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Earth Materials Review Questions #2
1) List the three types of volcano, give their principal features, the type of magma that feeds
them, and the geologic settings in which they occur.
Cinder cones can occur in any setting where magma is present and often are built up on other
volcanoes. The form when the gas content in the lava is high enough to explosively eject small
fragments of lava high into the air, causing to cool into cinders when fall back to build up the
cone of loose material that makes up the volcano. They usually are small and don’t last as long
as the other two types. Cinder cones can develop from both felsic and mafic magma.
Shield volcanoes form from high temperature, low-viscosity, mafic magma. The gas content
typically is low and the lava flows out of the volcano and over the surrounding countryside for
long distances. Thus, shield volcanoes are much wider than they are tall, although they can get
quite high. The biggest are over 100 kilometres wide and many nearly several kilometres high.
They are the typical volcano of mantle hot spots.
Composite volcanoes, or strato-volcanoes, form from less hot, more viscous magma which
usually is of a more intermediate chemical composition. The lower viscosity causes the lava to
build up tall cone-shaped volcanic mountains. The magma often has a higher gas content (often
water and carbon dioxide) which can result in more explosive ash eruptions (and pyroclastic
flows). Thus, the volcanic cone usually is built up of alternating layers of lava and ash. These
volcanoes are typical of subduction zones where differential melting of water-rich rocks produces
intermediate, gas rich magma. This can become more silica and gas rich as it incorporates silica
rich material, water, and CO2 if it passes through continental crust.
2) What are the implications of the following sedimentary structures:
a) mud cracks
frequent exposure to air and drying out in warm conditions.
b) symmetrical ripples and internal cross lamination
These are formed in shallow water where waves at the surface affect the sediment.
c) asymmetrical ripples and internal cross lamination
These are formed by moderately strong currents flowing over the sediment.
d) the presence of intact fossils
These imply that the organism’s remains have not been moved much from the place
where the organisms lived.
e) the presence of broken-up and randomly arranged fossils
These imply that the organism’s remains have been moved long distances and/or by
energetic conditions. The environment where they were deposited may not reflect
the conditions in which the organisms lived.
f) the presence of fossil burrows
These generally cannot be moved with out destroying them, so they show where
organisms actually lived. They also show the types of activities that were carried
out in the environment where they were made.
g) upwardly decreasing grain size
This develops where water current speed gradually decreases.
h) the presence of minerals such as mica and orthoclase feldspar fragments
These show that the sediment has not experienced much chemical weathering.
i) well rounded sand grains
This shows that the sediment grains have experienced prolonged periods of movement
around on the earth’s prior to being deposited.
j) angular sand grains
These also show that the sediment grains have not experienced much mechanical
weathering and have not been moved around much before being deposited.
k) well sorted versus poorly sorted sand grain sizes
Well sorted grain sizes imply long periods of consistent current speed.
Poorly sorted grain sizes imply short duration of current and rapidly changing current
speed.
l) sandstone versus shale
Sandstone implies higher current or wave energy to move sand sized particles around
and remove smaller clay and silt sized particles. Shale implied slow settling of small
sediment fragments in nearly still water.
3) You find two sandstones; one with ripple cross lamination and another with horizontal
lamination. Which was deposited in the faster current?
The one with the horizontal lamination.
4) What does a rock bed represent in general terms?
Some event in earth history.
5) What is the likelihood that any particular layer of sediment on a river bottom actually gets
preserved in a rock?
Very low. Most sediment layers are reworked many times as sediment is deposited, on
average, at very slow rates: fractions of millimetres per year.
6) How do clastic, chemical, and biological sedimentary rocks differ in terms of structure and
formation?
Clastic sediment forms from the mechanical weathering of rocks into small fragments which are
moved about on the earth’s surface by wind, water, and other means before being deposited,
compressed, and cemented together to form rock.
Chemical sedimentary rocks form when minerals precipitate out of aqueous solution. They
usually form interlocking crystals rather than grains.
Biological sedimentary rock forms from the skeletal remains of once-living organisms. These
skeletal fragments can behave as sediment grains like clastic sediment, but the shapes of the
grains are more complex. Often the minerals involved can be easily dissolved and thus the
sediment can cement itself together.
7) a) What is the difference between weathering and erosion?
Weathering is the transformation of rock material into something else.
Erosion is the transportation of weathered rock material to another location.
b) What is the difference between chemical and physical weathering?
Chemical weathering is the chemical transformation of rock materials into new minerals
(often including the dissolving of some of the former mineral components)
Physical weathering is the breaking of rock material into smaller fragments.
c) What are the steps for turning any rock into a sedimentary rock (think rock cycle)?
1) Weathering of rock, 2) transportation of rock material as fragments or dissolved material, 3)
deposition by settling out of fragments or precipitation of crystals, 4) compaction of grains or
crystals, and 5) cementing of grains or crystals into a solid mass.
8) What sorts of events are likely to be over represented in the sedimentary rock record?
The more violent events that create layers that are thicker than normal events can erode or
modify before the next violent event. Thinner layers made by every-day events are more likely to
be eroded or modified by the violent events which can affect much deeper sediment layers.
9) a) What is the difference between micrite limestone and the limestone out of which the south
building is made?
The north building is made of small sediment grains that were transported by currents producing
sedimentary structures within the rock. There are some fossil fragments, but mostly it is made of
small sediment grains. The south building is composed mostly of fossils fragments that look to
have been dumped into their deposits in thick masses with no obvious layering or internal
sedimentary structures.
b) Is micrite usually a biological or chemical sediment?
Usually the source of the material is biological - often the calcite skeletons of algae.
10) What is the ultimate source of the dissolved ions that get precipitated into biological and
chemical sedimentary rocks?
Igneous rocks.
11) What is the effect of increasing aqueous dissolved CO2 concentration on the solubility of
carbonate sediment and sedimentary rock in contact with that water?
The solubility increases.
12) a) What is chalk?
A soft calcium carbonate rock made from coccoliths: small skeletal disks from single-celled
algae.
b) What is diatomite?
A soft silica rock made from diatom skeletons (these also were single celled algae).
c) What practical uses do we have for each of these sedimentary rocks?
Writing on blackboards and filtering water.
13) Why can't a basalt be formed by the partial melting of a granite while granite can be formed
by partial melting of a basalt?
Most rocks contain a variety of minerals. When a rock is partially melted, only the lower
melting-point minerals melt at first. These are the more silica-rich minerals. Thus, partial
melting of a rock always produces a magma which is more silica-rich than the rock that was
partially melted.
14) Why are most sand grains made of quartz and why are they all of such similar size?
Quartz is indefinitely stable at the earth’s surface and most other volcanic minerals are not. Most
other stable sedimentary minerals are too soft to form sand-sized fragments that last for any
significant length of time. Quartz is the last mineral to crystalize out of a magma, so the grains
can only grow into the void spaces that are left between the mineral grains that already have
formed. These void spaces usually are quite small at this late stage of the cooling process.
15) Describe the features that you would look for to identify a package of sedimentary rocks
formed in a meandering river?
Thick, lens-shaped layers of cross-bedded
sandstones (often with a eroded base with large
pebbles) (the river channel), overlain by thinner
sandstone beds (the point bar), also with cross
laminations, overlain by shale and siltstone
with occasional thin, crossbedded standstone
(the flood plane).
16) What might be different about the sandstone formed from the erosion of a granite batholith as
opposed to a mass of gneiss from the canadian shield?
The sand made from the metamorphic rock would contain some minerals indicative of
metamorphic rock, while that sand from the granite would contain some mica and feldspar, but
not diagnostic metamorphic minerals.
17) What are the first, second, and third largest reservoirs of carbon on the earth?
The mantle, followed by carbonate sedimentary rock, followed by fossil fuel precursor
molecules.
18) Is the fast carbon cycle in a state of dynamic equilibrium now? If not, why not?
No, we are adding extra CO2 to the atmosphere by burning fossil fuels faster than new fossil
fuels are being formed.
19) a) What is karst topography?
Carbonate rock with sharp-edged cavities formed by the dissolving of the rock minerals.
b) What might occur in caves under where karst is developing?
Stalactites and stalagmites caused by precipitation of carbonate minerals when dissolved
CO2 escapes from the water entering the caves and lowering the mineral solubility.
20) How are limestone and dolostone different and what is the easiest way to tell them apart?
Calcite is CaCO3 while dolomite (the mineral that makes dolostone) is MgCa(CO3)2
Dolostone often weathers a more tan colour and will not react to HCl(aq) (calcite will)
unless it is powdered first.
21) When metamorphosed, how do the characters of sandstone and limestone (chemistry and
texture) tend to change, if at all?
Their minerals stay the same, but the grains partially dissolve and reprecipitate resulting in more
cementation, growth of larger crystals, and filling in of void spaces.
22) In what geological setting would you expect to find...
a) low pressure, high temperature metamorphism?
exposure of rock to magma and lava near the earth’s surface.
b) high pressure, low temperature metamorphism?
at subduction zones where plates descend to great depth, but heat up slowly.
c) high pressure, high temperature metamorphism?
deep burial under mountains
23) a) What process is generally responsible for contact metamorphism?
Volcanism: intrusion of magma into or extrusion of lava onto cold rocks near the surface.
b) Why might this produce more profound effects when it happens close to the surface?
Rocks at great depth usually are fairly hot already.
24) Why are accessory index minerals important for understanding metamorphic rocks?
They form under particular ranges of temperature and pressure.
25) a) What are foliation and mineral banding?
Foliation is the parallel alignment of phyllosilicate minerals (perpendicular to direction in which
they are being compressed) which allows the rock to be more easily split along that direction.
Banding occurs as minerals separate into separate layers within the rock as it is being
compressed.
b) What causes them? See a)
c) What needs to be part of the protolith rock for foliation to occur?
It must contain the material needed to form phyllosilicate minerals.
26) Which has been more severely metamorphosed: a phyllite or a gneiss?
A gneiss.