Download Course Learning Outcomes for Unit IV Reading Assignment Igneous

UNIT IV STUDY GUIDE
Igneous Activity and
Geologic Time
Course Learning Outcomes for Unit IV
Upon completion of this unit, students should be able to:
5. Demonstrate how earthquakes and volcanoes are driven by various geological forces.
5.1 Relate the type of volcanic activity to the major processes that generate magma from solid rock.
6. Explain the principles and techniques used by geologists to construct the geologic time scale.
6.1 Discuss principles used to determine the relative age of rock layers and how the layers relate to
each other.
6.2 Explain how radioactive isotopes can allow geologists to determine numerical dates.
Reading Assignment
Chapter 7:
Volcanoes and Other Igneous Activity
Chapter 8:
Geologic Time
United States Geological Survey. (1997). Other volcanic structures. Retrieved from
http://pubs.usgs.gov/gip/volc/structures.html
United States Geological Survey. (1999). The nature of volcanoes. Retrieved from
http://pubs.usgs.gov/gip/volc/nature.html
United States Geological Survey. (2001). Relative time scale. Retrieved from
http://pubs.usgs.gov/gip/geotime/relative.html
United States Geological Survey. (2001). Radiometric time scale. Retrieved from
http://pubs.usgs.gov/gip/geotime/radiometric.html
United States Geological Survey. (2010). Types of volcano hazards. Retrieved from
http://volcanoes.usgs.gov/hazards/index.php
United States Geological Survey. (2011). Principal types of volcanoes. Retrieved from
http://pubs.usgs.gov/gip/volc/types.html
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UNIT x STUDY GUIDE
Title
Unit Lesson
On May 18, 1980, Mount St. Helens erupted. This was
the largest historic eruption in North America. Part of
the peak was blown away in the violent eruption,
lowering the mountain by more than 1,300 ft.
Destruction from the blast demolished everything for
230 square miles. Ash traveled 22,000 miles and
affected hundreds of communities in the weeks that
followed. (United States Geological Survey [USGS],
2005).
Volcanoes are one way that we can directly observe
processes that occur far beneath the Earth’s surface.
Most of the earth is made up of solid rock. However,
this rock can undergo partial melting, which makes it
more buoyant, causing it to rise to the surface. This
partial melting may occur when there is a release in
Spirit Lake on the slopes of Mt. St. Helens in Washington
pressure (like at divergent plate boundaries), or when
State, USA. This photo was taken two years after the
water is added (like at subduction zones), or with
eruption and shows the ruined lake filled with debris from
increasing temperature (associated with continental
the eruption. (U.S. Army Corps of Engineers, 1982)
collisions). The “Ring of Fire” describes the pattern of
volcanoes found around the Pacific Basin. This activity is a result of oceanic subduction.
Volcanoes refer to "the opening or vent through which the molten rock and associated gases are expelled”
(USGS, 1999). Pressure can push magma to the surface. When this pressure is released, magma erupts in
the form of lava, gas, and pyroclastic materials (ash and hardened rock). If magma continues to erupt, the
lava can harden and build to create a mountain, known as a volcano (USGS, 1999). There are three main
types of volcanoes: shield volcanoes (broad domed structures created by basaltic lava), cinder cones (steep
and symmetrical structures built of pyroclastic material with little lava flow), and composite cones (also known
as stratovolcanoes, which are large and build from layers of ash and andesitic lava) (USGS, 2011).
Composite volcanoes are the most explosive and violent (Mt. St. Helens).
Different types of volcanoes differ in their eruption patterns, leading to the variety of volcanic forms. For
example, shield volcanoes usually originate on the ocean floor. The broad domed structure forms due to the
very hot lava that is released. Because the lava is so hot and viscous, it travels very quickly and far from the
vent. Because cinder cones have such deep craters, the temperature of the lava has already cooled
significantly by the time it is ejected. As lava fragments are ejected from the vent, they harden in flight to form
pyroclastic fragments. These fragments, or scoria, build up to form a cone-like structure around the vent. A
composite volcano produces a silica-rich andesitic magma, which is thick and viscous. Therefore, lava flows
slowly and does not reach more than a few kilometers from the vent. This creates the steep summits of these
volcanoes. Composite volcanoes also produce huge amounts of ash and pyroclastic materials (USGS, 2011).
During an eruption, volcanoes release lava, gases, and other pyroclastic materials. Because so many people
live in the shadow of these volcanoes, it is important to understand the different types of volcanic hazards.
The most deadly hazard associated with an erupting volcano is pyroclastic flow, which is a hot cloud of
expanding gas containing ash and pyroclastic materials. The speed and temperature make these flows
especially deadly to anything in its path. Lahars, which are mudflows of ash, can be equally destructive. Ash
and gas can also create respiratory problems and damage to homes and agriculture fields (USGS, 2010).
Landscapes often show evidence of past volcanic activity. A caldera is a crater formed by the collapse of the
magma chamber deep below the surface, after the magma has been ejected. Crater Lake in Oregon formed
from a volcano crater (USGS, 2010). Basaltic lavas that seep from fractures in the crust are known as
fissures. As the lava flows out of fissures to cover the crust, they create basalt plateaus. Often these plateaus
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will erode much slower than the surrounding landscape, leaving large mesas. UNIT
Shiprock,
in New
Mexico is the
x STUDY
GUIDE
remnant of the volcanic neck of a volcano. When magma solidifies beneath the
surface of the crust, it can
Title
form plutons (leading to batholiths and laccoliths), sills, and dikes. These will be exposed when the
surrounding area erodes away (USGS, 1997).
Approximately 640,000 years ago, a supervolcano erupted in what is now known as Yellowstone National
Park. The eruption sent ash as far as Missouri. The Caldera itself is approximately 40 miles across and likely
formed when a monstrous magma chamber erupted (National Park Service [NPS], 2014). The geologic
activity in the park suggests that this magma chamber still exists and might one day erupt again.
Geology and Geologic Time
Prior to modern dating methods, attempts to determine the age of the Earth were limited to placing rocks in
their sequence of formation and counting backwards through time. There are several principles that are still
used by geologists to try to build a timeline of Earth’s history. First is the principle of superposition—that older
rock will be found at the bottom of a sequences and newer rocks at the top (assuming no deformation has
taken place). A second principle is that of original horizontality—sediment was originally placed in relatively
flat layers. If geologists observe folded or tilted layers, there must have been a force that acted upon that rock
after the layers had formed. Similarly, the principle of cross-cutting states that features, such as faults or
igneous intrusions, which cut through a sequence of layers much have occurred after the layers of rock had
formed. The principle of inclusions simply states that any layer containing rock fragments from another layer
must be younger than the one that provided the rock fragment. Finally, uncomformities or interruptions in the
layers of rock show where there were interruptions in the sediment accumulation of a period. Click here to see
a diagram how using these principles can help understand the layers exposed in the Southwestern United
States. Fossils and the correlation of rock layers also help geologists to find layers of rock of that formed in
the same time and provide a geologic history of an area relative to other areas (USGS, 2001a).
Radioactivity was discovered around the turn of the century. In essence, it was discovered that certain
elements are unstable and will start to decay, losing neutrons, or electrons. This change in the number of
neutrons will change the atomic weight of the element, but the atomic number will remain the same (called an
isotope). The rate at which an element decays is constant, so by comparing the ratio of parent isotopes to
daughter (decayed) isotopes, it is possible to determine how old something is. This is called radiometric
dating and can give very precise ages for rocks containing certain isotopes (USGS, 2001b). With the
introduction of this technique, it then became possible to age the rocks of the Earth to determine when some
of these features formed. Geologists can now date the oldest known rocks to estimate that the Earth is
approximately 4.6 billion years old).
By using both dating methods (relative dating and radiometric dating), geologists have been able to develop a
geologic time scale. Essentially, they are able to divide Earth’s history into units of time. By comparing
common fossils, we can understand when certain plants and animals flourished on the Earth, or perhaps
identify periods of extinction. This geologic timescale is a work in progress, as we understand more and more
of our Earth’s history.
What do you see in your local topography? What types of rocks are exposed? Are there evidences of
mountain building, volcanoes, or earthquakes? By understanding the processes that form the layers of
exposed rock, we can get a better idea of the geologic history of an area. Hopefully, you will start to see
evidence of the geologic past in the landscapes around you.
References
National Park Service. (2014). Yellowstone volcano. Retrieved from
http://www.nps.gov/yell/learn/nature/volcano.htm
United States Army corps of Engineers. (1982). Spirit Lake two years post-eruption [Photograph]. Retrieved
from https://commons.wikimedia.org/wiki/File:Spirit_Lake_two_years_post-eruption.jpg
United States Geological Survey. (1997). Other volcanic structures. Retrieved from
http://pubs.usgs.gov/gip/volc/structures.html
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United States Geological Survey. (1999). The nature of volcanoes. Retrieved from
UNIT x STUDY GUIDE
http://pubs.usgs.gov/gip/volc/nature.html
Title
United States Geological Survey. (2001a). Relative time scale. Retrieved from
http://pubs.usgs.gov/gip/geotime/relative.html
United States Geological Survey. (2001b). Radiometric time scale. Retrieved from
http://pubs.usgs.gov/gip/geotime/radiometric.html
United States Geological Survey. (2005). Mount St. Helens – from the 1980 eruption to 2000. Retrieved from
http://pubs.usgs.gov/fs/2000/fs036-00/
United States Geological Survey. (2010). Types of volcano hazards. Retrieved from
http://volcanoes.usgs.gov/hazards/index.php
United States Geological Survey. (2011). Principal types of volcanoes. Retrieved from
http://pubs.usgs.gov/gip/volc/types.html
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