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Chapter 1
An Introduction to Geology
The Science of Geology
• Geology - the science that
pursues an understanding of
planet Earth
“If there is an interesting place you want to go, there is
interesting geology that you can study there”
(Cappadocia, Central Turkey).
The Science of Geology
• Physical geology - examines the
materials composing Earth and
seeks to understand the many
processes that operate beneath
and upon its surface
• Historical geology - seeks an
understanding of the origin of
Earth and its development
through time
Mersin ophiolite,
The Science of Geology
1.3: satellite image of Mt. Vesuvius, Italy.
• Geology, people, and the environment
• Many important relationships exist between
people and the natural environment
Problems and issues
addressed by
geology include
• Natural hazards,
resources, world
population growth,
and environmental
• Some historical views
of the Earth
• Aristotle, 300 BC;
• James Ussher, ca. 1600,
‘Earth was created in
4004 BC;’
• Catastrophism
• Earth’s features
formed through
sudden and violent
‘The Dog
Dwelling in Goreme, Cappadocia
The Science of Geology
Hutton, Kelvin, and the great
Earth debates.
• The beginnings of
modern geology
“All natural processes that affect the
Earth’s crust (erosion, deposition,
volcanic eruptions, faulting, glaciation
etc.) operate with the same intensity
and under the same set of physical
constraints now as in the geologic past.”
“(as to the age of Earth) we see no
vestige of a beginning, no prospect of
an end.”
These points are incorrect - why?
• ca. 1780, James Huton’s
Theory of the Earth;
• Uniformitarianism: “the
processes that operate
today have operated in
the past.”
• a uniformitarian view of
Earth requires a vast
amount of time….
James Hutton, 1726-1797
Hutton, Kelvin, Twain and the
great Earth debates.
Hutton, Lyell, Kelvin, and the
great Earth debates.
C. Lyell: ’Principles of Geology’
"The present is the key to the
past." (T&L p. 4)
Lord Kelvin, the greatest physicist of
his day… “Earth cannot be more than
100 million years old [on the basis of
a simple convective cooling model].”
Mark Twain, satirist and writer of the
time: “Scientific research has shed
considerable darkness on the subject of
the Age of the Earth, and if they
continue at their present pace, we’ll
soon know nothing about it!”
Uniformitarianism today:
Earth’s geological history can be
interpreted through processes
we see operating today. We can
also understand Earth’s present
by interpreting its past.
Sir Charles Lyell,
Geologic time
• Relative dating and the
geologic time scale
• Relative dating means that
dates are placed in their
proper sequence or order
without knowing their age in
• Fossils of pre-existing life,
such as this fish and
amphibian, are important
markers of relative time.
The principle of
succession is
useful for
Figure 1.8
• Geologists are
now able to
assign fairly
accurate dates to
events in Earth
history (better
than + 1% with
Auburn Noble Isotope Mass Analysis Lab
Figure 1.8
The nature of
scientific inquiry
• The magnitude of
geologic time
• Involves vast times
– millions or
billions of years
• An appreciation for
the magnitude of
geologic time is
important because
many processes are
very gradual
• Science assumes the natural
world is consistent and
• A goal of science is to discover
patterns in nature and use the
knowledge to make predictions
• Scientists collect data through
observation and measurements
Figure 1.8
The nature of
scientific inquiry
• How or why things happen is
explained using a
• Hypothesis – a tentative (or
untested) explanation
• Theory – a well-tested and
widely accepted view that the
scientific community agrees best
explains certain observable facts
• Scientific Laws provide brief usually mathematical statements to describe nature.
The nature of scientific inquiry…
methods of science
• The ‘Scientific method’ involves
• Gathering facts through
• Formulation of hypotheses and
theories, and laws.
• There is no fixed path that
scientists follow that leads to
scientific knowledge, yet the paths
begin with inquiry and are guided
by logic
Scientific laws are not ‘superior’ to theories.
A view of Earth
• Earth’s four spheres
• Hydrosphere
• Atmosphere
• Biosphere
• Solid Earth
Earth as a system
Earth System Science
• Aims to study Earth as a system
composed of numerous interacting
parts or subsystems
• Employs an interdisciplinary
approach to solve global
environmental problems
Systems have ‘negative’ and
‘positive’ feedback mechanisms
The hydrologic cycle is one of Earth’s many subsystems
Earth as a system
• The Earth system is
powered by:
o Solar energy drives most
external processes in the
• Atmosphere
• Hydrosphere
• At Earth’s surface
c.f. figure 1.13
Early evolution of Earth
• Origin of planet Earth
• Most researchers believe
that Earth and the other
planets formed at
essentially the same time
• Nebular hypothesis
– Rotating cloud called
the solar nebula
– Composed of
hydrogen and helium
– Nebula began to
contract about 5
billion years ago
Early evolution of Earth
*We see nebula today, and earth’s interior has the
same composition as early solar system materials.
*Samples of the Early Solar System
• Chondrites - contain spherical aggregates of
high-temperature minerals (olivine and
pyroxene, in ‘chondrules’); condensed from
the solar nebula with some ‘pre-solar’ grains
• Fe-Ni Meteorites
• Lunar samples
• Martian meteorites
Chondrites generally give
radiometric ages of ~ 4.5 Ga
Radioactive decay of isotopes (U234, U235, K40, etc.)
drives processes in Earth’s interior
o Lithosphere, Asthenosphere, Mesosphere, Core
• Origin of planet Earth
• Nebular hypothesis
– Assumes a flat, disk
shape with the
protosun (pre-Sun) at
the center
– Inner planets begin to
form from metallic
and rocky substances
– Larger outer planets
began forming from
fragments of ices
(H2O, CO2 , and
Early evolution of Earth
• Formation of Earth’s layered structure
• Metals (especially Fe and Ni) sank to the center
• Molten rock rose to produce a primitive crust
• Chemical segregation established the three basic
divisions of Earth’s interior: core, mantle, and
• Primitive atmosphere evolved from gases in
Earth’s interior
1 mm Chondrule
Prior to 2005, scientists felt that the bulk composition
of rocks in Earth’s interior was the same;
• using spectrometers more precise than previously
available, in 2005 scientists discovered important
chemical differences between the bulk Earth and the
Outer Core;
Inner Core.
the simple model
that Earth separated
from a uniform solar
nebula and then
changed only by
simple ‘segregation’
is incorrect.
c.f. 1.16
1 mm Chondrule
Earth’s internal structure
• Layers defined by composition
• Crust
• Mantle
• Core
• Layers defined by physical properties
Inner and Outer Core
How do we
know anything
about Earth’s
“We have never
sampled the
mantle directly….
“We have been able
to examine slivers
of the uppermost
mantle and
overlying oceanic
crust … in Cyprus,
Newfoundland and
Oman (p. 23).”
c.f. 1.16
Oceanic Crust section, Antakya, Turkey
(Syrian Antioch)
The face of Earth - continents
Note the shields… platforms… mountain belts
The face of Earth ocean basins
James Hutton,
~1780 ‘Theory
of the Earth’
Note abyssal plains, spreading ridges, seamounts, and trenches…
The rock cycle
c.f. 1.23
Rocks and the rock cycle
• Basic rock types
• Igneous rocks
• Cooling and solidification
of magma (molten rock)
• Examples include granite
and basalt
Rocks and the rock cycle
• Basic rock types
• Sedimentary rocks
• Accumulate in layers at
Earth’s surface
• Sediments are derived
from weathering of
preexisting rocks
• Examples include
sandstone and
Rocks and the rock cycle
• Basic rock types
• Metamorphic rocks
• Formed by “changing”
preexisting igneous,
sedimentary or other
metamorphic rocks
• Driving forces are increased
heat and pressure
• Examples include gneiss and
Plate Tectonics
(Chapter 2)