GEO 221 Study Guide - Spring 2018, Comprehensive Midterm Notes - Weathering, Thermohaline Circulation, The Age

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12 Oct 2018
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GEO 221
MIDTERM EXAM
STUDY GUIDE
Fall 2018
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Lecture 1: Introduction to Environmental Geology
The Study of Earth Hazard and resources; Geo = Earth and Logos = knowledge of.
Applied geology: Using knowledge of earth to benefit humanity, i.e reduce exposure and obtain
resources.
Processes: earthquakes, volcanoes, and erosion are each examples of internal and surface
earth processes. Internal Earth would be plate tectonics as a result of movement to heat. Surface
earth would be weathering and subsequent erosion and a result in movement of water.
Goals of environmental geology include:
1. To ensure continued availability of resources
2. To minimize problems associated with resource use
Geologic processes occurring become hazardous when people are surrounding. By studying
geologic record (rocks), geologists can study earth processes and history. Thereby giving the
ability to predict where an event may take place, what the effect is, and how often to expect an
event given the size. These processes produce resources that we need, including energy,
minerals, soil, air, water, and building material. The resources we use now took millions of years
to form making geologic processes non-renewable.
How long do we have for oil resources?
The answer depends on how much we are willing to pay and the extraction techniques followed
by subsequent environmental consequences. The environmental consequences of resource use
include extraction, refinement, use, and disposal techniques.
Composition and Structure of the Earth: Planetary accretion is formed by gravitational
attraction that impact between many smaller fragments. Kinetic energy converted on impact into
heat, resulting in melting of the earth. Molten earth is segregated into layers by density; dense
metals sank to the core, light silicates rose to form the crust, while heavy silicates remained in
the mantle. Earth is composed of >80% of chemical elements made of mostly iron, oxygen,
hydrogen, and nitrogen.
Continental crust is thicker and lighter (buoyant), comparable to granite. Oceanic crust is
thinner and denser, comparable to basalt. Temperature, composition, and pressure determine how
a material at a given depth will behave.
Below melting point: rigid, solid, brittle (like glass).
Close to melting point: soft, solid, ductile (like warm plastic).
Above melting point: liquid and lacks strength (like water).
The lithosphere is composed of the crust and uppermost mantle, which is composed of rock that
is rigid and brittle and rests below its melting point. The asthenosphere is the remainder of the
upper mantle, composed of rock that is soft and ductile that rests close to its melting point.
What is plate tectonics? The rigid and brittle lithosphere is broken into plates that move
by convection in the ductile asthenosphere and converge or diverge with other plates. The
motion of plates is driven by convection in earth’s interior. Interior is hot due to accretion and
radioactive decay; this heat moves towards the surface by convection in ductile layers. The core
is very hot, but it’s composition makes it too dense to rise. Instead, the mantle is heated and rises
towards the surface, forming new crust (oceanic) that becomes sea floor spreading. New oceanic
crust is produced by upwelling in the asthenosphere, creating ridges. This denser ocean crust
sinks beneath lighter continental crusts, called trenches, through a process called subduction.
The Discovery of Plate Tectonics: Three independent lines of evidence support and
validate the theory of plate tectonics, to include: continental drift (“Pangea” by Alfred Wegener),
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sea floor spreading, and subduction.
Evidence of continental drift is the fit of coastlines, distribution of fossils across the
globe, and similar types/ages of rocks on widely separated coastlines. The hypothesis of
continental drift was rejected initially because there was no explanation for how the continents
could move laterally. Discoveries about the earth’s magnetic field were uncovered in the 1950’s
to support this former hypothesis.
The sea floor was mapped and major topographic features that were discovered were
trenches and ridges. Trenches are the topographic representation of subductions, appearing near
the edge of continents. The age of the sea floor shows orderly distribution, as rocks are the
youngest along ridges and progressively get older away from ridges. Another interesting
observation of note is that there is no old oceanic crust nothing older than 200 million years.
The oldest rocks on continents are 4 billion years old. This distribution suggests that crust is
created at the ridges, and confirmed by changes in the magnetic field. Sea floor spreading is
explained by the production of magnetized polarity pattern that is symmetric in respect to the
ridge. Magnetic polarity switches with irregularity crust older with distance to the ridge. Harry
Hess is accredited with these observations. Sea floor spreading provides the mechanism for
continental drift, acting as a conveyor belt.
Earth’s magnetic field is generated by Earth’s rotation and molten iron outer core. There
are two poles of opposite polarity, and earth’s magnetic field flips this polarity. There is the
normal (north near the geographic North Pole) and reverse (south near the geographic South
Pole). As rocks crystallize, iron minerals align parallel to the field that is dominant at the time;
indicating polarity. Therefore, it can be said that rocks record changes in earth’s magnetic field.
Mapping the magnetization of rocks on the seafloor shows bands of alternating normal and
reverse polarity.
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