Download Lab 4

Geology 101
Name(s):
Lab 4: Igneous rocks and volcanic hazards
Needed: lab manual, samples R4 – R17 (Tubs 5 – 17)
As noted in the lab manual, igneous rocks are first separated into plutonic (intrusive)
or volcanic (extrusive) rocks. Then, plutonic rocks are identified by their characteristic
mineralogy, and volcanic rocks are identified by either their texture or their
characteristic mineralogy.
1. Samples R4 through R9 are drawn from the rock names under “rock types” on the
table on page 23 in the lab manual – actually, there is no “peridotite” sample. For each
of the rocks, identify its composition (“felsic”, “mafic” or “intermediate”), then its
predominant grain size (either “coarse” or “fine”). Finally, identify the rock name using
the flowcharts on pages 24 and 26.
Rock #
Felsic, mafic or
intermediate?
Predominant
grain size
Rock name
R4
R5
R6
R7
R8
R9
2. Look at samples R-10 and R-11, two very common rocks in the Cascades.
a. R-11 is called granodiorite. Is it mafic, felsic, intermediate, or in-between two of the
categories? If in-between, which two classifications is it in between? (Hint: look at the
name)
b. R-10 is called dacite. Same question as part a. (Hint: it is the extrusive equivalent of
granodiorite)
3. Why is it not weird that if you find a lot of one of these rocks in the Cascades, that you
find a lot of the other as well? Hint: your answer should use the phrases “magma
chamber” and “eruption”. A cross-section may help your explanation.
4. The rocks in the table below can be identified by their texture. Use the flowchart on
page 28 to aid your identification. Under the texture column, write the descriptive word
or words from that flowchart that helps you categorize the rock.
Rock #
R12
Texture
Rock name
R13
R14
R15
R16
5. Get a sample of rock R-17, which is cut and polished thunderegg. Thundereggs are a
type of geode, which make beautiful bookends and household decorations. The exterior
rock of these thundereggs, which are from the Priday Agate Beds near Redmond, Oregon, is
an andesitic basalt, and the interior material is milky quartz (I tell you this because I
don’t want you to perform the usual mineral tests on these!). Write a short history of how
this rock came to be; in other words, start from magma and tell me what happened to make
this rock. Hint: it’s not a one-event history.
Volcanoes
The viscosity of a fluid is its ability to resist flow; higher viscosity means that the fluid
doesn't flow very fast. Mafic magma is very fluid compared to more felsic magmas. As a
result, mafic lava travels further from the volcano than does felsic lava, which tends to pile
up near the eruption point. Felsic lava, because of its viscosity, also traps volatiles (gases).
6. Circle the correct choice below:
a. Which temperature magma would tend to have less viscosity (more fluidity)?
1200°C
900°C
600°C
b. Which temperature magma would tend to have more iron in its molten state?
1200°C
900°C
600°C
c. Which temperature magma would contain the least amount of undissolved gases, and
therefore be more fluid?
1200°C
900°C
600°C
d. So which word best describes the composition of the magma that satisfies the
temperature requirements above?
mafic
intermediate
felsic
7. Using the results of question 6, answer the following questions with mafic, felsic,
intermediate or any:
a. Which magma will likely erupt explosively?
b. Which magma will likely erupt effusively (quiescently)?
c. Which magma will produce volcanoes with steep slopes?
d. Which magma will produce volcanoes with gentle slopes?
The "2. Volcanism" raised relief model
8. Give the name of the rock most likely to be found on (make sure the name is for a
volcanic rock – “any” is an acceptable choice if the volcano’s composition is variable):
Volcano 21
Volcano 28
Volcano 25
9. Along the northern edge of the model, find features 37 and 38 and identify them as
either a “sill” or a “dike” or a “pluton”.
Feature 37 (the specific kind is called a laccolith)
Feature 38
10. How does feature 38 make it to the surface? In other words, how do features like 38
ever get exposed and allow geologists to study them?
Measuring eruptions
The Volcano Explosivity Index (VEI) was developed by volcanogists Chris Newhall and
Steve Self in 1982 to gauge the explosiveness of a particular eruption from
characteristics like the height of the ejecta plume, the total volume of ejecta, and other
factors. This could be used then to assess the danger a given volcano might pose to its
surrounding communities.
VEI
Volume of
ejecta (km3)
0.000001
Classification
Example
Hawaiian
Kilauea, now
0.00001
Hawaiian/
Strombolian
Stromboli, now;
Nyiragongo, 1982
2
Plume
height
<100 m
(< 0.1 km)
100 – 1000
m (0.1 – 1
km)
1 – 5 km
0.001
Colima, 1991; Galeras, 1992
3
4
3 – 15 km
10 – 25 km
0.01
0.1
5
6
>25 km
>25 km
1
10
7
8
>25 km
>25 km
100
>1000
Strombolian/
Vulcanian
Vulcanian
Vulcanian/
Plinian
Plinian
Plinian/
Ultra-Plinian
Ultra-Plinian
Ultra-Plinian
0
1
Galeras, 1924; Ruiz, 1985
Sakura-jima, 1914;
Galunggung, 1982
St. Helens, 1980
Krakatau, 1883; Vesuvius,
79AD
Tambora, 1815
Toba. 74000 yrs BP
The effect of the 1980 VEI 5 eruption of Mt. St. Helens are given at
http://vulcan.wr.usgs.gov/Volcanoes/MSH/May18/summary_may18_eruption.html;
one can only wonder at what effects a VEI 8 eruption might have!
Clearly, Mt. St. Helens is capable of a VEI 5 eruption. To translate this into human
effects, two researchers, John Ewert and Christopher Harpel, came up with a Volcano
Population Index or VPI. Through measurements of the eruptions of Central American
volcanoes, which are similar to the Cascades volcanoes, they discovered that VEI 5
eruptions warrant evacuations of all people within 10 km (6 miles) of the volcano’s
crater. The VPI is the number of people who therefore are in need of evacuation for a
potential VEI 5 event.
11. Let’s assess the hazards that nearby Cascades volcanoes may have locally. Look at the
Mt. Rainier, Glacier Peak and Mt. Baker maps. Use the scale to measure 10 km away
from the crater in any direction (so a circle of radius 10 km around the crater). In the
table, list any towns or cities within your boundary for these volcanoes.
Volcano
Towns and cities within
10 km
VPI10
Mt. Rainier
Glacier Peak
Mt. Baker
12. Use a census reference like http://www.ofm.wa.gov/pop/ to calculate the VPI for
each volcano. If there are no towns or cities within 10 km of a particular volcano, or else
they are not listed in the census, write “0”.
13. Compare and contrast this to the Central American country of El Salvador. Go to
http://www.agiweb.org/geotimes/apr04/feature_VPI.html which is an article authored
by Ewert and Harpel. Using figures from the article, complete the table:
Volcano
Towns and cities within
10 km
VPI10
Volcan San Salvador
Ilopango Caldera
14. The authors do caution about the limitations of using the VPI to predict volcanic
hazards. What two factors do they cite might cause the VPI to be a serious
underestimate of the true hazard?
What about those VEI 8 eruptions?
VEI 8 eruptions are called “super-eruptions”, and they earn their name through the
ejection of hundreds and possibly thousands of cubic kilometers of pyroclastic material,
usually called ignimbrite, a rhyolitic or dacitic tephra.
About 74,000 years ago (74000 yr BP), Toba volcano on the island of Sumatra in
Indonesia erupted over a two-week period, and causing a six-year decline in worldwide
temperatures plus significant acidification of rainfall. There is an ongoing debate over
whether that eruption nearly wiped out Homo sapiens:
http://www.sciencedaily.com/releases/2010/02/100227170841.htm
Go to http://www.andamans.org, and click on the “Toba” pull-down menu at the top.
There you will find the first five chapters of George Weber’s work on this eruption.
15. What is the evidence that Toba was, indeed, a VEI 8 event? What sort of tectonic
setting generated this eruption, and can this kind of setting produce a VEI 8 eruption?
16. Why isn’t there really a volcanic peak here any more? The hint is that these types of
volcanoes are called “resurgent calderas”. What is the evidence of the “resurgent”
part here?
Local volcanic hazards
In addition to ejecta and
explosions, volcanoes with
glaciers or snow or just a
lot of groundwater can
produce mudflows when
glacial meltwater combines
with the rock and sediment
on the mountain’s flanks.
These mudflows are known
by the Indonesian word
lahar.
The map to the left shows
the paths of two lahars that
descended from Mt.
Rainier. 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.
17. Notice that the lahar
paths are curved. Find a topographic map of this area and determine the names of the
specific geographical feature these lahars follow.
Osceola Mudflow —
Electron Mudflow —
18. Assume the Electron Mudflow (lahar) began at the summit of Mt. Rainier. The speed
of the front of a lahar has been measured for several recent events. The average speed
seems to be about 40 km/hr. How many hours after the initial event that triggered
the mud to start moving at the summit did it take for the lahar to get to Orting? Show
the detail of your calculations below. Hint: A piece of string may be handy here.
The final hazard discussed is tephra, or less formally, ash. This finely-pulverized rock
from a volcano can drift a long way in the wind. Tephra is a respiration hazard,
which can result in silicosis because the particles stick to the inner lining of the lung.
Tephra can also destroy engines, since the fine particles can cause pistons and valves
to seize (don’t drive through a tephra fall if you can help it!). Finally, if an eruption can
send tephra to the top of the troposphere, or even into the stratosphere, the fine
particles can block light and therefore reduce the amount of sunlight that reaches the
surface of the earth. In effect, a big eruption can cool the earth. Mt. Pinatubo in the
Philippines ejected enough material in its 1991 eruption to cool the northern
hemisphere by nearly 0.5°C for three years, which doesn’t sound like much. This
difference, though, shortened the growing season by nine days, which may have caused
some crops in northern countries to fail.
The US Geological Survey mapped the 1980 Mt. St. Helens ash plume. Go to
http://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_gallery_27.html and flip
over to image 6.
19. a. Which compass direction was the wind blowing on May 18, 1980?
b. On the same map, note the extent of some larger ash eruptions, like the Huckleberry
Ridge eruption of the Yellowstone supervolcano. If a similar eruption of
Yellowstone were to occur today, would Seattle be immediately affected? Would Seattle
feel any effects over time, and, if so, what would the effects be?
Igneous rocks and volcanoes summary
The distribution of the different types of volcanoes around the world seemed
inexplicable to earth scientists even in the last century. Plate tectonics, though,
provided the framework to allow explanations of why a particular type of volcano would
occur in a given area. You’ve seen (in Lab 2) some mechanisms that allow volcanologists
to predict what type of rock will be produced by volcanoes in certain plate tectonic
settings, and, in this lab, how destructive an eruption may be. Here’s a summary of the
volcanoes and associated rock types:
• a divergent plate boundary volcano (such as one on a mid-ocean ridge) produces
basalt
• a convergent plate boundary volcano (such as one located near a subduction zone)
produces andesite or dacite, with minor basalt
• an oceanic hotspot volcano, located on the interior of an oceanic plate, produces
basalt
• a continental hotspot volcano, located in the interior of a continental plate,
produces rhyolite, andesite, dacite or basalt
20. Given those rules, fill in the table below. You may wish to search the Web or other
sources to see what each of the volcanoes looks like.
Volcano
Tectonic
setting
Yellowstone
(Wyoming)
Continental
hotspot
Vesuvius
(Italy)
Subduction
zone
Cerro Azul
(Galapagos
Islands)
Interior of
oceanic plate
Name of rock the VEI
volcano is
estimate
primarily made
of
Estimate of effect
on climate
(global, regional
or local effect?)
What type of volcano (shield, composite or resurgent caldera) is each?
Yellowstone —
Vesuvius —
Cerro Azul —