Download Short Course in Basic Geology Gregory A. Miles This short course

Short Course in Basic Geology
Gregory A. Miles
This short course consists of three of the most essential subjects in geology: plate tectonics, minerals and
rocks, and geologic time. If you have not had a course in basic geology, this short course will help you
extract much more information from your textbook. It is in no way comprehensive, so if you need additional
information on a subject not covered here, consult a standard general geology textbook.
There are several links to web sites in this short course. The web sites come from three excellent sources:
Lynn S. Fichter of James Madison University, the University of California at Berkeley Museum of
Paleontology, and the U.S. Geological Survey. You will learn a lot more about geology if you take the time
to explore these sites as you work through the short course. If this short course is a mandatory part of the
University of Oregon Distance Education class you are taking (Geology 213 or Geology 308), please be
aware that exploring the web sites listed here is optional (but a great learning experience!).
If you need more information on geological terminology, the glossary at
http://www.ucmp.berkeley.edu/glossary/gloss2geol.html will help you.
OVERVIEW OF PLATE TECTONICS
I.
II.
III.
The outer "shell" of the Earth, called the lithosphere, is fragmented into seven large pieces and more
than a dozen small pieces. These pieces (called "plates") are in motion relative to each other--motion
that is responsible for mountain building activity and for much the volcanic and earthquake activity
on Earth (we call this activity "tectonics"). Four types of relative plate motion are possible:
spreading apart (called divergence), sideways slippage (called transform motion), convergence
where one plate slides under another (called subduction), and collision. Although geologists do not
yet know all the details, we do know that this system is driven by the Earth's internal heat, and we
think plate movement is due to convection (heat-driven, roughly circular paths of motion) within the
Earth.
Let's look at two ways of subdividing the Earth. This will help us understand what the plates are and
how they move.
1. Subdivision of the Earth's interior based on composition.
a. Crust (composed of relatively light rock; density=3.0). The crust is a thin layer that covers
all parts of the Earth's exterior; it makes up less than 1 percent of the volume of the Earth.
There are two basic types of crust:
 continental crust (density=2.7) is thicker and less dense than oceanic crust,
and is composed of a mixture of many rock types--mainly granite,
metamorphic rocks, and sedimentary rocks;
 oceanic crust (density=3.2) is mainly composed of basalt.
b. Mantle (composed of relatively heavy rock; density=4 to 5). The mantle makes up
more than 80 percent of the volume of the Earth.
c. Core (composed of metal--mainly iron; density=about 10).
2. Subdivision of the Earth's interior based on physical properties--mainly how seismic waves
behave as they travel through the Earth.
. Lithosphere--uppermost mantle + the crust; brittle rock.
a. Asthenosphere--upper mantle; weak, plastic rock.
b. Mesosphere--middle and lower mantle; solid rock.
c. Outer core--liquid metal.
d. Inner core--solid metal.
Overview of the concept.
1. Remember: plates are large portions of the lithosphere that are bounded by spreading
centers, transform faults, subduction zones, and collision zones; tectonics refers to largescale Earth movements and the results of those movements.
2. Early ideas.
. As soon as good maps of the world became available, people like Francis Bacon
noticed that continents such as South America and Africa fit together like pieces of
a jigsaw puzzle. Although there was little or no scientific evidence available, it was
proposed that the continents had been joined together at some time in the past.
a. In the early 20th Century, Alfred Wegener proposed an idea called continental
drift. Wegener:
 fit the continents together into one large landmass called Pangea--this
landmass formed from the convergence of previously scattered continents
in ancient times (roughly at the time when the dinosaurs first appeared on
Earth);
 proposed that Pangea later broke up and the continents moved ("drifted") to
their present positions on Earth;
 cited extensive evidence from the rock record and fossil record of the Earth
that strongly supported his idea. Although Wegener's idea made sense (we
now know that he was basically correct), there was no mechanism known
that would allow the continents to "drift". Geologists rejected his ideas until
1960, when the mechanism was finally recognized.
3. Basic mechanisms of plate tectonics.
The following image from the web will help you understand the location and appearance of
the tectonic features discussed here:
http://pubs.usgs.gov/gip/dynamic/graphics/Fig13.gif
.
Lithospheric plates diverge at spreading centers; basaltic magma rises, "fills in the
gap", and forms new lithosphere. Oceanic spreading centers occur at mid-ocean
ridges, which form a nearly continuous chain of submarine mountains
approximately 40,000 miles long. Continental spreading centers occur at
continental rifts.
 Any continent located on a moving plate moves ("drifts") along with the
plate, conveyer-belt style.
 Rocks and fossils that were formed in the same area in ancient times may
become widely separated when continents move apart.
a. Some plates move sideways relative to adjacent plates along breaks in the Earth
called transform faults. Most transform faults occur along the mid-ocean ridge
system.
b. Lithospheric plates may be destroyed at subduction zones, where one plate slides
beneath another.
 Down-dragging of the plates forms a trench at the top of the subduction
zone.
 Intense heat melts some of the subducting plate; part of the resulting magma
comes to the surface and forms a chain of volcanoes (like the Cascade
Range) all along the length of the subduction zone.
 Uneven movements in the subduction zone result in sudden slippage of the
plates and the release of seismic waves that cause large earthquakes.
 Constant plate destruction results in the sea floors of the Earth being no
older than about 180 million years old--only about 4 percent of the age of
the Earth.
Uplift of plate margins along subduction zones results in the growth of
mountain ranges.
c. Plates capped by continental crust are too light to subduct; therefore, when two such
plate converge, a collision results, and large mountain ranges such as the Himalayas
and Alps form.

Accreted terranes (also called exotic terranes) are pieces of crust that formed in one
location, but were later transported by moving plates to another location. Many of these are
small scraps of crust that were rafted to a subduction zone, but were too light to subduct so
they accreted to ("plastered onto") the adjacent continental margin as miniature collision
blocks. Adjacent terranes are separated from each other by sharp boundaries, and have
markedly different geologic characteristics. The geologic "foundation" of the western
margin of North America (including Oregon) is a crazy-quilt of accreted terranes that are
covered in many areas by younger volcanic and sedimentary rocks.
4. In summary, plate tectonics provides a unifying concept that explains:
. mountain building;
a. much of the volcanic activity on Earth;
b. distribution of many earthquakes and volcanoes;
c. movement of continents over geologic time;
d. the anomalous age of the sea floor;
e. the anomalous distribution of some fossils and rocks.
Here is a web site where you can get more information on plate tectonics:
http://pubs.usgs.gov/publications/text/dynamic.html
OVERVIEW OF MINERALS AND ROCKS
MINERALS
I.
II.
III.
IV.
Why study minerals? First, almost all rocks are composed of minerals; therefore, we must know
something about minerals before we can classify rocks and understand how rocks form and differ
from each other. Second, many minerals--ore minerals and gems--are important because of their
economic worth.
A mineral is a naturally occurring, inorganic solid having an orderly internal atomic structure and a
chemical composition that varies only within specific limits.
Silicate minerals are those that contain at least the elements silicon (Si) and oxygen (O) combined
together. Quartz (silica, SiO2) is the simplest silicate mineral. All others contain not only of Si and
O, but also iron, magnesium, sodium, calcium, or other elements. It's no wonder silicate minerals are
so abundant in the Earth's crust: the crust consists of 46.6% oxygen and 27.7% silicon.
The ferromagnesian minerals (ferro=iron) constitute an important group of dark (typically black)
silicate minerals that are rich in iron and magnesium. We will look at ferromagnesian minerals again
when we discuss igneous rocks.
Here is a really good basic web site on minerals: http://csmres.jmu.edu/geollab/fichter/Minerals/index.html
This web site has an alphabetical listing of common minerals:
http://csmres.jmu.edu/geollab/fichter/Minerals/Minalpha.html Click on a mineral name to bring up an
image and a description.
ROCKS
I.
II.
Rocks are aggregates (collections) of minerals or mineral matter. Based on their origin, rocks may
be classified as igneous (form from magma), sedimentary (form from particles), or metamorphic
(form when heat, pressure, and chemical alteration change solid rock bodies). Rocks in each
"family" may be further classified based on their texture (size, shape, and arrangement of constituent
minerals) and chemical composition.
Rock names used in the following discussions are explained in the glossaries and web sites
provided. Most of the names are also defined in a standard college dictionary.
IGNEOUS ROCKS
I.
II.
III.
Igneous rocks are those that have formed from the cooling and consolidation of magma (molten
rock material that forms in some places beneath the Earth's surface). Most of these rocks consist of
masses of crystals, but some are composed of volcanic glass or volcanic fragments (pyroclastic
material such as volcanic ash, pumice, and cinders). When magma cools and hardens beneath the
Earth's surface, the result is an igneous intrusive rock. When it cools and hardens above the Earth's
surface as lava or pyroclastic material, the result is an igneous extrusive rock.
Intrusive rocks are usually coarse-grained because the magma is insulated from heat loss by the
surrounding rock and the crystals have had a long time to grow. In general, extrusive rocks are finegrained because the crystals have had only a short time to grow. If cooling is extremely rapid, no
crystals are able to grow, and the extrusive rock is glassy.
Here is the basic classification for fine-grained and coarse-grained igneous rocks.
CHANGES IN CHEMICAL COMPOSITION:
1. Decreasing silica -------------------------------------------->
2. Increasing ferromagnesian (dark) minerals-------------------->
TEXTURE
ROCK TYPES...
Fine-grained----->rhyolite---andesite--basalt---(none)
Coarse-grained-->granite----diorite----gabbro---peridotite
Note: the average chemical composition of rhyolite and granite (or of andesite and diorite,
or of basalt and gabbro) is the same; the only fundamental difference between the two rocks
is the crystal size.
Here is a web site that shows the basic classification of igneous rocks with nice enlarged pictures of
each rock:
http://csmres.jmu.edu/geollab/fichter/IgnRx/simpclass3.gif
We can even place glassy and pyroclastic rocks in the table, but it takes a little more work (most are
in the rhyolite-to-andesite range). The most common glassy rocks are obsidian, which is solid glass,
and pumice, which is solidified lava foam that is very porous because it contains abundant vesicles
("frozen gas bubbles"). Both rocks typically have the composition of rhyolite. Common pyroclastic
rocks include tuff, which is composed of consolidated volcanic ash and other small pyroclastic
material, and volcanic breccia, which composed of consolidated large, angular volcanic material.
IV.
Igneous intrusions (or plutons) are masses of igneous rock formed when magma cools below the
Earth's surface. The most common types are batholiths, which are large (mountain-range-sized)
intrusions composed of granite-like rocks; dikes, which are shaped like a table top and cut across the
rocks they intrude; and sills, which are also shaped like a table top, but do not cut across the rocks
they intrude.
Lava is magma that has reached the Earth's surface as a liquid. Some common types of lava flows
include the following.
a. Large fissure-eruption "flood" basalt flows. Very liquid magma is extruded from a fissure
(large fracture in the Earth's crust) and spreads out over a large area.
b. Aa flows. These are viscous, slow-moving lava flows in which the basaltic lava contains
relatively little gas. As the flow moves, the exterior hard crust breaks into masses of angular
blocks.
c. Pahoehoe flows. These are fluid, faster-moving flows in which the basaltic lava contains
much more gas. Its hardened surface may be fairly smooth or may be rope-like.
A volcano is a vent ("hole") in the Earth's crust from which lava, pyroclastic material, and gases are
erupted. It is also the mountain or hill that results from these volcanic eruptions.
V.
VI.
Here is a great basic web site on igneous rocks: http://csmres.jmu.edu/geollab/fichter/IgnRx/IgHome.html
This web site contains a basic short course on igneous rocks:
http://csmres.jmu.edu/geollab/fichter/IgnRx/Introigrx.html
GLOSSARY OF COMMON IGNEOUS ROCKS
This web site has an alphabetical listing of common igneous rocks:
http://csmres.jmu.edu/geollab/fichter/IgnRx/IgAlphabetical.html . Click on a rock name to bring up an
image and a description.
•
•
•
•
•
•
•
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•
•
Andesite -- A medium gray, dense, very fine-grained rock. A common rock of large subduction
zone volcanoes.
Basalt -- Very fine-grained, dense, and black. May contain frozen gas bubbles or scattered larger
crystals. Basalt is a very common lava-flow rock.
Diorite -- Similar in appearance to granite, but with more dark ferromagnesian minerals. It is the
coarse-grained equivalent of andesite.
Gabbro -- A coarse-grained, dark igneous rock. It is the intrusive equivalent of basalt.
Granite -- Large crystals of light-colored minerals are easily visible.
Obsidian -- Volcanic glass which usually has the composition of rhyolite. It has a shiny, glassy
appearance and conchoidal (shell-shaped) fracture.
Peridotite -- A dark, green-black, coarse-grained intrusive rock that is very high in ferromagnesian
mineral content.
Pumice -- A very light-weight, vesicular, glassy rock; commonly white to buff in color, but may be
black; may float on water; usually has the composition of rhyolite.
Rhyolite -- Very fine-grained light-colored rock that has the same mineral composition as granite.
Often it is faintly banded.
Tuff -- Fine, compacted volcanic ash; may be mixed with pumice fragments; light-weight; often
white, buff, or gray in color.
Volcanic breccia -- A coarse-grained rock composed of larger, angular volcanic fragments
cemented in a finer-grained mass of volcanic material.
SEDIMENTARY ROCKS
I.
Sedimentary rocks are those in which particles (sediment) have been compacted or cemented to
form solid rock bodies. They are classified into three major types:
1. Clastic. The particles are from the breakdown of existing rocks (particles are gravel, sand,
mud, etc.). Examples: conglomerate, sandstone, siltstone, shale.
2. Chemical precipitates. The particles are crystals that have precipitated from sea water or
lake water (salt, gypsum, etc.). Examples: rock salt, rock gypsum, some limestone.
3. Organic. The particles are parts of living things (limbs, leaves, shell, etc.). Examples: coal,
most limestone.
For a somewhat different approach to classifying sedimentary rocks, take a look at:
http://csmres.jmu.edu/geollab/fichter/SedRx/sedclass.html
II.
Worldwide, clastic sedimentary rocks are far more abundant than chemical precipitate and organic
sedimentary rocks. Let's take a closer look at the clastics. They form as a result of four major
processes.
a. Weathering: the physical and chemical breakdown of rock at the Earth's surface.
b. Erosion: picking up and transporting the weathered products (mud, silt, sand, and gravel).
c. Deposition: the settling of sediment in a relatively quiet area.
d. Compaction and cementation: turning the sediment into sedimentary rock by compacting
and naturally cementing the particles together.
III.
Clastic sedimentary rocks are further classified according to their grain size.
. Conglomerate forms from rounded gravel.
a. Sandstone forms from sand.
b. Siltstone forms from silt.
c. Shale forms from "mud". Shale usually splits into thin layers.
IV.
Characteristics of sedimentary rocks (these can also be found in igneous and metamorphic rocks, but
are more common in sedimentary rocks).
0. Stratification (layering).
1. Often contain fossils.
2. Form by processes active at the Earth's surface.
Here is a great basic web site on sedimentary rocks:
http://csmres.jmu.edu/geollab/fichter/SedRx/index.html
This web site contains a basic short course on sedimentary rocks:
http://csmres.jmu.edu/geollab/fichter/SedRx/SimpModl.html
GLOSSARY OF COMMON SEDIMENTARY ROCKS
This web site has an alphabetical listing of common sedimentary rocks:
http://csmres.jmu.edu/geollab/fichter/SedRx/Sedalphab.html
Click on a rock name to bring up an image and a description.
•
•
Coal -- A rock that is composed of concentrated and compressed fossil plants remains. It is typically
black, but some is brown.
Conglomerate -- Rock composed mainly of rounded (water-worn) rock fragments larger than 2 mm
(roughly the size of a BB); fragments are usually composed of harder rocks such as chert, quartzite,
and basalt, rather than softer rocks that abrade more easily; composition and color vary with
sediment source.
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•
•
•
•
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Limestone -- Rock composed mainly of the mineral calcite. It may be crystalline due to growth of
small calcite crystals during and after deposition. Most limestone is ultimately derived from the
calcareous hard parts of organisms.
Quartz sandstone -- A common variety of sandstone composed mainly of quartz grains.
Rock gypsum -- A soft white crystalline rock composed of the mineral gypsum which has
precipitated from sea water or lake water.
Rock salt -- A rock composed of the mineral halite (common table salt). It forms from the extreme
evaporation of sea water or lake water.
Shale -- A fine-grained rock composed of microscopic particles of clay minerals, quartz, and mica;
can be split into flat layers. Its color is variable.
Siltstone -- A rock composed of silt (clastic particles that are smaller than sand but larger than clay).
METAMORPHIC ROCKS
I.
Metamorphic rocks are those that have changed form in the solid state due to heat, pressure, and
chemical alteration. They can originate from the metamorphism of any kind of existing rock and
form a complete spectrum from low-grade (lower temperature and pressure conditions) to
intermediate-grade to high-grade rocks.
II.
RESULTS OF METAMORPHISM.
1. Recrystallization.
a.
Growth of new minerals: existing minerals in a rock may break down under high temperature and
pressure, and their chemical elements may move around and join other elements to form new minerals.
Example: clay minerals may alter to micas.
b. Enlargement of minerals: crystals of existing minerals grow to a larger size without
changing composition. Example: fine-grained limestone becomes coarse-grained
when it is metamorphosed to marble.
2. Alignment of minerals.
.
Existing platy or elongate minerals may rotate to a new position
perpendicular to the direction of pressure.
a. Also, newly forming platy or elongate minerals will usually grow in this orientation.
III.
The principal processes of metamorphism are:
1. Regional metamorphism: associated with convergent plate margins. Much pressure is
involved and large areas are impacted.
2. Contact metamorphism: associated with intense heat and chemical alteration surrounding
an igneous intrusion. A relatively small, thin "shell" of rock is metamorphosed around the
intrusion.
IV.
Classification of metamorphic rocks:
1. Foliated rocks are those in which the constituent mineral grains are aligned. Examples:
slate, phyllite, schist, and gneiss.
2. Nonfoliated rocks lack mineral grain alignment. Examples: marble, quartzite, serpentine,
and greenstone.
Here is a great basic web site on metamorphic rocks:
http://csmres.jmu.edu/geollab/fichter/MetaRx/index.html
This web site has a good introduction to metamorphic processes:
http://csmres.jmu.edu/geollab/fichter/MetaRx/Metaintro.html
GLOSSARY OF COMMON METAMORPHIC ROCKS
This site has an alphabetical listing of common metamorphic rocks:
http://csmres.jmu.edu/geollab/fichter/MetaRx/Metaalphab.html. Click on a rock name to bring up an image
and a description.
Gneiss A high-grade metamorphic rock which usually has a composition similar to granite. It is
characterized by alternating bands of light and dark minerals.
Greenstone A rock formed from the low-grade metamorphism of basalt, gabbro, and related rocks. The
green color results from the alteration of ferromagnesian minerals.
Marble Marble is formed from the metamorphism of limestone or similar rocks. It is soft and reacts with
dilute acid. Its color is variable and depends on the type and amount of impurities.
Phyllite A fine-grained metamorphic rock which is intermediate in grain size between slate and schist (see
below). The smooth surfaces have a satin-like sheen caused mainly by microscopic flakes of mica. It forms
from the metamorphism of fine-grained clastic sedimentary rocks such as shale or from the further
metamorphism of slate. It may have the same color and general appearance as slate, but is distinguished by
the sheen.
Quartzite An extremely hard rock composed of metamorphosed quartz sandstone. The individual grains are
so tightly held together that breakage of the rock occurs through the grains just as easily as around them. The
color is commonly white or some light shade of brown or gray.
Schist Minerals making up this rock are strongly aligned and are usually large enough to see with the naked
eye. The color and composition are variable. Schists are named for their dominant constituent minerals (for
example, talc schist or garnet-mica schist).
Serpentine A shiny green to black rock which on fractured surfaces resembles plastic. Often forms from the
metamorphism of igneous rocks such as peridotite.
Slate A fine-grained metamorphic rock that is harder and more brittle than shale, which it resembles. Like
shale, it splits into flat sheets. It forms from the metamorphism of fine-grained clastic sedimentary rocks,
especially shale. The color is variable, but is commonly gray, black, or red.
OUTLINE OF GEOLOGIC TIME
I.
II.
Geologists measure time in two basic ways:
1. Relative time (order of events).
2. Absolute time (time measured in units, such as years). We will not examine the derivation
of absolute time in this discussion.
Relative time is determined by following a few basic principles, most of which are simple common
sense applied to geologic situations.
1. Principle of superposition: in an undisturbed succession of rock layers, the oldest layer is
on the bottom and the youngest layer is on the top.
2. Principle of original horizontality: rock layers are originally deposited in an
approximately horizontal position; if they are not now horizontal, they have been disturbed
by tectonic forces (large-scale Earth forces).
III.
3. Principle of cross-cutting relationships: a feature or a surface that cuts across a second
feature or surface is younger than the second feature or surface. (Another way to think of it
is that the second feature or surface must be older because it had to have existed in order to
be cross-cut.)
4. Principle of fossil succession: fossils occur in a systematic, predictable succession in the
rock record of the Earth. They do not occur randomly and haphazardly. This was
demonstrated by William Smith about 1800. Organisms evolve, then eventually become
extinct. Their duration of existence on Earth is called their range. If we can determine the
range of a fossil, we can use the fossil to determine the age of rock that contains the fossil.
Expanding this idea to include the ranges of hundreds of fossils has helped geologists
develop the geologic time scale.
Now let's look at the geologic time scale.
1. The Earth is 4.6 billion years old. Geologists break this vast amount of time into smaller
segments so they can communicate better with each other about when geologic events
occurred.
2. The largest segments are called eons, which include the:
Phanerozoic (Younger. Fossils are abundant.)
Precambrian (Older. Fossils are relatively rare.)
The Precambrian informal eon takes up 7/8 of Earth's history. Precambrian rocks are
difficult to correlate worldwide because fossils are relatively rare. For this reason we will
not discuss Precambrian time further. The Phanerozoic Eon takes up the last 1/8 of Earth's
history and is readily subdivided by using fossils and superposition. Most of the geologic
time scale consists of time terms from the Phanerozoic Eon.
3. In our daily communications about time, we find it convenient to use a hierarchy of time
segments like this:
years
months
weeks
days
hours
minutes.
4. Similarly, geologists find it convenient to break up the Phanerozoic into a hierarchy of time
segments like this:
eras
periods
epochs.
In this system, each time segment is given its own name--just like each year, for example, is
given its own name ("nineteen eighty-seven", and so on). This hierarchy of time segments
and names is summarized in a diagram called the geologic time scale.
5. From the geologic time scale you should learn the eras and periods of the Phanerozoic Eon
and the epochs of the Cenozoic Era. You should also know that Precambrian time takes up
the first seven-eighths of Earth history. It is very important that you learn the time scale,
since you will see the time terms over and over again in readings. Be sure to learn all the
names shown below in their proper order (you do not have to learn any of the numbers).
GEOLOGIC TIME SCALE
You might also have a copy of the geologic time scale in your book. Note that the time scale
is written in the order of superposition.
EONS
ERAS
PERIODS
EPOCHS
Quaternary
Holocene (=the last
10,000 years)
Pleistocene
Cenozoic
Pliocene
Miocene
Tertiary
Oligocene
Eocene
Paleocene
65
million
years
ago
PHANEROZOIC
Cretaceous
Mesozoic
Jurassic
Triassic
245
million
years
ago
Permian
Pennsylvanian
Mississippian
Paleozoic
Devonian
(Miss.+
Penn.=Carboniferous
in some time scales.)
Silurian
Ordovician
Cambrian
544
million
years
ago
PRECAMBRIAN (the
first seven-eighths of
Earth history).
The following website shows a nice version of the geologic time scale:
http://www.ucmp.berkeley.edu/help/timeform.html. This is a small part of the incredible website maintained
by the UC Berkeley Museum of Paleontology. Click on any time term to see more information about that
time in Earth history. For additional information on geologic time, click on the links "Introduction to
Geology" and "Navigating our Geology Wing."