Download Walter - Department of Earth Sciences

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WALTER LIBRARY
Besides being the home of the Science and Engineering collections, Walter Library is one
of the more beautiful buildings on campus. Its walls host a diverse array of building
stones that formed in a variety of geologic settings.
Lab groups should confine themselves to the main foyer, to avoid bothering library
patrons.
INSIDE
The walls, floors, and trim of the main foyer are lined by a variety of rock types whose
origins run the gamut of igneous, sedimentary and metamorphic environments.
LIMESTONE - Change Link Name to: FLOOR
The foyer’s floor showcases Walter’s variety of building stones.
A border of dark green rock called serpentine (see “Serpentine’ link on the ‘Inside Walter
Library’ page) surrounds the floor. Two bands of polished rock parallel this serpentine
border. An outer band consists of a mosaic of metamorphic and sedimentary rock
fragments while the inner band is composed of red marble. Gray limestone covers the rest
of the foyer floor, displaying a pattern of dark irregular lines caused stylolites.
2ND
Although the limestone covering the floor is a solid rock, it is composed of the mineral
calcite that reacts to stress. Under stress, limestone will recrystallize and some of the
calcite will even dissolve. Only the calcite crystals dissolve, so any insoluble impurities
in the calcite are left behind to form stylolites.
3RD
Geologists can use stylolites’ orientation to determine the direction of the stress that
produced them, as stylolites form perpendicular to the direction of maximum stress.
If rocks are simply buried, as was the case with this limestone, then the only stress on the
rock is the weight of overlying rock layers and horizontal stylolites form. In tectonically
active areas though, stress can be applied from many directions. Convergent plate motion
may even form stylolites that are vertical.
DOLOSTONE - Change Link Name to: WALLS
Buff to light brown dolostone slabs with a trim of dark green serpentine rock line the
foyer’s walls. At first glance, it appears as if there are two dolostone varieties present, but
the only real difference lies in how the rock’s surface has been treated. In this photo,
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rough unpolished dolostone was used for the wall left of the serpentine section, while the
dolostone on the right was polished to a matte finish.
2ND
A network of fossil burrows betrays the dolostone’s marine origin and provides a mottled
texture that increases the rock’s value as a decorative stone.
Although dolostone is considered to be a sedimentary rock, it typically forms as an
alteration of marine limestone, rather than directly from seawater. The dolostone’s
burrows suggest this rock originated as a limestone formed in a shallow marine
environment. After deposition and burial, the limestone altered to dolostone as
magnesium-rich groundwater moved through it.
3RD
Close up, a mottled pattern of burrows dominates the dolostone’s texture. It is difficult to
see detailed structures though, because the rock’s fabric recrystalized during the process
of becoming dolostone. Small holes scattered across the wall probably formed as fossil
shells dissolved.
SERPENTINE
Serpentine rock is found throughout Walter Library as columns of dark green rock and a
border around floor areas.
A metamorphic rock, serpentine’s beauty is the product of a complex tectonic history.
Volcanism along the crest of deep-sea divergent spreading zones created the precursor
rock, basalt, from which serpentine forms. Millions of years later, as slivers of seafloor
crust were caught up in converging plate boundaries, basalt may be metamorphosed into
serpentine.
2ND
With feet for scale, you can see why serpentine is prized for its beauty. A complex
pattern of white streaks provides a striking contrast with the rock’s dark green matrix.
3RD
The columns in the foyer provide unworn examples of serpentine’s texture. A mosaic of
white seams bears witness to how the original seafloor was crushed and broken during
metamorphism as plates converged together. As this metamorphism occurred at depth,
the fact that this rock was quarried at the Earth’s surface also tells a story of significant
uplift and erosion. So serpentine, a single rock, reflects a complex history of diverging
seafloor volcanism, convergent mountain building as an ocean basin was subducted away
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to allow continental masses to collide, and the slow erosion and uplift to expose the root
rocks of a now-vanished, ancient mountain belt.
OUTSIDE
At the mall entrance to Walter Library, the stairs, benches, and sidewalk borders are
composed of the same igneous rock. Blocks of another type of igneous rock were also
used to form decorative patterns in the mall’s concrete sidewalks.
STAIRS
Interlocking crystals make up the stone blocks that were used for Walter Library’s stairs.
This crystal pattern not only increases the rock’s worth as decorative stone, it also shows
that the rock formed from the cooling of a magma, deep beneath the Earth’s surface.
2nd
We can tell that the rock formed deep beneath the Earth’s surface, as the crystals making
up the rock are large enough to be seen without magnification. Even from a distance you
can tell that the rock is composed of a variety of minerals. If the magma had reached the
Earth’s surface to erupt as lava, it would have cooled so quickly that its crystals would be
too small to be seen.
3rd
Close up, it is possible to begin to distinguish individual minerals, despite the rough
texture of the worn steps. A combination of black amphibole, pink potassium feldspar,
white plagioclase and gray to clear quartz identifies this rock as a variety of granite.
BENCHES
The granite used for Walter Library’s stairs was also used for the mall benches. As the
benches have seen less wear and tear, the rock texture is easier to see on their surface
than on the stairs. Choosing the benches also decreases the possibility that someone will
trip over you while you are studying the stone’s surface.
2nd
Even from a distance, it is possible to distinguish black amphibole crystals from the
pinkish potassium feldspar and white plagioclase crystals. Small clear quartz crystals,
which distinguish this rock as granite, are not apparent until you get closer.
3rd
Crystals large enough to be seen with the naked eye confirm that this rock formed from
magma cooling well beneath the Earth’s surface. Magmas that contain enough silica to
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form quartz crystals typically occur deep within convergent continent collision zones. So
the presence of granite at the Earth’s surface may be the last evidence of a mountain
range that has long since been eroded away.
4th
A darker mass of small crystals in the photo’s center is distinct from the surrounding rock
matrix. This is a xenolith, a fragment of the country rock through which the magma rose.
Similar xenoliths can be found in many of the mall’s granite blocks.
WALKWAY
Decorative patterns of red stone blocks are set in the concrete sidewalks of the mall area.
2nd
Walking along the sidewalk, it is easy for people to overlook the decorative stones
beneath their feet. These stones have a history though. Even the simple fact that they
were quarried at the Earth’s surface is evidence of the Earth’s dynamic nature.
3rd
Composed of a mixture of coarse reddish potassium feldspar and quartz crystals, these
paving stones are granite blocks from the St. Cloud area. They originally formed from the
slow cooling of a silica-rich magma, as a convergent collision boundary built a mountain
range across Minnesota. Millions of years later, as the ancient mountains were eroded
away rocks that had formed deep within the roots of the mountain range rose to be
exposed at the Earth’s surface. These blocks bear mute testimony to both a time when
Minnesota was an active convergent plate boundary, and the immense amount of time
during which an ancient range was worn away.