Assignment 5-1 PHY103
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PHY103
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Geology
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Jan 9, 2024
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Sara Rigby
Assignment 5-1
PHY103
I.
Earthquakes
An earthquake is a rapid release of energy cause by the movement of crustal plates that
generates waves of vibration (Lutgens, 2021). Along the faults or weak regions of tectonic plates,
stress builds up gradually over hundreds of years. This is all released in an instant with a slipping
or breaking motion, releasing the shockwaves we know as an earthquake. Earthquakes are
happening consistently in the deep parts of the Earth, but they do not often reach the surface
(Lutgens, 2021).
There are two main categories of seismic waves, body waves, and surface waves (Lutgens,
2021). Body waves can move through the interior of the Earth, while surface waves remain along
the upper layers of the crust. Body waves are broken down into further categories, primary or P
waves and secondary or S waves. P waves are the first waves to be recorded, followed by
secondary and lastly surface waves. Out of all three of these types of waves, the most damaging
are the surface waves because they have the greatest amplitude (Lutgens, 2021). Seismographs
measure these waves using inertia, the body of the instrument moves with the waves while the
sensor above it remains suspended and stationary, the difference between the two is what ends up
being recorded. (Lutgens, 2021). The seismograph uses the nature of the different types of waves
to triangulate where the epicenter of the quake occurred.
The intensity of an earthquake is the measurement of the amount of ground shaking at a
location. The magnitude of an earthquake is an estimate of the how much energy was released
(Lutgens, 2021). The Richter scale uses the maximum amplitude of the seismic waves and the
distance of the waves to the seismograph to measure the intensity of the quake. The moment
magnitude scale was created to help differentiate between larger earthquakes, it uses the strength
of the rock, amount of slippage, and area of the fault to measure the magnitude of the energy
released (Lutgens, 2021).
Faults are weakened points in rock where a fracture has occurred, and movement is
occurring on both sides of it. Faults can be placed into four common categories. A normal fault
moves in the same direction as the fault itself. The hanging wall block moves down relative to
the footwall (Lutgens, 2021). This type of fault is most common in divergent boundaries. In a
reverse fault, the motion is the opposite of a normal fault, instead the hanging wall block moves
up relative to the footwall (Lutgens, 2021).
A thrust fault is the same as a reverse fault, but it
occurs at a much lower angle. Convergent boundaries are most often associated with reverse and
thrust faults. Strike-slip faults move in the direction of the fault trace (Lutgens, 2021). Transform
faults are where strike-slip faults are located.
When Earthquakes occur, they often leave a path of death and devastation in their wake.
As time has gone on humans have tried to prepare themselves for these natural disasters. In
studying earthquakes, scientists have learned what causes the most destruction and have tried to
prevent it to the best of their abilities. Intensity and length of shaking is one of the top three
destruction factors (Lutgens, 2021). When an earthquake shakes the earth, that includes buildings
as well. Tall buildings don’t have much support to keep them from swaying, they will often
collapse, burying the people inside. The structure of the ground beneath the building itself can be
a big factor in how it is affected in a quake. If sediment is waterlogged, severe shaking can cause
liquefaction, reducing its ability to support buildings. Landslides and tsunamis are also triggered
by earthquakes (Lutgens, 2021). Both act like waves, burying anything in its path, the biggest
difference is, one is made of the earth itself and the other is made of water.
The circum-Pacific belt is the most seismically active area in the world.
This includes some
of the coastline along the Pacific Coast (Lutgens, 2021). Specifically, the San Andreas Fault near
the cost of California has been known to produce destructive earthquakes
.
The San Andreas
fault is a transform fault. The two plates located here are slipping past each other and a release of
energy that is long overdue could cause extreme devastation. An earthquake here could easily
rupture gas lines leading to fires and cause landslides. This is mostly due to the large population
that lives right along this fault. Because of the unpredictability of earthquakes, it is impossible to
be completely prepared for this and it could take many people by surprise.
I live in Eastern Washington. We have had earthquakes in my lifetime, but only one of them
has ever been large enough for me to feel. The epicenter often lies across the Cascade Mountains
and is far enough away that we would not see much damage here. However, they are frequent
and the Juan de Fuca plate off the coast is a hotspot for seismic activity in the US. We do have
several faults that are closer to us, but they are not as violent or frequent as the ones on the coast.
There is also earthquakes that emanate from the many volcanoes in the Cascade range.
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Earthquakes are happening often here, and it has always seemed like we’re just waiting for that
next big one.
II.
Volcanoes
Volcanoes are sites where magma has broken through the surface of the Earth. The way a
volcano forms is greatly influenced by the characteristics of the magma itself. The two main
factors that affect magma are viscosity and temperature (Lutgens, 2021). Viscosity is the
resistance of the magma, often silica rich magma is more viscous. As the temperature of the
magma increases the viscosity does as well (Lutgens, 2021). Volcanoes are common throughout
the world, mainly occurring along fault lines.
There are three basic types of volcanoes. Shield volcanoes mainly occur along divergent
faults and hotspots (Lutgens, 2021). They are broad and slightly domed, much like the name
implies, forming from low viscous magma. They are often made up of basalts. Eruptions of
shield volcanoes are gentle and successive. Cinder cone volcanoes are relatively small compared
to the other two types, yet they are steep sided. They are mainly composed of pyroclastic debris
and form from highly viscous magma (Lutgens, 2021). They are highly explosive and often form
from single eruptive events. The last type of volcano is a strato or composite volcano. These
often appear to look like mountains and are usually found among mountain ranges. They form
along convergent plate boundaries. Composite volcanos are like a mixture of a shield and cinder
volcano (Lutgens, 2021). They often have both pyroclastic and lava flows and are formed from
an intermediate viscous magma.
There are several hazards that are associated with volcanoes. The deadliest is the
pyroclastic flow, it is a thick mixture of extremely hot gas and pyroclastic material that moves
downhill at incredible speeds (Lutgens, 2021). These flows envelope everything in its path.
Another hazard is a lahar, these are mudflows that form on volcanoes (Lutgens, 2021). They are
often formed from quick glacial melt when the eruption occurs, and they often flow into stream
valleys and destroy everything in its path. Volcanic ash is another threat that is created by
volcanoes. A plume of smoke in the atmosphere can last for weeks or even months. It can impede
air travel and it can affect those that inhale it, including choking out plants and animals from
sunlight and air.
Where I live in eastern Washington, I am very close to many famous volcanoes and have
visited Mt. St. Helens and Mt. Rainier and many more. Living near the Cascade Range has given
me an appreciation for the magnificent beauty and horrifying destruction a volcano can bring.
When Mt. St. Helens blew my mom lived in the same town where we still live. She said she
could hear or see anything, but suddenly, the sky was as dark as is it was at night, yet it was the
middle of the day. Many people driving along I-90 were stopped at a standstill as their car
engines were choked out from the ash in the air. This is still where the risk for us lies today, as
Mt. Rainier is long overdue for an eruption.
References
Lutgens, F. K., & Tarbuck, E. J. (2021).
Foundations of Earth Science
(Ninth Edition). Pearson
Education Inc.
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