Lab 6 - Volcanism online finished
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Earth Sciences 1023/2123
Lab #6
Volcanism
ONLINE
1. Introduction:
Today’s lab will focus on volcanoes and volcanic rocks, but we also will learn about intrusive igneous
rocks, and the hazards associated with volcanic eruptions.
Igneous rocks
are one of the three main rock types, and are formed by the cooling and solidification of
magma either within
the Earth’s crust ( intrusive
or plutonic
igneous rocks) or at the Earth’s surface
(
extrusive
or volcanic
igneous rocks). In intrusive
igneous rocks, the crystals are large enough to see with
the naked eye as they had time to grow in a slowly cooling magma. In contrast, extrusive
rocks are
typically glassy or fine-grained due to rapid cooling in lava at the Earth’s surface.
1
Volcanoes generate a wide variety of phenomena that can alter the Earth’s surface and atmosphere and
endanger people and property. While most volcanic hazards are associated with eruptions, some, like
landslides, can occur when a volcano is quiescent. Small volcanic events may pose a hazard only within a
few kilometres of the volcano, while large events can directly or indirectly endanger people and property
tens to hundreds of kilometres away, and even globally in extreme cases.
2
Hazard assessment
is the determination of the threat that a geological event poses to human lives and
property and is one of the more difficult problems involved in dealing with geological hazards. For
example, a volcano erupting in Antarctica is only a geological phenomenon
, and generally not a
geological hazard
, because it poses no threat to lives or property – unless of course it is near one of the
remote research stations! The same volcano erupting in London is a very different matter. The difficulty
associated with hazard assessment is in determining how much of a threat any hazard represents. If
geologists underestimate a threat then many people can be killed and properties and businesses can be
destroyed. On the other hand, if geologists overestimate a threat then they can lose credibility resulting in
people not heeding future warnings.
3
Therefore, it’s important to understand the scale of threat associated
with various volcanic hazards.
2. Objectives
By the end of this laboratory, you should:
1) Know the basic properties of intrusive and extrusive igneous rocks;
2) Be able to identify some samples of intrusive and extrusive igneous rocks;
3) Be able to identify where various rock types might be found around a non-active volcano;
4) Understand the hazards associated with volcanic eruptions.
1Definitions from the Glossary of Geology, 5th Ed. (2005). American Geological Institute, USA. 779 pp.
2Reference: Department of Earth and Space Sciences, University of Washington.
3Reference: Department of Geology, University of Maryland.
1
3. Identification and classification of rocks
The following information was included in Lab #2, and is reprinted here for your convenience.
The classification of rock types and the names of individual rocks are largely determined by composition
and texture
. The identification of rocks is not difficult if it is approached logically.
Composition
refers to the minerals that are found within a rock and is largely manifest as colour.
Texture
refers to the size, shape and arrangement of grains within a rock.
It is important to note that some properties of rocks are characteristic of more than one type. For example,
there are examples of igneous, metamorphic, and sedimentary rocks that may all be described as “dark
and fine-grained”. Therefore, it is essential to classify a rock based on more than one of its properties.
3.1. Igneous rock identification
The types of minerals formed in an igneous rock depend upon the chemical composition of the magma
from which they crystallized. The minerals formed in igneous rocks can be arranged into 4 groups:
1)
Felsic:
Generally light coloured rocks – “Fel” from feldspar, and “sic” from silicon. Felsic
rocks are made up of abundant silicon, aluminium and alkalis (potassium, sodium, calcium) due
to the presence of abundant feldspar and quartz.
2)
Mafic:
These are generally dark coloured rocks that take their name from “Ma” from
magnesium and “fic” for iron. The rocks have abundant magnesium and iron, due to the presence
of abundant iron-magnesium-bearing minerals such as amphibole and/or pyroxene.
3)
Intermediate:
These rocks, as their name implies, are intermediate in colour and contain a balance between the felsic and mafic minerals.
4)
Ultramafic:
These rocks are very dark coloured and are composed predominantly of the ironmagnesium minerals pyroxene and olivine.
Thus the COMPOSITION of an igneous rock can be estimated by observing the proportion of dark- to light-coloured minerals (Table 1).
The TEXTURES of igneous rocks are largely controlled by cooling rate. There are 7 main textures to consider:
1.
Course to medium grained (phaneritic):
In these rocks, the majority of the minerals are of
uniform size and identifiable with the naked eye. Medium grained = 1–5 mm diameter grain size;
course grained > 5 mm diameter. This texture occurs when magma cools slowly within the Earth
(i.e., in intrusive igneous rocks).
2.
Fine grained (aphanitic):
These rocks comprise very small crystals that are not distinguishable
with the naked eye. Grain size is < 1mm diameter. This texture develops when molten material
cools quickly either close to, or on the Earth’s surface.
3.
Glassy
: These rocks resemble dark glass and contain no visible crystals. Glassy rocks indicate rapid cooling and form only in extrusive igneous rocks.
4.
Porphyritic:
Such rocks contain a mix of minerals with two main grain sizes: large mineral
grains (phenocrysts) are set in a matrix of fine grained minerals. This texture is caused by magma
having two different rates of cooling. For example, if a magma is cooling and crystallizing deep
in the Earth’s crust and then erupts, it could form a porphyritic texture.
5.
Pegmatitic:
These rocks are composed of conspicuous large crystals often > 1cm in diameter.
2
6.
Fragmental:
As their name implies, these rocks are made up of broken, angular volcanic
fragments distinguishable with the naked eye. They are generally the result of explosive volcanic
eruptions. Explosive volcanic rocks are sometimes known as pyroclastic.
7.
Vesicular
: These rocks comprise spherical to ovoid air spaces (vesicles), formed by escaping gas
when a lava cools. The surrounding rock may be glassy and they are also indicative of rapid cooling.
Table 1.
Classification of igneous rocks.
4. Volcanic Hazards
2
Eruption Columns and Clouds
An explosive eruption blasts molten and solid rock fragments (tephra) into the air with tremendous force.
The largest fragments (bombs) fall back to the ground near the vent, usually within 3 km. The smallest
rock fragments (ash) continue rising into the air, forming a huge, billowing eruption column. Volcanic ash
is composed of fragments of rock, minerals, and glass that are less than 2 mm in diameter.
Eruption columns can be enormous in size and grow rapidly, reaching more than 20 km above a volcano
in less than 30 minutes. Once in the air, the volcanic ash and gas form an eruption cloud. Eruption clouds
pose a serious hazard to aviation. During the past 15 years about 80 commercial jets have been damaged
by inadvertently flying into ash, and several have nearly crashed. Large eruption clouds can travel
hundreds of miles downwind from a volcano, resulting in ash fall over enormous areas. Ash from the May
18, 1980, eruption of Mount St. Helens was deposited over 22,000 square miles of the western United
States. With increasing distance downwind from a volcano, the ash particles become smaller and the
thickness of the resulting layer decreases. Minor ashfall can be a nuisance to people and damage crops,
electronics, and machinery; heavy ashfall can collapse buildings.
3
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Pyroclastic Flows
High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during
explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows,
which can reach 815
o
C and move at 160–240 km/h, are capable of knocking down and burning
everything in their paths. A more energetic and dilute mixture of searing gas and rock fragments is called
a pyroclastic surge. Surges move easily up and over ridges; flows tend to follow valleys.
Lava Flows and Domes
Molten rock (magma) that pours or oozes onto the Earth's surface is called lava. The higher a lava's silica
content, the more viscous it becomes. For example, low- silica basalt lava can form fast-moving (15–50
km per hour), narrow lava streams or spread out in broad sheets up to several miles wide. Between 1983
and 1993, basalt lava flows erupted at Kilauea Volcano in Hawaii destroyed nearly 200 houses and
severed the coast highway along the volcano's south flank.
In contrast, higher-silica andesite and dacite lava flows tend to be thick, move slowly, and travel short
distances from a vent. Dacite and rhyolite lava flows often form mound-shaped features called domes.
Between 1980 and 1986, Mount St. Helens built a lava dome about 300 m high and 1000 m in diameter.
Renewed activity since October, 2004 has built a second new dome that continues to grow:
(see http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/LIDAR/framework.html
for recent laser
altimetry data of the growth of the new dome)
Lahars (Debris Flows or Mudflows)
Lahars are mixtures of water, rock, sand, and mud that rush down valleys leading away from a volcano.
They can travel over 80 km downstream, commonly reaching speeds between 30 to 65 km per hour.
Sometimes they contain so much rock debris (60-90% by weight) that they look like fast-moving rivers of
wet concrete. Close to the volcano they have the strength to rip huge boulders, trees, and houses from the
ground and carry them downvalley. Further downstream they simply entomb everything in mud.
Historically, lahars have been one of the most deadly volcanic hazards.
Lahars can form in a variety of ways, either during an eruption or when a volcano is quiet. Some
examples include the following: (1) rapid release of water from the breakout of a summit crater lake; (2)
generation of water by melting snow and ice, especially when a pyroclastic flow erodes a glacier; (3)
flooding following intense rainfall; and (4) transformation of a volcanic landslide into a lahar as it travels
downstream.
Volcanic Landslides (Debris Avalanches)
A landslide is a rapid downslope movement of rock and/or soil. In glaciated regions, landslides can also
include snow and ice. Landslides range in size from small movements of loose debris on the surface of a
volcano to massive failures of the entire summit or flanks of a volcano. Volcanic landslides are not always
associated with eruptions; heavy rainfall or a large regional earthquake can trigger a landslide on steep
slopes. Volcanoes are susceptible to landslides because they are composed of layers of weak, fragmented,
volcanic rocks that tower above the surrounding terrane. Furthermore, some of these rocks have been
altered to soft, slippery, clay minerals by hot, acidic ground water inside the volcano. At least five large
landslides swept down the slopes of Mount Rainier during the past 6,000 years. The largest volcanic
landslide in historical time occurred at Mount St. Helens on May 18, 1980.
4
5. Exercise
1.
The diagram below shows an actively erupting volcano. Years after eruption has ceased, magma,
lava, ash, and pyroclastic deposits will cool and solidify into various rock types, in, under, and
around the volcano. Imagine you visited this volcano 100 years after eruption, and collected 5
groups of rock samples.
a.
Study the 5 groups of samples (A, B, C, D, E on next pages), and make note of textures
and composition. Then, based on your observations, decide whether the rock is intrusive
or extrusive, and assign a rock name to each group of samples. (10 marks)
Sample
Textures
Composition
Intrusive/Extrusive
Rock Name
A
phaneritic
Intermediate
Instusive
Diorite
B
aphanitic
Mafic
Extrusive
Basalt
C
Fragmental
Felsic
Extrusive
Volcanic Tuff
D
Vesicular
Felsic
Extrusive
Pumice
E
Glassy
Ultramafic
Intrusive
Obsidian
5
6
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8
9
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10
b.
On the diagram below, please indicate the areas where each of the 5 sample rocks
(A,B,C,D,E) were likely collected. Remember, samples were collected AFTER eruption
ceased and cooling took place (3 marks).
C
D
E
B
A
Figure 1.
Cross-section of an erupting volcano.
2.
The government of Columbia has hired you to assess the danger posed by a recent eruption.
Although volcanic activity has subsided, several feet of ash have fallen near the volcano and the
rainy season is beginning. The town of Armero is located 20 km from the base of the volcano in a
narrow river valley and surrounded by small hills.
a.
What is the greatest volcanic hazard under these conditions? (1 mark)
Lahars
b.
Why? (1 mark)
Especially during the rainy season, rivers can experience growth of up to 4 times. This with the inclusion of the village being surrounded by valleys and small hills creates a perfect equation of rainwater mixing with volcanic substances following the river path to the village.
c.
Suggest a course of action to help the 23,000 inhabitants of Armero. (1 mark)
1.
Move, its just far safer to just get out the way
2.
Divert the river away from the village, whether that be through dams or other means redirecting the river would substantially increase the likelihood of the village surviving
11
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3.
The May 18, 1980 eruption of Mount St. Helens generated a horizontally directed series of
explosions that formed a lateral blast. This blast destroyed an area of 370 square kilometers. Trees
2 m in diameter were mowed down like blades of grass as far as 24 km from the volcano. The
blast exhibited characteristics of both pyroclastic flows and surges.
On the day of the main Mt. St. Helens eruption, seismometers 6 km from the blast origin stopped
transmitting 77 seconds after the eruption, when they were overridden by a dense pyroclastic
blast cloud. Based on this information, how fast (in km per hour) was the blast cloud moving?
Show your math. (1 mark)
Speed (S): S = 6km / 77s = 77.92 m/s = 280.5 km/h
4.
Figure 2 shows the paths of two lahars that descended from Mt. Rainier in the past. What
makes this a little scary is that neither of these events seems to be related to a major eruption
of the volcano – a lahar can happen simply because of increased heat flow from some deep
magmatic source melting the glaciers.
Figure 2. (left)
Osceola and Electron Mudflows, Mt. Rainier, Washington (right)
Geological
map of the area
https://www.dnr.wa.gov/programs-and-
services/geology/geologichazards/volcanoes-and-lahars#washington-volcanoes.3
12
The Osceola Mudflow downstream from Mt. Rainier, Washington, (~5000 years ago) is the
maximum lahar at Mt. Rainier (see Figure 2) and is believed to have a recurrence interval
~10,000 years. It would have had a mean flow velocity of ~40 meters per second (m/s) at the
base of the volcano, ~20 m/s at the Lowland boundary, and 10 m/s on the Lowland.
a.
Assuming an average recurrence interval of 10,000 years for an Osceola-sized lahar
event, how would you advise city planners in nearby Enumclaw, WA, regarding site
location within city limits for an elementary school? a nuclear power plant? (2 marks)
(There isn’t a right or wrong answer, but please explain your recommendations)
I would recommend that they create said facilities on highest ground possible, especially for a nuclear reactor so a lahar would have minimal but still very significant status on the buildings integrity and operational status should a lahar take place. As well as since if put on a hill lahars would most likely flow around these facilities further reducing its impact.
b.
From the map and using a piece of string (or equivalent), measure the distance the Osceola Mudflow traveled. Assume the lahar began at the summit of Mt. Rainier. (1 mark)
275kms approximately The speed of the front of a lahar has been measured for several recent events. The
average speed seems to be about 40 m/s. Using this speed, how long after the initial event
that triggered the mud to start moving at the summit did it take for the lahar to get to
Greenwater? to Auburn? (Good thing that no one was living around there at the time!) (1
mark)
Green water approx. 137.5 kms away from summit
Time (T): T = 137 / 40m/s = 3425 sec
= 57.08 min = 57 min 5 sec
Auburn Approx 255 Kms from summit
Time (T): T = 255 / 40m/s = 6375 sec
= 106.3 min = 106 min 15 sec = 1.771 hour = 1 hour 46 min 15 sec
5.
Case I lahars (e.g., Electron Mudflow) have a recurrence interval of 500-1000 years. Even one
event equal to or greater than a flow with a 1000-year recurrence interval has a 9.5% probability
of occurring at least once in the next century.
What would you recommend to Orting city planners regarding the site location for an elementary
school within city limits? Would it differ from your advice to Enumclaw regarding their
elementary school? Explain. (2 marks)
In regard to Orting City, Id recommend that they construct their facilities on the valley ridges away from the edge. Seemingly within city limits this would prevent said facilities from being hit at all from a Lahar.
Difference being that Orting is enclosed closely by the valley while Enumclaw is surrounded by flatland dead centre of the valley.
TOTAL: 23 marks
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