentary rocks
like arkose, breccia and red shale.
Beware: Complications can occur.
For example, if divergent tectonics
(e.g., rifting) occurs in the middle of an ocean basin, you would get pillow basalts
associated with deep marine sedimentary rocks like chalk, greywacke and black shale.
Were it to occur within a continental setting, those basalts will be associated with
granites, arkose and breccias. There is one other paleotectonic setting that should be
mentioned here. It is the setting when you
have no active tectonics (e.g., no
volcanoes, earthquakes etc.. The Gulf
Coast is like this today. Passive
continental margins are those coastlines
that have no active tectonics. The rocks
formed here included quartz arenite
sandstone, limestone, green shales etc. In
this type of tectonic setting, no igneous
rocks can form.
Age control. One of the most useful
things that sedimentary rocks do is to
provide information about geological age.
This is primarily because they can contain fossils. Fossils are studied by paleontologists
and good ones (like Dr. Clark) can determine what they are and when they lived. If you
find a specific “beastie” in a sedimentary rock, and if you know the age limits of that
beastie, then you automatically know the age of the rock. There are other ways to date
rocks, and we will get to them over the next couple of days.
Important terms/concepts from today’s lecture
(Google any terms that you are not familiar with)
Grain size (gravel, sand, silt, clay)
Sedimentary Rocks; Siliciclastic, Biochemical, Chemical, Organic, Evaporite
Depositional Environment (AKA Paleoenvironment)
Rock Cycle
Weathering
Lithification
GY 112 Lecture Notes
D. Haywick (2006)
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Pressure
Paleocurrent
Paleoclimate
Paleotectonic Setting (divergent, convergent, passive continental margin)
Fossil
Paleontology
Sedimentology
This is the complete list of rocks that you will encounter in the Lab component of GY 112
Sedimentary Rocks
Quartz arenite
lithic sandstone
greywacke
arkose
conglomerate
breccia
red shale
black shale
green shale
siltstone*
fossiliferous limestone
oolite (oolitic limestone)
non-fossiliferous limestone
dolostone
coal
amber
halite
Igneous Rocks
Granite*
Basalt*
Metamorphic Rocks
Schist*
*-you will not see these rocks on lab exams in GY 112