Download What Types of Igneous Processes Are Occurring Here?

I N V E S T I G AT I O N
What Types of Igneous Processes
Are Occurring Here?
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IGNEOUS ACTIVITY IS NOT DISTRIBUTED UNIFORMLY ON EARTH. As a result, some regions are more
likely to experience volcanic eruptions and other igneous activity. In this exercise, you will investigate five
sites to interpret the types of igneous rocks likely to be present, the style of eruption, and the probable causes
of melting.
Goals of This Exercise:
• Use the regional features of an ocean and two continents to infer the tectonic setting and cause of melting at five sites.
• Observe and identify nine rock types and infer the cooling history of each rock based on its texture.
• For the volcanic rocks, predict the viscosity of the magma and what type of eruption probably formed the rock.
Tectonic Settings of Igneous Activity
The perspective view below shows two continents and an intervening ocean basin. The area has five sites, labeled
A, B, C, D, and E, where igneous activity has been observed. For each site, investigate the igneous processes
responsible for the activity and enter your results in the worksheet or online using the steps listed below.
1. Use the features on this map to infer whether the tectonic setting of each site is associated with a plate boundary and, if
so, which type of plate boundary is present. The possible tectonic settings for this region are: (1) an oceanic or continental
divergent boundary, (2) one of the three types of convergent boundaries, or (3) a hot spot in a continent, ocean, or both.
However, not all of these settings are present in this area.
2. For each site, determine the likely cause of melting. The options are (1) decompression melting, (2) melting by adding water,
and (3) melting of continental crust caused by an influx of mantle-derived magmas. More than one of these causes might apply
to each site. Think about the kinds of igneous rocks you would expect to find at each site, including those that solidify at
depth (plutonic) and those erupted onto the surface (volcanic). Your instructor may ask you to list the predicted rock types.
Site A: A line of volcanic islands
and submarine mountains.
Broad volcanoes on the islands
are erupting dark volcanic
rocks. [Sample 1]
Site B: Circular volcanic
depressions (calderas)
on land, which are filled
with light-colored volcanic ash and other
volcanic rocks.
[Samples 2 and 3]
Site C: A mid-ocean
ridge that zigzags
across the ocean
floor. The sample
is dark colored
and is from a
lumpy lava flow
on the seafloor.
[Sample 4]
Site D: Volcanoes on top of a mountain belt near the edge of the
continent. The volcanoes erupt
light-colored and gray volcanic
rocks. The mountain range also
exposes older intrusive rocks,
some with coarse crystals.
[Samples 5, 6, and 7]
Site E: A chain of volcanic
islands form an island arc
adjacent to an oceanic
trench. The volcanoes
erupt gray volcanic
ash, sometimes
explosively, and
lava flows. There
are also some
intrusive rocks.
[Samples 8
and 9]
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Igneous Environments
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Predicting the Types of Igneous Rocks and Eruptions at Each Site
The photographs below show nine different rocks. Your instructor may provide you with samples of each rock or
may substitute a different suite of rocks. Observe each of these rocks and complete the steps below.
1. Your instructor may have you write a short description of each photograph or of actual samples.
2. On the worksheet, indicate (1) whether each rock shown is coarsely crystalline, finely crystalline, or has other distinctive igneous
textures, (2) whether it is probably mafic, intermediate, or felsic, and (3) the name you would apply to such a rock. For certain
samples, some of this information is provided in the text accompanying the photograph below.
3. Predict the cooling and solidification history (slow, moderate, fast, slow then fast, or slow cooling in the presence of water) for
each rock sample based on its texture.
4. For each site, use the rock samples that you interpret to be volcanic to predict whether the magma for that site has a high
or low viscosity, and what type of volcanic eruption probably formed the rock sample.
5. Your instructor may have you use the various types of information to explain how the samples are consistent with the tectonic
setting of each site. Alternatively, your instructor may have you infer the entire sequence of events including (1) what caused
the initial melting event, (2) what processes might have occurred in the magma chamber, (3) where and how the rock cooled
and solidified, and (4) whether uplift and erosion are required to expose the rock.
Descriptions of Rocks
Some descriptions are provided, because it is
more difficult to describe and identify rocks from a
photograph than if you could pick up and closely
examine each rock. Make your own observations
first, and then read the hints here.
Rock 2. This light-colored rock is finely crystalline and not glassy. It contains some finegrained gray crystals and a few small holes.
A chemical analysis documents that it is a
felsic rock. It was collected on a volcano.
Rock 1. This rock is
dark colored, has no
visible crystals, and
is not glassy. It does
have many vesicles
(holes) and is a volcanic rock. A chemical analysis revealed
a relatively low silica
content.
Rock 3. This brown-colored rock
contains large, light-colored lenses of
flattened pumice in a finely crystalline
matrix. Under the microscope, the
matrix contains compacted volcanic
ash, along with
small crystals and
fragments of other
rocks. The crystals
are mostly quartz
and K-feldspar.
Rock 6. This rock
has very large crystals, some of which
are 5 to 10 cm long.
Most of the rock
is K-feldspar and
quartz, and the dark
crystals are a type
of amphibole that
is most common in
felsic rocks.
Rock 4. This rock
is finely crystalline
and lacks visible
crystals. It has a
dark-gray color. A
chemical analysis
of the sample
indicates that it
is a mafic rock.
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Rock 7. This medium-grained rock is a plutonic rock. It has a
salt-and-pepper appearance, caused by the presence of felsic
minerals (feldspar and quartz) and mafic minerals (mostly biotite mica). It is intermediate in composition.
Rock 8. This rock has scattered visible crystals of
amphibole and biotite in a medium-gray, finely crystalline matrix. It reportedly was collected either on a volcano or from dikes exposed near a volcano.
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Rock 5. This porous
rock in the center
of the photograph
has numerous holes
and is extremely
lightweight. It is
light colored and is
composed of volcanic glass. There
are no visible crystals, but the rock is
likely to be felsic in
composition.
Rock 9. This rock
contains large crystals of K-feldspar
in a brown-colored
matrix of small
to medium-sized
crystals. There is
no glass, ash, or
vesicles, so the rock
is probably not a
volcanic rock.