Historical Geology = the study of changes to Earth and life in time and space.
I. Time
A. How much time?
 Oldest meteorites and oldest moon rocks are 4.6 billion years old
4,600,000,000 years
 Oldest rocks found so far on Earth are zircon grains from a sandstone
in western Australia, dated at 4.1 to 4.2 bilion years old. Previously,
the oldest Earth rocks were 3.96 billion years old, from the Northwest
Territories of Canada.
B. How do we know?
Radioactive materials serve as geologic clocks.
C. What happened on the Earth during this long period of time?
Many natural events:
o meteorite impacts
o volcanic eruptions and lava flows
o mountain building
o earthquakes
o erosion
o slow movement of continents (plate tectonics)
o formation and destruction of ocean basins (plate tectonics)
o glaciations
o climatic changes
All of these natural events are still going on today - We see evidence in
the rock record that these events have been occurring for a long time.
Geologists call this uniformitarianism.
The physical laws governing the universe operate uniformly through time.
Some events which occurred in the past, and left a record in the rocks, ARE
NOT OCCURRING TODAY, or have not occurred in the human lifespan:
 Huge meteorite imapcts
 Large glacial ice sheets
Other events occur so slowly that they are difficult to measure:
 Plate Tectonics - sea floor spreading, continental drift
 Erosion of mountain ranges
Still other events are short lived but very catastrophic:
 Volcanic eruptions
 Earthquakes
 Floods
 Mudflows, avalanches, etc (mass wasting)
Evidence of all of these events is preserved in the geologic record.
D. Layers of the Earth – the earth is divided into 3 basic layers:
1. Crust
- predominantly composed of O & Si
2. Mantle
- predominantly composed of Fe & Mg
3. Core
- predominantly composed of Fe & Ni
- very hot
The Crust
a. Continental Crust
i) About 35 km thick; 60 km thick in mountain ranges
ii) mostly of granite composition
b. Oceanic Crust
i) About 5-12 km thick
ii) it has a layered structure
 Pillow basalts = basaltic rock that erupted below water
 “sheeted dikes” = interconnected basaltic ridges
 Gabbro = course grained equivalent of basalt; cooled slowly
The Mantle
a. Lithosphere = the outer mantle + the crust
 ~ the outermost 100 km of Earth
 Divided into tectonic or lithospheric plates that cover the
surface of the Earth
b. Asthenosphere = the inner mantle
 Depth of 100 – 250 km
 Seismic wave velocity decreases (b/c less dense)
 Rocks are at, or near melting point
 Plastic-type behavior = solids that flow (magmas)
 Convection in this layer moves the tectonic plates
The Core
a. Outer Core
 Molten Fe (85%) with some Ni
b. Inner Core
 Solid Fe (85%) with some Ni
 Also contains lighter elements such as Si, S, C, O
E. What do you mean by geologic record?
The geologic record consists of rock units, each of which records
some event or series of events that occurred in the past.
1. What are rocks?
a. Rocks are defined as aggregates of one or more minerals.
2. What are minerals?
a. naturally occurring
b. inorganic
c. solids
d. definite chemical composition
e. characteristic internal crystal structure
F. How do rocks form?
 By cooling, hardening, and crystallizing from hot, molten lava
o Igneous rocks
 By forming from the compaction and cementation of sediments
o Terrigenous, clastic or detrital sedimentary rocks
 By forming from the precipitation of dissolved chemicals in water
o Chemical and biochemical sedimentary rocks
 By forming from accumulated organic matter
o Organic sedimentary rocks (coals)
 By the alteration of pre-existing igneous or sedimentary rocks by heat
and pressure
o Metamorphic rocks
II. The Rock Cycle
Rocks are always on the move through the rock cycle!
o All rock (except for meteorites) that is on Earth today is made
of the same stuff as the rocks that dinosaurs and other ancient
life forms walked, crawled or swam over
o While the stuff that rocks are made from stays the same, the
rocks do not.
o Over millions of years, rocks are recycled into other rocks.
o Moving tectonic plates help to destroy and form many types of
rocks.
Types of Rocks
Type
Characteristics
Examples
Igneous rocks are created when molten material
such as magma (within the Earth) or lava (on the
surface) cools and hardens. The hot material
crystallizes into different minerals. The properties
and sizes of the various crystals depend on the
magma's composition and its rate of cooling.
Granite
Obsidian
Basalt
Pumice
Andesite
Diorite
Rhyolite
Sedimentary rocks are made up of sediments
eroded from igneous, metamorphic, other
sedimentary rocks, and even the remains of dead
plants and animals. These materials are deposited
in layers, or strata, and then are squeezed and
compressed into rock. Most fossils are found in
sedimentary rocks.
Sandstone
Shale
Conglomerate
Limestone
Chert
Coal
Gypsum
Metamorphic rocks are produced when
sedimentary or igneous rocks are transformed by
heat and/or pressure. The word "metamorphic"
comes from the Greek language, which means "to
change form."
Marble
Slate
Quartzite
Schist
Gneiss
Igneous rocks
Sedimentary rocks
Metamorphic rocks
A. Sedimentary Rock
1. Sedimentary rocks make up about ¾ of the rocks at the Earth’s surface.
 They form at the surface in environments such as beaches, rivers, the
ocean, and anywhere that sand, mud, and other types of sediment
collect.
 They preserve the record of the environments that existed when
they formed.
 By studying them, scientists can examine how climate and
environments changed through history, as well as study ancient life
preserved within them.
2. Clastic sedimentary rocks = Sedimentary rocks that are made from the
broken bits of other rocks.
 Broken bits of rock are called sediment.
o It is the sand you find at the beach, the mud in a lake bottom,
the pebbles in a river, even the dust on furniture.
 Sediment can form rock over time if the “pieces” are cemented
together.
3. Chemical sedimentary rocks are made of mineral crystals, such as halite
and gypsum, formed by chemical processes.
4. Organic sedimentary rocks are the remains of living things such as
clamshells, plankton, bones, and plants.
B. Igneous Rocks
1. Igneous rocks form when molten rock cools and becomes solid.
 Molten rock is called magma when it is below the Earth’s surface
and called lava when it is above.
2. They are divided into 2 groups, based on where the rock forms.
a. Intrusive igneous rocks (plutonic rocks) = rocks that form below
the Earth’s surface
 They form when magma enters an underground chamber
 They cool very slowly
 They form rocks with large crystals
b. Extrusive igneous rocks (volcanic rocks) = rocks that form above
the Earth’s surface
 Form when lava cools quickly at or above the Earth’s surface
C. Metamorphic Rocks
1. Form when they are in a place that is very hot and pressure is high.
 Generally occurs where tectonic plates are colliding
 The collisions squash the rocks and hot pools of magma heat
them deep underground
 The crystals in the rock usually change due to the intense heat
and pressure
2. Foliation = When flat minerals, like mica, become lined up perpendicular
(at a right angle) to the direction of the pressure.
 Rocks can be classified as either foliated or non-foliated
3. Any type of rock can be metamorphosed.
a. Contact metamorphism = when the rocks are changed in small areas
b. Regional metamorphism = when the rocks are changed in very
large areas
III. Fundamental principles of Geology
Most sedimentary rocks occur in the form of layers called beds or
strata. Each layer is the result of the deposition of sediment during
some natural event (such as a flood or storm).
A. Steno's Laws Named for Nicholaus Steno, a Danish physician living in
Florence, Italy in the 1600's.
1. Principle of Superposition
1. Oldest rocks on the bottom
2. Younger rocks on top
2. Principle of Original Horizontality
1. Sediments are deposited in flat layers
3. Principle of Original Lateral Continuity
1. Sediments are deposited over a large area in a continuous sheet
B. Other basic principles of Geology which we can use for relative dating
(or determining which rocks are older or younger)
1. Principle of intrusive relationships
The intrusion is younger than the rocks it cuts.
2. Principle of cross-cutting relationships
The fault is younger than the rocks it cuts.
3. Principle of components or inclusions
Note the irregular erosional surface. This is an unconformity.
The clasts (in the bed above the unconformity) are derived from the
underlying (older) bed.
The gravel clasts are older than the layer which contains them.
The layer containing the gravel must be younger than the layer from
which the clasts originate.
The principle of components or inclusions also applies to xenoliths.
A xenolith is a fragment of country rocks which has been broken off
during an intrusion, and has become surrounded by magma. The xenolith
is older than the igneous rock which contains it.
4. Principle of fossil succession
Fossils occur in a consistent vertical order in sedimentary rocks all
over the world.
Unconformities = buried surfaces of erosion or non-deposition
1. Angular unconformities
Implies tectonic deformation and erosion of underlying strata.
2. Nonconformity
Sedimentary strata overlying igneous or metamorphic rocks (in an
erosional - not intrusive- contact)
3. Disconformity
An irregular surface of erosion betwen two units of parallel strata
4. Paraconformity
A planar surface between two parallel units of sedimentary rock,
representing a period of non-deposition, but no erosion.
1. Angular Unconformity
Angular Unconformity, Grand Canyon, Arizona. Angular unconformities are ones in which the
overlying and underlying rocks dip at different angles. We can therefore infer that the underlying
rocks were tilted and eroded before the younger rocks were deposited. In the Grand Canyon, the
Cambrian Tapeats Sandstone is nearly flat-lying, and overlies tilted Proterozoic sedimentary rock.
Elsewhere, it overlies Proterozoic schist and granite as a nonconformity.
2. Nonconformity
Nonconformity, Grand Canyon, Arizona. Nonconformities are types of unconformities in which
sedimentary rock depositionally overlies metamorphic or intrusive igneous rock. In this photo,
Cambrian Tapeats sandstone overlies Proterozoic schist, with an age greater than 1.7 Ga--which
means that the erosional surface between the two represents more than a billion years of geologic
time! Elsewhere in the Grand Canyon, the Tapeats Sandstone overlies tilted Proterozoic sedimentary
rock to form an Angular Unconformity.
3. Disconformity
Disconformity, Death Valley, California. Disconformities are unconformities in which the
younger material is roughly parallel to the contact--and thus appears to be concordant. This
photograph shows the rocks being parallel on the left side. However, on the right it shows the
gravel cutting down into the marble to indicate erosion. Photo is approximately 1 meter across.
4. Paraconformity
The separation in a paraconformity is a simple bedding plane with no obvious buried erosional
surface. Because the beds above and below a paraconformity are parallel and no erosional surface
is evident, this unconformity can only be recognized because of the temporal gap (hiatus) in the
rock record.
 It tends to be delineated by a contrast in color change 
Rocks (Supplement)
Igneous Rocks
I.
Where they form
A. Intrusive rocks are formed within the earth (below the surface)
B. Extrusive rocks are formed at or above the Earth’s surface
II. Mineral Composition
A. Felsic rocks are light-colored, high silica contents, and contain quartz and feldspars
(such as granite)
B. Mafic rocks are dark-colored, have low silica contents, and are rich in iron and
magnesium (such as gabbro)
III. Grain Size
A. Large crystals form when the rock cools slowly
1. intrusive igneous rocks may have crystals larger than 1 cm
B. Small crystals form when rock cools rapidly
1. extrusive igneous rocks have small crystals or no visible mineral grains at all
IV. Texture
A. irregular crystal shape or no mineral grain
B. Fractional crystallization - well-shaped crystals form
C. Porphyritic texture – characterized by large, well-formed crystals surrounded by
finer-grained crystals (such as granite)
Sedimentary Rocks
I.
II.
III.
How they are formed
A. Weathering – wearing away of the rock which breaks them into smaller pieces
1. Chemical weathering – occurs when the minerals in a rock are dissolved or
otherwise chemically changed
2. Physical weathering – rock fragments break off along fractures or grain
boundaries; minerals remain chemically unchanged
3. Weathering produces rock and mineral fragments known as clastic sediments
B. Erosion – there are four main agents for this:
1. wind, water, gravity, & glaciers
2. Deposition – when sediments are laid down on the ground or sink to the
bottoms of bodies of water
Lithification = The physical and chemical processes that transform sediments into
sedimentary rocks.
A. It begins with compaction – usually caused by the overlying sediments
B. Heat – the temperature in Earth’s crust increases with depth by about 30°C per
kilometer
C. Cementation – occurs when mineral growth cements sediment grains together into
solid rock
Types of sedimentary rocks
A. Clastic sedimentary rocks – formed from the abundant deposits of loose sediment
found on Earth’s surface
1. Can be course-grained, medium-grained, or fine-grained
B. Chemical sedimentary rocks – during chemical weathering, minerals can be
dissolved and carried into lakes and oceans
C. Organic sedimentary rocks – form from the remains of once-living things
Metamorphic Rocks
I.
II.
III.
Causes of metamorphism
A. pressure & temperature
Types of metamorphism
A. Regional metamorphism – large regions are affected
1. generally high temperature and high pressure over a large region of Earth’s
crust
B. Contact metamorphism – small regions are affected
1. generally high temperature and low (to medium) pressure
Texture
A. Foliated – characterized by wavy layers and bands of minerals
B. Nonfoliated – lack mineral grains with long axes in one direction; composed
mainly of minerals that form with blocky crystal shapes
Minerals
Objectives:
1) Define a mineral
2) Describe how minerals form
3) Identify the most common elements in Earth’s crust
Mineral = A naturally occurring, inorganic solid with a specific chemical composition and a
definite crystalline structure.
Crystal = A solid in which the atoms are arrange in repeating patterns.
6 Major Crystal Systems:
1) Cubic (ex. = Pyrite)
4) Orthorhombic (ex. = Topaz)
2) Tetragonal (ex. = Wulfenite)
5) Monoclinic (ex. = Gypsum)
3) Hexagonal (ex. = Pyromorphite)
6) Triclinic (ex. = Feldspar)
Magma = Molten material found beneath the Earth’s surface. (It is called lava above ground.)
Silicates = Minerals that contain silicon (Si) and oxygen (O), and usually one or more other
elements. Silicates make up ~96% of the minerals found in Earth’s crust.
Ways Minerals are Formed:
1) from magma
a. large crystals – formed from cooling slowly beneath the Earth’s surface
b. small crystals – formed by cooling rapidly on the Earth’s surface
2) from solution
a. supersaturation – formed when a solution becomes supersaturated and mineral
crystals precipitate out of solution
b. evaporation – formed when liquids evaporate out of solution and elements left
behind arrange into crystals
Mineral Groups:
1) Native Elements [ex. = copper (Cu) , aluminum (Al) , silver (Ag)]
2) Oxides and Hydroxides [ex. = hematite (Fe2O3) , magnetite (Fe3O4) , brucite (Mg[OH]2)]
3) Halides [ex. = halite (NaCl)]
4) Carbonates [ex. = calcite (CaCO3) , dolomite (CaMg(CO3)2) , rhodochrosite (MnCO3)]
5) Sulfates [ex. = anhydrite (CaSO4)]
6) Silicates [ex. = olivine (Mg2SiO4) , feldspar (KAlSi3O8), quartz (SiO2)]
7) Sulfides [ex. = pyrite (FeS2)]
Mineral Identification:
1) color
 sometimes caused by the presence of trace elements or compounds within a mineral
 a milky appearance is cause by the numerous bubbles of gas and liquid trapped
within the crystal
 color is one of the least reliable clues to a mineral’s identity!!!
2) luster = the way that a mineral reflects light from its surface
 described as being:
a. metallic – shiny
b. nonmetallic – dull, pearly, waxy, or silky
3) texture = how a mineral feels to the touch
 smooth, rough, ragged, greasy, soapy, or glassy
4) streak = the color of a mineral when it is broken up and powdered
 tested by rubbing a mineral across an unglazed porcelain plate
 a mineral’s streak rarely changes; even if it is weathered or its color varies
 can only be used on minerals that are softer than porcelain; therefore, can’t be used
to test all minerals (but reliable on those that it can be used on)
5) hardness = a measure of how easily a mineral can be scratched
 one of the most useful tests for identifying minerals
 Mohs Hardness Scale [1 (talc) – 10 (diamond)]
6) cleavage = when a mineral splits relatively easily and evenly along one or more planes
& fracture = when a mineral breaks with rough or jagged edges
 determined by atomic arrangement; minerals break along planes where atomic
bonding is weak
7) density = mass per unit of volume (D = m/V)
& specific gravity = the ratio of the weight of a substance to the weight of an equal
volume of water at 4°C
 density reflects the atomic weight and structure of a mineral
 a particularly useful tool b/c density is not dependent on the size or shape of a
mineral
8) special properties
 double refraction = the refraction of a single ray of light into two rays (creating the
appearance of two images) when it passes through the mineral
 fizz test = certain minerals will react to HCl and fizz (releasing CO2)
 magnetism
Mineral Uses:
1) Ore = a mineral that contains a useful substance that can be mined for a profit
2) Gem = valuable minerals that are prized for their rarity and beauty
 Sometimes, the presence of trace elements can make one variety of a mineral
more colorful, and thus, more prized than other varieties of the same mineral