Introduction to Geology
Definition: Geology -the study of examines the Earth, its form and composition, and the changes
which it has undergone and is undergoing
2 main branches of geology:
1. Physical geology -examines the materials of the Earth and seeks to understand the processes and
forces acting beneath upon the Earth’s surface
2. Historical geology -examines the origin of the Earth and its development through time
-involves the study of fossils and their sequence in rock beds
-must understand how Earth works before studying Earth history
-Aristotle’s notions of geology erroneous, but widely accepted for centuries
Definition: Catastrophism -the concept that the Earth was shaped by catastrophic events of a
short-term nature
-catastrophists believed Earth was only approx. 6000 years old
Definition: Uniformitarianism -the concept that the processes that have shaped the Earth
in the geologic past are essentially the same as those operating today
-first proposed in late 1700’s by James Hutton (“father of modern geology”)
-later supported and advanced by Charles Lyall in 1800’s
-not all geological processes hold same importance and operate at same rate through
different time periods
-Earth is approx. 4.6 billion years old
-use radiometric dating to age Earth
Definition: Radiometric Dating -procedure of calculating the absolute ages of rocks and minerals that
contain certain radioactive elements, such as carbon and uranium
Geologic Time Scale:
-geologic time scale developed using relative dating
Defintion: Relative Dating -rocks are placed in their proper sequence or order in relation to each
other
-no absolute ages of rocks (ie. Formation date) are known
-use law of superposition
Definition: Law of Superposition
-in any undeformed sequence of sedimentary rocks or
surface-deposited igneous materials, each layer is older than
the one above it and younger than the one below it
-fossils are essential for geologic time scale
Definition: Fossils -remains or traces or organisms preserved from the geologic past
Definition: Principle of Faunal (Fossil) Succession -fossil organisms succeed one another in a
definite and determinable order, and any time period can be recognized by its fossil content
-different units of time and duration depend on the amount and variety of fossils
found in a particular period
Structure of the Earth:
-environment divided into 4 major parts (or “spheres”):
1. Atmosphere
-region of air (78% N2; 21% O2) for breathing and protection from
Sun’s harmful radiation
-energy exchanges between atmosphere & earth’s surface and atmosphere &
space produce weather and climate
2. Hydrosphere
-region of water (ie. oceans, lakes, glaciers, groundwater)
-water continually moves from ocean to air to land and back to ocean
(hydrological or water cycle)
3. Geosphere -divided into 4 major
regions:
A. crust -light, outer shell; 5 – 40 km
thick (least dense)
B. mantle -solid or fluid, rocky layer;
max. thickness of 2885 km
C. outer core -molten, metallic layer;
2270 km thick
D. inner core -solid, iron-rich layer;
radius of 1216 km
4. Biosphere – all life on Earth
Definition: Lithosphere
(“sphere of rock”)
-includes the crust
(continental & oceanic) and
uppermost mantle (down to
approx. 100 km)
Definiton: Asthenosphere -hot,
weak zone capable of gradual
flow
-located between 100 – 700 km
below lithosphere
The Theory of Plate Tectonics:
-Earth driven by heat engines (or devices that convert heat energy to mechanical energy)
-2 heat engines: external & internal
-external heat engine provides energy for atmospheric and oceanic circulation (weather & climate)
-internal heat engine driven by heat for hot interior moving toward the cooler exterior; it produces
moving continental plates and earthquakes
-rock deep inside Earth can deform under intense heat and pressure
-hot, less dense rock rises slowly toward surface
-cooler, more dense rock at surface sinks downward
-this process repeats to form a convective flow
Rising hot, less dense rock
Falling cool, more dense rock
Definition:
Theory of Plate Tectonics
-theory that proposes that
Earth’s outer shell consists
of individual plates which
interact in various ways and
thus produce earthquakes,
volcanoes, mountains and
the crust itself
-originally proposed by
Alfred Wegener
(1912)
-based on earlier theory of
continental drift
-plate tectonic theory used to
explain geological structures and
events
-lithosphere broken into individual pieces called plates
-plates are constantly in slow motion
-motion caused by internal heat engine (see above)
-as hot material moves upward, it spreads laterally and moves plates
-this movement generates earthquakes, volcanoes, orogenesis
Definition: Orogenesis
-periods of intense deformation of rocks in a region
-mountain formation results
Interactions Between Tectonic Plates:
-3 types of plate boundaries:
1. Divergent boundaries
– zones where plates move apart, leaving a gap between them
– sea-floor spreading occurs where gap fills with molten rock and cools, repeatedly
adding more oceanic lithosphere
2. Convergent boundaries
– zones where plates move together, causing one to go beneath the other (oceanic crust)
OR where plate collide (continental crust)
– subduction zones occur where oceanic lithosphere is being consumed
– as a plate moves downward, subducted material melts under high temperature and
pressure and moves up to overriding plate (volcanic activity)
3. Transform boundaries
– zones where plates slide past each other, scraping and deforming as they pass
– earthquake activity occurs
The Rock Cycle:
-concept map that explains origin of 3 basic rock types: igneous, metamorphic &
sedimentary
Definition: Igneous rock -rock that forms by the crystallization of molten magma
Definition: Magma -a body of molten rock found at a depth
Definition: Crystallization -the formation and growth of a crystalline solid from a liquid or gas
-igneous undergo weathering
Definition: Weathering -the disintegration and decomposition of rock at or near the surface of the
Earth
-the weathered material is picked up, transported and deposited by agents of erosion
Definition: Erosion -the incorporation and transportation of material by a mobile agent, such as
water, wind or ice
Definition: Sediment -loose particles created by weathering and erosion of rock, by chemical
precipitation from solution in water, or from the secretions of organisms, and transported by water,
wind or ice
-sediment is deposited in beds and undergo lithification to form sedimentary rock
Definition: Lithification -the process of converting sediment to solid rock
-if sedimentary rock is buried deep in Earth or involved in orogenesis, great pressures
and heat will form metamorphic rock
-metamorphic rock can then form igneous rock (magma) under increased pressure and
heat
-“shortcuts” in rock cycle model:
 igneous rock may be transformed directly to metamorphic rock (high temp. & pressure)
 exposed metamorphic or sedimentary rock can be weathered and eroded to later form new
sedimentary rock
-rock cycle first proposed by James Hutton
-very little known about rock formation
-later plate tectonics used to explain cycle
The Rock Cycle
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The Rock Cycle as it relates to the Theory of
Plate Tectonics