adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Geology, What is it?
Geology is the study of the Earth. It includes not only the surface
process which have shaped the earth's surface, but the study of the ocean
floors, and the interior of the Earth. It is not only the study of the
Earth as we see it today, but the history of the Earth as it has evolved
to its present condition.
Thus, organisms have only been witness to about 15% of Earth's history.
Thus, for us to have an understanding of the earth upon which we live, we
must look at processes
that occur today, and interpret what must have happened in the past.
One of the major difficulties we have is with the time scale. Try to
imagine 1 million years-- That's 50,000 times longer than most of you have
lived. It seems like a long time doesn't it? Yet, to geologists, 1 million
years is a relatively short period of time. More discussion about time in
later lectures. But one thing we have to remember when studying the earth
is that things that seem like they take a long time to us, may take only a
short time to earth.
A river deposits about 1mm of sediment (mud) each year. How
thick is the mud after 100 years? -- 10 cm hardly noticeable over your
What if the river keeps depositing that same 1 mm/yr for 10
million years? Answer 10,000
Things can change drastically!
Why Study the Earth?
We're part of it. Dust to Dust. Humans have the capability to make
rapid changes. All construction from houses to roads to dams are
effected by the Earth, and thus require some geologic knowledge. All
life depends on the Earth for food and nourishment. The Earth is there
everyday of our lives.
Energy and Mineral
resources that we depend on for our lifestyle come from the
Earth. At present no other source is available.
Geologic Hazards -- Earthquakes, volcanic
eruptions, hurricanes / cyclones,
landslides, could affect us at any time. A better understanding of the
Earth is necessary to prepare for these eventualities.
Curiosity-- We have a better understanding of things happening
around us. Science in general. I'll try to relate geology (and science
in general) to our everyday lives.
Earth Materials and Processes
The materials that make up the Earth are mainly rocks (including soil,
sand, silt, dust) . Rocks in turn are composed of minerals.
Minerals are composed of atoms, Processes range from those that occur
rapidly to those that occur slowly
Examples of slow processes
Formation of rocks
Chemical breakdown of rock to form
Chemical cementation of sand grains together to form rock
Recrystallization to rock to form a different rock (metamorphism)
Most surface rocks started out as igneous rocks- rocks produced by
crystallization from a liquid. When igneous
rocks are exposed at the surface they are subject to weathering
(chemical and mechanical processes that reduce rocks to particles).
Erosion moves particles into rivers and oceans where they are deposited
to become sedimentary
rocks. Sedimentary rocks can be buried or pushed to deeper levels
in the Earth, where changes in pressure and temperature cause them to
become metamorphic rocks. At high temperatures metamorphic
rocks may melt to become magmas. Magmas rise to the surface,
crystallize to become igneous rocks and the processes starts over.
External Processes Erosion- rocks are broken down (weathered)
fragments which are then carried by wind, water, ice and gravity.
External because erosion operates at the Earth's surface. The energy
source for this process is solar and gravitational.
Internal Processes Processes that produce magmas, volcanoes,
earthquakes and build mountain ranges. Energy comes from the interior of
the Earth, Most from radioactive
decay - nuclear energy.
Principle of Uniformitarianism
Processes that are operating during the present are the same processes
that have operated in the past. i.e. the present is the key to the past.
If we look at processes that occur today, we can infer that
the same processes operated in the past.
Rates -- rates of processes may change over time for example a
river might deposit 1 mm of sediment /yr at if we look at it today.
but, a storm could produce higher runoff and carry more sediment
tomorrow. Another example: the internal heat of the Earth may have
been greater in the past than in the present -- rates of processes
that depend on the amount of heat available may have changed through
Observations -- we may not have observed in human history
all possible processes. Mt. St. Helens Example. Size of Earthquakes
Perhaps a better way of stating the Principle of Uniformitarianism is
that the laws of nature have not changed through time. Thus, if we
understand the physical and chemical laws of nature, these should
govern all processes that have taken place in the past, are taking place
in the present, and will take place in the future.
All processes that occur on or within the Earth require energy. Energy
can exist in many different forms:
Gravitational energy -- Energy released when an object falls from
higher elevations to lower elevations.
Heat energy -- Energy exhibited by moving atoms, the more heat
energy an object has, the higher its temperature.
Chemical Energy -- Energy released by breaking or forming chemical
Radiant Energy -- Energy carried by electromagnetic waves
(light). Most of the Sun's energy reaches the Earth in this
Atomic Energy -- Energy stored or released in binding of atoms
together. Most of the energy generated within the Earth comes from
Heat Moves through material by the following modes:
Conduction - atoms vibrate against each other and these vibrations
move from high temperature areas (rapid vibrations) to low
temperature areas (slower vibrations).- Heat from Earth's
interior moves through the solid crust by this mode of heat
Convection - Heat moves with the material, thus the material must
be able to move. The mantle of the Earth appears to transfer
heat by this method, and heat is transferred in the atmosphere by
Radiation - Heat moves with electromagnetic radiation
(light) Heat from the Sun or from a fire is transferred by
Temperature and pressure increase with depth in the Earth. Near
the surface of the Earth the rate of increase in temperature (called the
Geothermal Gradient) ranges from 15 to 35oC per
kilometer. Temperature at the center of the Earth is about
The Earth -- What is it?
The Earth has a radius of about 6371 km, although it is about 22 km
larger at equator than at poles.
Internal Structure of the Earth:
Density, (mass/volume), Temperature, and Pressure increase with depth in
Crust - variable thickness and composition
Continental 10 - 50 km thick
Oceanic 8 - 10 km thick
Mantle - 3488 km thick, made up of a rock called peridotite
Core - 2883 km radius, made up of Iron (Fe) and small amount
of Nickel (Ni)
Layers of Differing Physical Properties
Lithosphere - about 100 km thick (deeper beneath continents)
Asthenosphere - about 250 km thick to depth of 350 km -
solid rock, but soft and flows easily.
Mesosphere - about 2500 km thick, solid rock, but still
capable of flowing.
Outer Core - 2250 km thick, Fe and Ni
liquid Inner core - 1230 km radius, Fe and Ni, solid
All of the above is known from the way seismic (earthquake waves) pass
through the Earth as we will discuss later in the course.
Surface Features of the Earth
Oceans cover 71 % of Earth's surface -- average depth 3.7
Land covers remaining surface with average of 0.8 km above sea level
Continental Shelf, Slope, and rise
Tectonics = movement and deformation of the crust, incorporates
Plates: are lithospheric plates -about 100 km thick, which move around
on top of the asthenosphere.
Divergent Boundaries occur at Oceanic Ridges, where new Oceanic
lithosphere is formed and moves away from the ridge in opposite
Convergent Boundaries occur where oceanic lithosphere is pushed
back into the mantle, marked by oceanic trenches and subduction zones.
Two types are possible -
When two plates of oceanic lithosphere converge oceanic lithosphere is
subducted beneath oceanic lithosphere. When ocean lithosphere runs into
a plate with continental lithosphere, the oceanic lithosphere is
subducted beneath the continental lithosphere.
Transform Boundaries occur where two plates slide past one another
horizontally. The San Andreas Fault, in California is a transform
Continental rifting may create a new divergent margin and evolve into an
oceanic ridge, such as is occurring in East Africa and between the
African Plate and the Arabian Plate.
Continental Collisions: may occur at a convergent boundary when plates
of continental lithosphere collide to join two plates together, such as
has occurred recently where the Indian Plate has collided with the
Eurasian Plate to form the Himalaya Mountains.
Plate tectonics explains why earthquakes occur where they do, why
volcanoes occur where they do, how mountain ranges form, as well as many
other aspects of the Earth. It is such an important theory in
understanding how the Earth works.