Geology &
Earth Science
1. Introduction
2. Geologic Time
3. The Earth’s Spheres
4. Plate Tectonics
5. Earthquakes and Seismology
6. Minerals
7. Rocks and Their Environments
8. Oceans and Shorelines
9. Deserts and Wind
10. Glaciers and Glaciation
11. Other Erosive Forces
12. The Water Cycle
13. Streams
14. Groundwater
15. Climate Change
Geology &
Earth Science
Introduction
Geology, from the Greek geo (“Earth”) and logia (“study”), is
the scientific study of the origin, history, and structure of the Earth.
Geologic Time
Geologists
consider time from the formation of the Earth to today, following a geologic timescale that breaks Earth
history into manageable pieces. Geologic time is divided and subdivided into eons, eras, periods, epochs,and ages:
·
The boundaries on the geologic timescale are set by
major events that have been preserved in the rock record. For example, the mass
extinction of dinosaurs and some marine invertebrate species 65 million years
ago forms the Cretaceous/Tertiary boundary, or K/T boundary.
Relative
Age
Early
geologists used general rock characteristics to create broad time divisions.
1.
Fossils: The preserved remains of extinct organisms. Deeply
buried rocks contain fossils that look very different from organisms that
inhabit the Earth today. Many geologic time subdivisions are based on the first
or last appearance of a certain fossil called an index fossil.
o Index fossil: The remains of a single species that was so
widespread that its fossils enable geologists to correlate environments and
time.
2.
Stratigraphy: The 17th-century Danish-Italian scientist Nicolaus
Steno formulated three principles in order to decipher a chronology based on
layering in sediments.
A.
Principle of superposition: The lowest layer in
sedimentary rock strata(layers) is the
oldest, and the layers above it are progressively younger.
B.
Principle of original horizontality: Sediments are
horizontally stratified when they are first deposited because they settle from
air or water under the influence of gravity.
C.
Principle of original lateral continuity: Sediment layers
originally extended in all directions until they thinned to nothing at the
edges. This principle is useful for tracing sequences (laterally extensive
sedimentary strata) across miles of terrain.
Unconformities: Gaps or
discontinuities in the rock record. Geologists study unconformities in an
attempt to match up the rock record across different places.
.
Angular unconformity: Contact between
strata that are oriented at different angles. Angular unconformities arise from
erosion, uplift, and tilting of rock.
A.
Disconformity (parallel unconformity): Contact between
strata that are oriented in the same angle but actually were deposited at very
different times.
B.
Nonconformity: Contact between two different rock types, like
sedimentary rock and volcanic rock.
Magnetic polarity reversals: Magnetic minerals in
rocks on the sea floor and on land record the orientation of Earth’s magnetic
field at the time during which the rock crystallized. Sometimes, as it is
today, the magnetic field is oriented “normally,” so that a compass needle
points north. The magnetic field periodically reverses, however, and rocks that
crystallize during those times record the reversal.
o Polarity reversal scale: A time scale based on
the principle that the sea floor is composed of stretches of rock of normal
polarity interspersed with rocks of reversed polarity. Each strip of rock was
formed over the length of time during which the magnetic field was in that orientation.
These strips correlate with other magnetic rocks from that time interval around
the world.
Technological
innovations have given geologists the means to determineabsolute age—that is, to put exact dates on events in the
rock record. For example, geologists have long recognized the K/T boundary as
the boundary after which no dinosaur fossils are found. By dating the rocks in
which the last dinosaur fossils appear, geologists have located the boundary at
65 million years ago.
1.
Isotopes: Some elements occur in several species, or isotopes, that
have different atomic weights. For example, the element carbon can exist as any
of three isotopes: 12C, 13C, and 14C.
2.
Radiometric dating: Geologists determine
absolute age based on analysis of isotopic decay—the principle
that isotopes of certain elementsdecay, or
change over time to a more stable isotope of a different element.
A.
Radioactive (unstable) isotopes: Isotopes with
unstable nuclei. Radioactive isotopes change over time to a stable configuration,
emitting particles or electromagnetic waves as they decay.
i.
The original parent isotope → (decays to) the daughter isotope, the decay product.
ii.
Decay rate (commonly referred to in terms of half-life) is
the time it takes half of a given amount of a parent isotope to decay to its
daughter product.
B.
Because isotopes have a variety of decay rates,
different isotopes are useful for dating rocks of different ages.
.
Uranium-235 is useful for dating old rocks because its
half-life is 700 million years.
i.
Carbon-14 is useful for dating young objects because its
half-life is only 5,730 years.
Half-lives of radioactive elements commonly used for dating
rocks:
Parent
|
Daughter
|
Half-life (billions of years)
|
|||||||||||||||||||||||||||
Uranium-235
|
Lead-207
|
0.704
|
|||||||||||||||||||||||||||
Uranium-238
|
Lead-206
|
4.47
|
|||||||||||||||||||||||||||
Potassium-40
|
Argon-40
|
1.25
|
|||||||||||||||||||||||||||
Rubidium-87
|
Strontium-87
|
48.8
|
|||||||||||||||||||||||||||
Samarium-147
|
Neodymium-143
|
106.0
|
|||||||||||||||||||||||||||
Thorium-232
|
Lead-208
|
14.0
|
|||||||||||||||||||||||||||
The Earth’s Spheres
Our
planet formed 4.5 billion years ago. Since then, it has developed and modified
four main physical environments that interact strongly with one another.
1.
Atmosphere: The layer of gases that surrounds the Earth. The
atmosphere protects us from the sun’s intense heat and radiation, provides the
air we breathe, and produces weather.
2.
Hydrosphere: The Earth’s water. The hydrosphere includes all the
liquid and frozen water of the Earth’s oceans and land (groundwater), as well
as water vapor in the atmosphere.
3.
Biosphere: All organisms living on and inside the Earth’s
surface.
4.
Lithosphere: The rigid, relatively cool rocky zone immediately
under the Earth’s surface. The lithosphere includes the Earth’s crust and
part of the upper mantle. The asthenosphere is the region in the upper mantle
(beneath the lithosphere) where rocks melt to form magma (molten
rock). The asthenosphere is less rigid than the lithosphere and is able to
flow. Movement of the lithosphere is directly connected to flow within the
asthenosphere.
The
Earth’s Interior
The
Earth’s interior is divided as follows:
1.
Crust (5–40 km thick): The thin outer skin of the planet.
2.
Mantle (2,885 km thick): The origin of most magma.
3.
Core (3,486 km thick): A dense, metal-rich ball inside
the Earth. The core is composed of the liquid outer core and solid inner core.
Plate Tectonics
Plates are the slabs of the
Earth’s crust that make up the lithosphere.
Plates
and the Earth’s Crust
The
Earth’s crust is composed of the continental crust (30–100 km thick;
forms the continents) and the oceanic crust (about 10 km thick;
denser than continental crust; mostly covered by oceans).
Plate
Tectonic Theory
Geologists
developed plate tectonic theory as a model of
movement on Earth’s crust on the surface of our planet. Observations and
measurements of the processes that lead to and result from this movement
support the plate tectonic model.
·
Continental drift: In the early 1900s,
scientists noticed that, based on the continents’ shapes, it looked like the
continents could fit snugly together. Geologists proposed that the continents
gradually float around on the surface of the planet, bumping into each other
and pulling apart.
·
Wilson cycle: In the 1960s, J. Tuzo Wilson proposed that
landmasses, over time, repeatedly join to form a supercontinent—an amalgamation of all the
continents into one big mass—and subsequently split apart.
Isostasy
Isostasy: The concept that the
crust “floats” on the heavier mantle in gravitational balance, like a block of
ice in water.
·
Mountains have “roots” that enable them to stay in
balance; bigger mountains have bigger roots.
·
When a great load is removed from Earth’s surface
(like when a glacier melts), the crust rebounds, or gently rises, to maintain isostatic equilibrium.
Plate
Boundaries
The plates meet at plate boundaries, which are the sites of most earthquakes, volcanoes,
and mountain formation. There are three types of plate boundaries:
1.
Convergent boundary: The margin between two plates that are moving toward each other.
Plate convergence leads to ocean-ocean, ocean-continent, or continent-continent
collision.
A. Subduction: Dense oceanic crust sinks beneath
less dense continental crust at a convergent boundary. In this setting, a
deep oceanic trenchforms along the coast above the subduction
zone, and volcanoes arise on the continental plate. An example
of ocean continent convergence is seen today in the Aleutian
Arc of Alaska.
B. This convergence eventually leads
to continent-continent collision and mountain formation as two
landmasses crumple into each other. A classic example of this mountain
formation is the convergence between India and Asia, which continues to build
the Himalayan chain and the tallest mountain in the world, Mt. Everest.
C. “Ring of Fire”: The circumference of the
Pacific Ocean, bounded by subduction zones at the edges of the Pacific plate,
that is the site of many volcanoes.
Divergent boundary (spreading center): The margin between two plates, usually both
oceanic, that are moving away from each other. Plates grow at spreading
centers, which are often coincident with mid-ocean ridges like
the Mid-Atlantic Ridge. At a mid-ocean ridge, magma rises from the
asthenosphere, pushing the plates apart and accreting, or
sticking onto, the sides of the plates. The plates widen in parallel strips as
they diverge from each other. This is also the source of magnetic striping on
the sea floor (see Magnetic polarity
reversals).
Transform boundary: The margin between two plates that are sliding past each other.
Transform boundaries are prominent features on sea floors, where they connect
offset mid ocean ridge segments. The most famous transform boundary is along
the San Andreas Fault in California, where the Pacific and North American
plates slide past each other.
Deformation
Rock layers crumple when the Earth’s
crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the
diagram below) or faults (fractures in the crust).
Earthquakes and Seismology
Faults
Fault: A fracture in the
Earth’s crust caused by stress. There are several different types of faults:
1.
Normal fault: A fault in which the hanging wall (the block of crust above the fault) moves down
relative to the footwall (the block of crust below the fault) as a result of
extension.
2.
Reverse fault: A fault in which the hanging wall moves up relative
to the footwall as a result of compression.
3.
Strike-slip fault: A fault in which two
blocks of crust slide past each other on the same plane. The San Andreas Fault
is a strike-slip fault.
Earthquakes
Earthquake: A vibration of the
Earth caused by slippage along a fault.
1.
Hypocenter (focus): The exact location of
an earthquake (often far below the surface).
2.
Epicenter: The point on the Earth’s surface directly above the
hypocenter.
3.
Foreshocks: Small earthquakes that commonly precede a major
earthquake.
4.
Aftershocks: Small earthquakes that commonly occur after a major
earthquake.
Seismic
Waves
Energy
travels away from an earthquake’s focus in waves, both through the Earth and
along its surface. Different types of seismic waves include:
1.
Surface waves: Seismic waves that travel along the Earth’s surface.
2.
Body waves: Seismic waves that travel through the Earth’s
interior. There are two types:
A.
Primary waves (P waves): Body waves that
compress and expand rock in the direction the waves travel (like a slinky).
B.
Secondary waves (S waves): Body waves that shake
material at right angles to the direction the waves travel (like shaking a
rope). Solid rock transmits S waves, but gases and liquids do not.
Measuring
Earthquakes
Geologists
use the Richter scale to assign magnitude to earthquakes by
assessing the amplitude (height) of the largest seismic wave each earthquake
creates. Each additional unit of magnitude denotes a tenfold increase in the
power of the earthquake (e.g., a magnitude 7.0 earthquake is ten times more
powerful than a magnitude 6.0).
Locating
Earthquakes
The exact location of an
earthquake’s epicenter is determined throughtriangulation, which
requires several seismometers (instruments that record seismic waves) stationed
around the world.
1.
Seismometers
record P wave arrival first, followed by S wave arrival.
A. The time difference in arrival is
used to calculate the distance from the seismometer to the earthquake
epicenter.
B. However, this measurement tells
only the distance to the earthquake, not the direction in which it lies.
To
determine location, each of three stations draws a circle around their station
location with the radius of the distance it calculated. The epicenter is at the
intersection of the three circles.
Earthquake
Aftermath
In
addition to causing great destruction at the epicenter, an earthquake sometimes
triggers other natural disasters.
1.
Tsunami: A massive wave created when an earthquake shakes
coastal or undersea land. Tsunamis have a short height but a long length (seeShorelines), causing amplification of tides.
A.
Tsunamis are especially dangerous because they
cause low tides to be very low. While people are walking the freshly exposed
beach, the high tide comes in quickly and much higher than normal.
B.
A tsunami can move thousands of miles across the
ocean at hundreds of miles per hour. An earthquake in Japan, for instance, can
send a tsunami all the way to Hawaii.
Landslide: A fast-moving wall of dirt and mud that an
earthquake shakes loose. Landslides are primarily a problem in hilly, populated
regions like Southern California.
Earthquake
Aftermath
In
addition to causing great destruction at the epicenter, an earthquake sometimes
triggers other natural disasters.
1.
Tsunami: A massive wave created when an earthquake shakes
coastal or undersea land. Tsunamis have a short height but a long length (seeShorelines), causing amplification of tides.
A.
Tsunamis are especially dangerous because they
cause low tides to be very low. While people are walking the freshly exposed
beach, the high tide comes in quickly and much higher than normal.
B.
A tsunami can move thousands of miles across the
ocean at hundreds of miles per hour. An earthquake in Japan, for instance, can
send a tsunami all the way to Hawaii.
Landslide: A fast-moving wall of dirt and mud that an
earthquake shakes loose. Landslides are primarily a problem in hilly, populated
regions like Southern California.
Minerals
Minerals are earth materials
that have four main characteristics: they aresolid, inorganic, naturally occurring, and have a definite chemical structure.
Mineral
Properties
Minerals
are identifiable based on a number of specific properties:
1.
Crystal form: The outward expression of a mineral’s chemical
structure. For example, quartz has a hexagonal, or 6-sided, crystal form.
2.
Cleavage: Planes of weakness in the mineral’s crystal lattice
along which the mineral tends to break. Cleavage faces are usually flat
surfaces.
3.
Fracture: If a mineral lacks cleavage, it fractures in an
irregular, jagged manner.
4.
Hardness: The resistance of a mineral to being scratched.
Geologists use the Mohs scale to
assign each mineral a hardness between 1 (softest) and 10 (hardest).
The Mohs hardness scale
Mineral
|
Talc
|
Gypsum
|
Calcite
|
Fluorite
|
Apatite
|
Feldspar
|
Quartz
|
Topaz
|
Corundum
|
diamond
|
Hardness
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
Streak: The color a mineral
leaves when rubbed across a piece of unglazed porcelain.
A mineral’s visible color is not a reliable
diagnostic property. A single mineral may vary in color from sample to sample,
but its streak color does not. For example, quartz may be clear, gray, purple,
or pink, but its streak is always colorless.
Luster: The way light
reflects off a mineral’s surface. Luster may be described as vitreous (glassy), metallic, pearly, silky, or dull.
Specific gravity: The comparison of a
mineral’s weight to the weight of an equal volume of water (water’s specific
gravity is 1). The greater a mineral’s specific gravity, the greater its
density.
Other diagnostic properties: Some minerals are magnetic, some tastesalty, and some fizz when hydrochloric acid is dropped on them.
Mineral
Groups
1.
Silicates: The most common mineral group. Silicates have a
framework of silicon (Si) and oxygen (O), the two most common elements in the
Earth’s crust.
A.
Silicon-oxygen tetrahedron: The basic silicate
structure, which consists of four oxygen atoms around a central silicon atom.
B.
Silicate minerals can form from:
i.
A single tetrahedron (e.g., olivine)
ii.
Single chains (pyroxenes, e.g., augite)
iii.
Double chains (amphiboles, e.g. hornblende)
iv.
Sheets (micas, e.g., muscovite, biotite)
v.
Three-dimensional networks (e.g., feldspar, quartz)
Nonsilicates: Less common but also important rock-forming
minerals.
.
Carbonates: Contain carbon and oxygen in a carbonate group (CO3). Calcite (CaCO3), which forms
limestone and marble, is a common carbonate.
A.
Oxides: Usually consist of
oxygen and another element. Common oxides include ice (H2O) and magnetite (Fe3O4).
B.
Sulfides: Contain sulfur ions. Pyrite, or “fool’s gold,” is a common sulfide.
C.
Sulfates: Contain sulfur and oxygen in a sulfate group (SO4). Gypsum, a
material used in buildings, is a common sulfate.
D.
Halides: Contain a “salt” ion such as Na, Cl, or F. Halite,
or common table salt (NaCl), is a halide.
E.
Native elements: Minerals that exist
in pure elemental form. Native elements include gold (Au), silver (Ag), and copper (Cu).
Common
Rock-Forming Materials
1.
Felsic minerals: Comprise over 50% of
the Earth’s crust. Felsic minerals are silicates that are light in color,
contain little iron and magnesium, and have abundant silica.
A.
Quartz (SiO2): Has vitreous luster;
lacks cleavage but has conchoidalfracture (smooth, curved fracture like that
of glass); lacks streak; and is usually gray in color but can be pink, purple,
or black.
B.
Feldspars:Potassium feldspar (KAlSi3O8) and plagioclase((Ca,Na)AlSi3O8) both have distinct
cleavage planes that meet at about a 90° angle. Potassium feldspar usually is
cream or pink in color, whereas plagioclase usually is in a range between white
and light gray.
C.
Mica: A family of sheet silicates, including silvery muscovite and blackbiotite. Micas are important minerals and often
give rocks a sparkly appearance.
Mafic minerals: Contain iron and/or
magnesium, making them dark.
.
Olivine ((Fe, Mg)2SiO4): Has glassy luster,
conchoidal fracture, and is usually dark green. Olivine is a major component of
the upper mantle.
A.
Pyroxenes: Usually dark green to black, with distinctive
cleavage planes that meet at right angles. Pyroxenes form a group of chemically
complex minerals, the most common of which is augite, which
are common in oceanic crust.
B.
Amphiboles: A complex group, distinguished from pyroxenes on
sight by their cleavage planes, which meet at 60° and 120°. The most common ishornblende.
Rocks and Their Environments
Rocks are aggregates of
minerals.
Igneous
Rocks
1.
As magma (molten material) cools, ions arrange
themselves into orderly patterns during crystallization. There are two types of crystallization:
A. Volcanic (extrusive): Magma crystallizes
quickly at spreading centers and from volcanic eruptions.
B. Plutonic (intrusive): Magma crystallizes
slowly deep below the Earth’s surface.
Magma’s rate of cooling affects crystal size and mineral composition. Fast
cooling results in smaller crystals, more mafic; slow cooling results in larger
crystals, more felsic.
.
Glass: No crystals. Forms when magma cools too rapidly to
form crystals.
A. Fine-grained (aphanitic): Crystals too small to
distinguish individual minerals with the unaided eye. Gas bubbles leave openings
or vesicles.Aphanitic rocks form quickly at Earth’s
surface or in the upper crust (volcanic).
B. Coarse-grained (phaneritic): Crystals large enough
to distinguish minerals with the naked eye. Phaneritic rocks form in a slowly
cooling magma chamber deep in the crust (plutonic).
C. Porphyritic: Large crystals in a matrix of smaller crystals.
Porphyritic rocks form when magma crystallizes rapidly, forming a fine-grained
matrix, but then moves to a slower-cooling environment before all the melt has
crystallized. The remaining melt forms large crystals.
Bowen’s reaction series: The geologist N. L.
Bowen (1887–1956) created a chart showing the series in which different
minerals crystallize from cooling magma:
o On the left side: Mafic minerals begin to crystallize.
After each mineral crystallizes, it reacts with the remaining magma to form the
next mineral in the series.
o On the right side: Felsic, calcium-rich minerals
crystallize to form early feldspars, which then react with sodium in the
remaining magma to form more sodium-rich feldspars.
o At the bottom of the series: When magma
crystallization is nearly complete, the remaining magma is mostly SiO2, and quartz forms.
Volcanoes
1.
Volcanoes form
where magma burns through the crust, at subduction zones, at spreading centers,
or at “hot spots” like Hawaii.
A. Successive eruptions build a cone
of hardened lava. Eruptions are explosive (pyroclastic) if the
magma is gas-rich and felsic, slow if the magma is gas-poor and mafic.
B. Although volcanoes typically form
at subduction zones or spreading centers, they also may form within a plate, as
in the Yellowstone region of Wyoming.
Volcano morphology
.
Crater: The
pit inside a volcano. A crater more than 1 km wide is called acaldera.
A.
Vent: A
pipelike structure connecting the underground magma chamber to the crater.
Types of volcanoes
Shield
volcano: A
broad, slightly domed structure typically built of liquid basalt. The Hawaiian
volcanoes are shield volcanoes.
Composite
cone (stratovolcano): A
large, nearly symmetrical cone made of alternating lava flows and pyroclastic
volcanic debris.
Cinder
cone: A
generally small volcano with steep sides, built from ejected lava fragments and
often in groups near larger volcanoes.
Volcanic rocks
Basalt: Dark green to black,
fine-grained, mostly pyroxene and plagioclase feldspar, with some olivine. The
ocean floor is mostly basalt.
Tuff: Hardened ash from an
explosive volcano.
Plutons are
the site of plutonic rock formation. Most magma in the Earth is deep underground,
in chambers that cool slowly or rise slowly to intrude into preexisting rock.
.
Plutonic rocks
i.
Gabbro: Has
a basaltic composition (mafic) but large grain size.
ii.
Granite: A
phaneritic igneous rock with 25–35% quartz and more than 50% feldspar, with
hornblende, muscovite and biotite.
A.
Pluton forms
.
Batholith: A
large expanse of granitic rock (more than 100 km2). Batholiths
frequently form the cores of mountains, exposed only after much of the ground
surface erodes.
i.
Sill: A
lateral layer of igneous rock formed when fluid basaltic magma rises from a
magma chamber and squeezes into horizontal strata.
ii.
Dike: A
vertical or angled layer of igneous rock that cuts across other rock layers,
usually by injection into fractures.
Metamorphic
Rocks
1.
High temperature, high pressure, or variable
chemical conditions can change country (preexisting) rocks through the
process of metamorphism.Rocks remain
solid during the process.
A.
Regional metamorphism: An extensive volume
of the crust is metamorphosed, usually by intensive compression at convergent
boundaries.
B.
Contact metamorphism: Intruding magma heats
cold country rock nearby and causes it to recrystallize.
C.
Metasomatism: Hot fluids dissolve original minerals, and then
chemical reactions cause new minerals to grow.
Rocks undergo both mineral and textural changes
during metamorphism.
.
Mineral changes: During metamorphism,
two minerals can react, and their ions can diffuse across grain boundaries,
resulting in a new mineral. Alternatively, complex minerals may break down into
simpler ones.
A.
Textural changes: Rocks gain foliation (alignment)
as minerals align into bands. With increasing temperature and pressure, grain
size increases and texture coarsens.
Classification: Metamorphic rocks are
classified by strength of metamorphism. The following are listed in order from
weak to strong metamorphism:
.
Foliated rocks:
i.
Slate: A fine-grained rock, usually made of metamorphosed
fine sediments.
ii.
Phyllite: Similar to slate but slightly coarser-grained, and
shiny due to high mica content.
iii.
Schist: A coarse-textured metamorphic rock, with minerals
aligned in parallel bands, containing more than 50% platy minerals (minerals
with a planar, layered structure) like mica.
iv.
Gneiss: Bands of abundant coarse grains, mostly feldspar
and quartz, alternated with bands of flaky minerals.
A.
Nonfoliated rocks:
.
Marble: Metamorphosed limestone with a sugary texture.
Marble is composed of interlocking calcite grains.
i.
Quartzite: Metamorphosed quartz sandstone. Quartzite is very
hard and is composed of interlocking quartz grains.
ii.
Hornfels: Fine-grained rock altered in contact zones around
igneous intrusions.
Sedimentary
Rocks
1.
When
weather and other forces of erosion wear away rocks, sediments
form. Those sediments can be compacted, through lithification, to
form sedimentary rocks.
A. Erosion: The transport of material
around Earth’s surface by a mobile agent like water or wind. Erosion and
weathering form sediments and soil.
i.
Mechanical (physical) weathering: Rocks break into smaller pieces, with each
piece retaining the original mineral composition.
§ Frost wedging: Water freezes and expands
in a rock, breaking off fragments.
§ Unloading: Erosion removes material
from above buried rock. Pieces pop off in response to the lowered pressure.
§ Biological activity: Roots wedge into and widen
rock fractures, or animals burrow into soil and expose rock to the surface.
ii.
Chemical weathering: Rocks break down chemically, and their constituent minerals alter
during the process.
§ Oxidation: Water (H2O) is the strongest chemical weathering agent. It
causes iron-rich rocks to oxidize, or rust.
§ Ionization: CO2 + H2O → carbonic
acid, which breaks granite down into clay minerals.
B. Lithification: After erosion and
weathering, sediments cement to form sedimentary rocks.
Sedimentary settings: Sedimentary rocks can form anywhere on or
just below the Earth’s surface, in dry or wet environments.
Classification: Sedimentary rocks may be classified in several different ways:
.
Based on origin:
.
Detrital sediments: Sediments that are fragments of broken-down rock. They are listed
here in order of decreasing grain size:
§ Breccia: Lithified angular blocks of
rock.
§ Conglomerate: Lithified round rock
fragments, pebble-sized and larger.
§ Sandstone: Cemented sand.
§ Shale: Compacted clay, mud, or
silt.
i.
Chemical and biochemical sediments: Sediments that form from minerals that
precipitate from water, either physically or biologically (as organisms pull
elements out of water to make their skeletons):
§ Limestone (CaCO3): Formed from cemented
fragments of any size of shell.
§ Chert: Cemented shells made of
silica.
ii.
Evaporites: Sediments
that form as water evaporates from a closed basin and the solution becomes
supersaturated with certain elements, which then precipitate out as minerals
like halite.
iii.
Coal: An
organic material that nonetheless is considered a sedimentary rock because it
consists of compacted plant matter.
A.
Based on grain size and sorting:
.
Grain size: The
physical size of individual grains that make up sedimentary rock.
§ Gravel (>2 mm) forms
conglomerate, breccia
§ Sand (1/16–2 mm) forms
sandstone, greywacke
§ Mud (<1/16 mm) forms shale,
mudstone
i.
Sorting: The
degree of variety of grain size and composition within a sedimentary rock.
§ Well sorted: One grain size and type
dominates the rock’s composition.
§ Poorly sorted: The rock is composed of
grains of many sizes and compositions.
B.
Based on sedimentary structure:
.
The
method of sediment deposition often imparts a distinctive pattern on a package
of sedimentary rocks.
i.
Bedding planes: The planes that separate strata (layers) of
sedimentary rock. Often, these planes are the planes along which the rock
breaks.
§ Cross-bedding: Wind or waves deposit
sediments in an upsweeping pattern.
§ Graded bedding: Grain size becomes coarser
or finer from the bottom to the top of a layer.
The Rock Cycle
1.
Heat,
pressure, erosion, and other forces are always at work on the Earth, changing
the composition of rocks: from igneous to sedimentary, sedimentary to
metamorphic, igneous to metamorphic, and so on.
2.
These
continual conversions from one type of rock to another are collectively termed
the rock cycle.
A. All rocks exposed at Earth’s
surface are subject to weathering, which leads to the
formation of sediments.
B. All sediments are subject to
burial, after which they are undergo lithification (cementation and
compaction) to form sedimentary rocks.
C. All rocks can be exposed to heat
and pressure, which often leads to formation of metamorphic rock.
D. When metamorphic rocks melt into
magma, the magma sometimes cools and crystallizes into igneous
rock.
Oceans and Shorelines
Continental
Edges and the Ocean Floor
The
edge of each continent gradually slopes downward underwater for a number of
miles offshore. After this gradual sloping, there is a sudden drop onto the
deep sea floor.
1.
Continental margin: The stretch of crust
from the shoreline to the deep sea. The continental margin includes the:
A.
Continental shelf: The zone of gently
sloping underwater ground where the water gradually deepens.
B.
Shelf break: The sudden drop at end of the continental shelf.
C.
Continental slope: The steep underwater
cliff after the shelf break.
D.
Continental rise: The gentle slope at
the base of the break. The continental rise is at the end of the continental
margin and leads to the abyssal plain.
E.
Abyssal plain: The very flat, deep-ocean floor.
Undersea features: Water and sediment
eroding from the continents leave imprints on the continental margin. The
continental slope can be unstable.
.
Submarine canyons: Deep, steep-sided
underwater valleys, possibly former river channels.
A.
Turbidity currents: Sediment-laden water
at the continental slope. Turbidity currents often flow downward, eroding the
slope and entraining (drawing along and transporting) more sediment.
B.
Turbidites (deep-sea fans): Sediments deposited
by turbidity currents on the continental rise. Turbidites typically are
composed of sequences of sediments that are coarse grained at the bottom and
fine-grained at the top.
Shorelines
1.
Tides: Daily
changes in ocean surface height caused by the gravitational attraction between
the Earth and the moon. This gravitational attraction leads to rhythmic rising
and falling of the waterline.
2.
Currents: Continuous
flows of water in one direction.
3.
Waves: Water
surface ripples generated by wind.
A. Wave crest: The top peak of a wave,
vs. trough, the lowest point between waves.
B. Wave height: The vertical distance from
trough to peak.
C. Wavelength: The horizontal distance
from one wave peak to the next.
D. Waves begin to slow when water
depth is 1/2 the wavelength. When wave height is 1/7 of the wavelength, the
wave breaks, or collapses. Breaking waves carve the shoreline.
Beach: The
sloping shoreline made of sediments moved by waves, tides, and currents.
.
Offshore: The
region below the low tide line.
A.
Foreshore: The
region between the low tide line and above the flat beach.
B.
Backshore: The
region landward of the beach face.
C.
Berm: A
small hill of sediments just above the flat beach area. Gentle waves build the
berm up in summer, and storms carve the berm away in winter.
Deserts and Wind
1.
Deserts form in land areas with low precipitation
(typically less than 25 cm of rain per year).
2.
Wind: The movement of air on the Earth’s surface stems
from the uneven distribution of solar heat. Hot air rises over the equator,
drops out moisture, and descends as cool, dry air over latitudes 30 N and 30 S.
Deserts are found at these latitudes.
Wind-Created
Features
Wind
is a strong sculpting agent. It carves away rocks and sediments and deposits
sediments elsewhere.
1.
Bed load: The sand grains and other particles that wind (or
water) carries on or just above the ground.
2.
Suspended load: The fine particles
that wind (or water) keeps aloft.
3.
Saltation: The “jumping” of sand grains due to strong wind.
Wind blowing perpendicular to a surface decreases the pressure on that surface.
When the inertia of a sand grain is overcome, it begins to roll. When it hits
other grains, they bounce into the air, where they are carried forward until
gravity pulls them back down.
4.
Deflation: A process by which wind carries fine particles away
and leaves a compact surface of larger pebbles.
5.
Dunes: Sand mounds or ridges that the wind creates. Dunes
have a steep side called a slip face. Types of dunes include:
A.
Barchan dune: A solitary dune shaped like a horseshoe, with its
tips pointing away from the wind. Barchan dunes form on flat surfaces where
sand supply is low.
B.
Transverse dune: A long ridge of sand
oriented perpendicular to the direction of the wind. Transverse dunes form
where wind is steady and sand is plentiful.
C.
Longitudinal dune: A dune that forms
parallel to wind direction, in places where sand supply is limited.
D.
Parabolic dune: A dune shaped like a
barchan dune but with its tips pointing into the wind. Parabolic dunes form on
beaches with abundant sand and are partly covered by vegetation.
Glaciers and Glaciation
Glacier: A large mass of ice,
formed on land by compaction of snow, that flows downhill from snow accumulated
at the head. Glaciers survive by accumulating more snow each year than they
lose to snowmelt.
1.
Alpine glacier (valley glacier): A glacier in
mountains that flows down channels previously eroded by streams.
2.
Piedmont glacier: A glacier that forms
when several valley glaciers flow out onto land at the front of a mountain
range and merge with one another.
3.
Ice sheet (continental glacier): A large expanse of
ice that flows in all directions.
Locations
Most
expanses of ice on Earth are at the extreme latitudes, where the weather
remains consistently cold. Greenland, in the north, houses
one huge ice sheet, and Antarctica, in the south, is the
site of the other. Together, these ice sheets cover 10% of the Earth’s land
surface. Smaller ice sheets are found at high altitudes, in places like Alaska,
Canada, and the Alps.
Formation
and Morphology
1.
Snow line: The line above which more snow falls than melts in
a year, creating permanent snow cover.
2.
Firn: Packed snow that turns into ice over time. Under
pressure, the boundaries of ice grains melt, and the grains refreeze together,
forming the interlocking ice crystals that comprise a glacier.
3.
Glacial terminus: The front toe of a
glacier.
4.
Ablation: The direct conversion of ice into vapor. Glaciers
lose mass by this process.
5.
Zone of fracture: The upper 35 meters
of glacial ice, where ice responds rigidly to stress by cracking.
6.
Zone of plastic flow: The area of glacial
ice 35 meters below the surface and deeper. In this zone, glaciers deform under
their own weight. Squeezing and flow therefore are greatest where the glacial
ice is thickest.
Erosion and
Landforms
Glaciers carve the landscape as they
flow and leave deposits in their wakes.
1.
Glaciers
cause erosion in several ways:
A. Plucking: Glacial melt water runs
into rock crevices, freezes, expands, and causes fragments to break off.
B. Abrasion: Rocks entrained in the
bottom of a glacier grind against the surface over which the glacier flows.
Abrasion causes glacial striations,parallel grooves worn into
bedrock.
Glacial
movements can create many different landforms:
.
Cirque: An
amphitheater-shaped scoop out of bedrock, caused by plucking.
A.
Arête: A
knife-edged rock ridge between adjoining cirques.
B.
Glacial trough: A valley with a U-shaped bottom, carved by a glacier.
C.
Fjord: A
glacial trough that extends into the sea.
D.
Till: Unlayered
sediments deposited by a glacier. Till often includes boulders, gravel, sand,
and clay together.
E.
Moraine: A
ridge of till left behind by a retreating (melting) glacier. Types of moraines
include:
i.
Terminal moraine: The ridge left behind at the farthest point a glacier reaches (the
line of maximum advance).
ii.
Lateral moraine: A long, narrow mound of till that forms parallel to the glacier’s
direction of motion (as a line within the glacier).
F.
Glacial erratic: A large boulder that a glacier carries from its place of origin
and drops in a different place.
G.
Drumlin: A
low, rounded hill of till, with a tapered end pointed in the direction the
glacier flowed. The other end has a steep, squared face.
H.
Outwash plain: A
large sheet of stratified sediment deposited by melt water streaming out of the
toe of a glacier.
I.
Esker: A
winding ridge of sediment left behind when a glacier melts.
J.
Kettle: A
scoured depression in an outwash plain, formed when a block of glacial ice is
left behind, buried, and melts.
Ice
Ages
Over
geologic time, the Earth has undergone periods of extreme cold during which ice
sheets expanded, and periods during which glacial ice melted (see Climate Change).
Other Erosive Forces
Mass wasting is movement of rocks
and soil caused by the loosening effects of water and the downward force of
gravity. There are several types of mass wasting.
1.
Creep: Slow earth movement that has effects seen only
after some time. Creep usually occurs where soil freezes, expands, thaws, and
settles. Evidence of creep is seen in tilted telephone poles and cemetery
headstones.
2.
Fall: The unrestrained fall of rock fragments off a cliff.
A rock fall createstalus—fields
of rock fragments that collect at the bottom of a cliff.
3.
Slide: The breaking off of rocks or soil from a plane of
weakness and their subsequent slide down the face of a steep slope.
4.
Slump: A type of slide that involves intact blocks of rock
sliding down a concave surface.
5.
Flow: The quick movement of water-soaked sediments down a
slope in one large mass. Flow occurs when these water-soaked sediments are
shaken.
6.
Solifluction: The flow of watery soil due to repeated thawing and
freezing, as happens to a dirt road after winter.
The Water Cycle
Hydrologic
Cycle
The
Earth’s water supply is always in motion, going through an unending cycle of
water running from the land to the sea, precipitating, evaporating, and reprecipitating.
Water is always recycled or moved from place to place—never completely
destroyed or created anew.
1.
Runoff: Water that flows off the land surface into rivers,
lakes, streams, and oceans.
2.
Transpiration: The process by which plants release the water they
absorb into the atmosphere.
3.
Water budget: Most of the Earth’s water is contained in the
oceans, but other reservoirs hold significant water as well. Geologists measure
the Earth’s water supply in terms of volume (measured in km3):
A.
Oceans: 1,350,000,000
B.
Glaciers: 27,500,000
C.
Groundwater: 8,200,000
D.
Lakes: 205,000
E.
Atmosphere: 13,000
F.
Streams: 1,700
Streams
Streams are channeled flows
of any amount of water. Although streams hold only a small percentage of the
Earth’s water at any given time, the energy of streams has done much to sculpt
the landscape. Stream energy is controlled by channel size and slope.
1.
Gradient: The steepness of land over which a stream flows. As
a stream flows down a slope, its potential energy converts to kinetic energy.
The steeper the gradient, the faster the stream flows.
2.
Base level: The lowest level to which a stream can erode its
channel. Oceans are considered the ultimate base level because they are the final destination
of streams. More often, local base levels like lakes, dams, or stream junctions
control stream flow.
3.
Cross section: The area of water in a cross-sectional slice of a
stream. For a flat stream, cross section is calculated by multiplying depth by
width. For a semi-circular stream, it is calculated using stream radius:
(1/2)πr2.
4.
Discharge: The volume of water that flows past a certain point
in a stream over a measured time interval. Discharge is calculated by
multiplying the cross section of the stream by the velocity of the stream.
Stream
Flow and Transport
1.
Water can flow within a stream in two ways:
A.
Laminar flow: In slow-moving streams, water flows in parallel
paths.
B.
Turbulent flow: In fast-moving
streams and in rough stream channels, water swirls around as it moves down a
gradient.
Capacity: The amount of sediment a stream can carry past a
certain point in a given time.
Competence: A measure of how strong a stream is, based on the
biggest size of an object the stream can move.
Saltation: Skipping and bouncing of particles on the bottom of
a stream caused by water flow pushing the particles.
Load: The material a stream carries. There are several
types:
.
Bed load: Heavy objects dragged along a stream bottom.
A.
Suspended load: Fine particles
carried suspended in a stream’s moving water.
B.
Dissolved load: Material (salt,
carbonate, or other ions) dissolved in the stream water.
Graded stream: A stream with a slope and channel that have
adjusted enough over time so that the stream has just enough energy to carry
its load, but no excess energy so that it erodes its banks.
Stream
Settings
1.
Alluvial fan: A gently sloping blanket ofalluvium, or
sediment deposited by a stream, where it exits a gully onto a flatter surface
2.
Flood plain: A plain surrounding a stream. Streams periodically
overflow their banks or move laterally across surrounding flood plains, leaving
layers of sediments in their wake.
3.
Delta: The mouth of a stream, where the stream slows due
to a gentler gradient and deposits much of its sediments as it moves to base
level.
4.
Tributary: A small stream that flows into a larger stream.
5.
Drainage system: All the land area
that contributes to a stream system. Acontinental divide is a ridge that separates streams
flowing in opposite directions on either side. For example, the Great Divide in
the United States follows the Rocky Mountains: all streams east of the divide
flow east to the Atlantic, whereas all streams west of the divide flow west to
the Pacific.
Stream Shapes
and Patterns
There are several types of streams
and drainage patterns, which are dictated by landforms and also shape those
landforms. Whereas glaciers carve flat-bottomed, U-shaped valleys, streams
carve sharp canyons, or V-shaped valleys.
1.
Braided stream: A stream that divides into smaller streams. When a stream gradient
decreases, its flow slows, causing the stream to branch into smaller
subchannels. Braided streams are common on alluvial fans and glacial outwash
plains.
2.
Meandering stream: A stream that carves a path sideways and forms wide loops,
called meanders, as it flows downstream. Often, when water in
a stream flows over a bump, ripples are created that deflect water toward one
side of the stream and carve into the side. This sideways flow creates a bend
in the channel, and water flowing out of this bend then deflects toward the
opposite side of the stream, carving a bend there.
A. Point bar: Sediment deposited in the
inner curves of a meandering stream. The stream moves slowest in these inner
curves, so the stream drops sediment here.
B. Oxbow lake: A lake that splits off from
a meandering stream when erosion carves a straight channel that cuts off the
flow into one of the stream’s meanders.
Streams
can follow several different drainage patterns:
.
Dendritic drainage: Several substreams branch out from a main stream in a treelike
pattern.
A.
Radial drainage: Streams run in all directions from a central high point.
B.
Rectangular drainage: Streams make right-angled turns, following
rectangular fracture patterns in the bedrock over which they flow.
C.
Trellis drainage: Tributaries flow perpendicular to the main channel, following
parallel beds of weak strata. Trellis drainage often occurs in tilted or folded
rocks.
Groundwater
Groundwater is surface water that
seeps into the ground. It constitutes 95% of the Earth’s supply of fresh water
(outside of glaciers) and feeds not only humans and crops but also streams and
lakes.
Groundwater
Distribution and Movement
1.
The ground under the surface of the Earth’s
landmasses is divided into two zones based on the presence or absence of
groundwater:
A.
Zone of aeration: The area just below
ground in which spaces between rocks and soil are filled with air.
B.
Zone of saturation: The area below the
zone of aeration in which the spaces between particles are filled with water.
The top level of the zone of saturation is called the water table. The
water in the zone of saturation is groundwater.
The porosity and permeability of soil and rock
dictate the accumulation and movement of groundwater.
.
Porosity: The ratio of open spaces to volume of material.
Porosity is quantified in percentages.
A.
Permeability: A measure of the ease with which sediments
transport water. Permeability is calculated as the volume of water that can
move through a cross-section of sediments in a given time. Permeability is
classified on a scale from very low to excellent.
§ Sand typically has 20% porosity and excellent
permeability.
§ Clay has a 50% porosity but poor permeability.
The water table roughly follows topography, rising
slightly beneath hills and depressed beneath stream channels. Where the land
surface cuts low, the water table intersects the land, typically at stream
channels.
.
Effluent stream: A stream that gains
water from the zone of saturation, typically in wet environments.
A.
Influent stream: A stream that loses
water to the water table, typically in dry environments.
B.
Hydraulic gradient: The slope of the
water table. The hydraulic gradient, along with the permeability of material
through which water flows, influences the speed of groundwater flow.
C.
Recharge area: An area (usually higher elevation) that receives
precipitation that soaks into the zone of saturation.
D.
Discharge area: An area (usually near
a stream) that receives groundwater from the zone of saturation and carries it
away.
E.
When recharge and discharge are in balance, the
water table remains steady.
Groundwater availability is affected by the types
of rock underground and the flux of water on the Earth’s surface.
.
Aquifer: An underground body of permeable rock or sediment
that conducts water. Aquifers typically are composed of sand or gravel.
A.
Confining bed: A laterally continuous sheet of rock that is
impermeable to water and prevents the escape of water from aquifers. Typically,
a confining bed is composed of shale (see Sedimentary Rocks).
i.
Confined aquifer: An aquifer between
two confining beds.
ii.
Unconfined aquifer: An aquifer above a
confining bed.
B.
Artesian well: A well drilled into a confined aquifer.
.
Confined aquifers cannot receive precipitation from
directly above; instead, water seeps in from far away on the sides, where the
top confining bed thins to nothing.
i.
As a result, a confined aquifer often has high
water pressure. When a well is drilled into the top confining bed, water gushes
upward out of the confined aquifer.
C.
Perched water table: A pocket of
groundwater stranded above the main water table by a confining bed beneath it
Geology
and Groundwater
Groundwater
can move in confined channels underground, carving out spaces like caves.
1.
Cave: A crevice in a rock large enough for a person to
enter. Caves usually form when acidic water flows through limestone formations
and dissolves the calcium carbonate. As this carbonate-rich water drips off a
cave ceiling, it forms:
A.
Stalactites: Calcite deposits that hang from a cave ceiling.
B.
Stalagmites: Calcite spears that point up from a cave floor.
When groundwater dissolves underground limestone,
strange topographical features may result:
.
Sinkhole: A depression that forms on the surface when the
roof of an underground cavern collapses.
A.
Karst topography: Irregular topography
on the surface that results when groundwater below flows through an extensive
area of limestone, carving underground channels and caverns until surface water
flows only underground. The land above these areas takes on irregular patterns
as it sinks into various holes and grooves.
Geysers and hot springs: Features that form
when magma exists near the surface of the Earth (e.g., near volcanoes) and
heats the groundwater. Some water turns to steam, expands, and erupts out of
holes in the ground in geysers. In other places, hot water trickles out of
springs.
Climate Change
The
Earth’s climate has changed considerably over the planet’s history. Scientists
have determined that these changes occur in cycles driven by a number of
factors.
Climate
Cycles
Evidence
suggests that the Earth has experienced climate cycles—alternating periods of extreme warmth and
cold—throughout geologic time.
1.
Periods of glacial climate, which have fostered the
growth of glaciers, have alternated with interglacial periods, during which temperatures are
so warm that glaciers melt.
2.
During the Ice Age around 2–3 million years ago,
ice sheets spread over much of the Earth’s land surface. About 55 million years
ago, however, air and sea temperatures were so warm that geologists think
glaciers melted away completely.
Driven by the
Earth’s Orbit
Most geologists believe that these
dramatic temperature changes result from variations in the Earth’s orbit
1.
In the
1920s, Serbian astrophysicistMilutin Milankovitch formulated a
model of climate cycles based on three properties of the Earth’s orbit:
A. Eccentricity: Changes in the shape of the
ellipse that the Earth traces as it orbits the sun. The ellipse is at its
longest once every 100,000 years.
B. Obliquity: The tilt of the Earth
toward the sun on its axis of rotation. The Earth’s obliquity shifts between
21.5 and 24.5 every 40,000 years. When the tilt is greatest, polar regions
receive more summer sunlight and less winter sunlight.
C. Precession: The wobble of the Earth on
its axis. Precession completes a full cycle every 26,000 years and affects the
intensity of sunlight that reaches the Earth’s polar regions.
These
three cycles regularly reinforce each other. At certain times, they combine to
maximize the input of solar radiation to the Earth, which leads to warming of
the Northern Hemisphere and glacial retreat. At other times, they combine to
minimize the heat that the Northern Hemisphere receives, leading to glacial
advance.
Driven
by Tectonics
1.
When the Earth’s tectonic plates form a
supercontinent at high latitudes (see Plate Tectonics), ice growth is
encouraged. This convergence of the continents is a rare event in Earth
history, however.
2.
More frequently, extensive volcanism leads to
outpouring of CO2 into the atmosphere, which traps heat
and leads to a greenhouse effect.
3.
Glaciers record climate change. Geologists are able
to drill cores out of glacial ice to measure the CO2 content of the atmosphere at different
times in the past. They have determined that the CO2 content of the Earth’s atmosphere has
fluctuated over time and that these fluctuations correspond to rising and
falling temperatures.
Driven
by Humans
Today,
as we burn fossil fuels that release CO2 into the atmosphere,
we are experiencing human-induced climate change.
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