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NORTHROP HOME
Inside and outside, Northrop Auditorium has rocks of interest to geologists. The igneous
rocks that border Northrop’s plaza formed as the root of an ancient Minnesota mountain
range. While the stairs, banisters, and walls of Northrop’s interior lobby are composed of
a variety of marine sedimentary and metamorphosed sedimentary rocks originally
deposited on ancient seafloors
INSIDE
The stairs, walls and railings of Northrop’s spacious lobby are formed of rock that
originated as ancient seabed deposits. Unlike the igneous rocks of Northrop’s outdoor
plaza, these sedimentary and metamorphosed sedimentary rocks did not form on an
active plate tectonic boundary, but in a plate interior as deposition occurred along a
passive continental margin or within a continental basin. Although they share a roughly
similar original setting, variations in their origin and subsequent geologic history give the
rocks very different appearances.
STAIRS
The stairs at either end of the lobby are built from blocks of slightly metamorphosed
rocks that could be classified as either metamorphic rock (marble) or sedimentary rock
(limestone). Which term used in part depends on the person describing them. From a
geological point of view, the rock is probably more correctly considered to be limestone.
Marble, a more luxurious sounding term, is favored by the building stone industry.
For the tour’s purpose, a more important concept is that there can be a complete
continuum of change from sediments’ original deposition and compaction, through their
lithification and subsequent alteration, up to the rock’s complete metamorphism. Nature
is remarkably nonchalant about our human attempts to classify its components.
2ND
Regardless of the term used, the rock you now see as stairs originally formed as marine
deposits on the floor of an ancient sea. After being deposited, the sediments were buried
by other layers of sediment and lithified into sedimentary rocks. As they were buried
deeper, these rocks recrystallized slightly to become even more cohesive. If a piece of the
rock were broken off now, the break would cut through the depositional grains, rather
than along their original surface. This recrystallized texture allows the rock to be cut into
shapes and is the reason why marble can be used to create fine sculpture.
3RD
Because the rock has been slightly recrystallized, it may be difficult to tell whether it is
crystalline or grainy. Part of the difficulty lies in that the rock formed from fossils that
were broken up by wave and current action into grains. Since the original environment
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was fairly high energy, no mud was present and the grains are easily mistaken as crystals.
A few elongated shapes remain though, attesting to the grains’ origin from fossil debris.
LABEL
A faint dark line runs from the photo’s upper left corner to its right margin. This is a
stylolite, the edge of a three-dimensional plane of insoluble material left behind as some
of the calcite fossil grains dissolved under pressure as the rock was buried.
WALLS
The bulk of Northrop's foyer walls are composed of dolostone, a carbonate rock made
primarily of the mineral dolomite. Dolostone forms from the alteration of limestone and
as the rock alters much of its original texture is lost. It is more difficult to distinguish fine
textural details in the walls than in the stairs and banisters.
2ND
Despite the loss of textural detail, we can still tell that the dolostone began as a marine
carbonate because the rock retains a pattern of tracks or pathways that formed as marine
organisms burrowed through seafloor mud. The activity of these burrowing organisms
imbued the rock with an irregular pattern that increases its worth as a decorative building
stone. Burrowing also reveals an important depositional difference between this rock and
that of the stairs and banisters. Unlike the grainy sediment that formed the stairs’ rock,
this dolostone originated as mud-rich sediment, deposited in a quiet, possibly deeper
water setting. Organisms burrowing through the mud were feeding on decaying organic
material found within the mud.
3RD
Closer yet, individual tracks from the burrowing organisms are harder to distinguish
because of the rock's coarsely recrystallized fabric. Small holes scattered through the rock
most likely formed as fossil shells dissolved during the rock's alteration from limestone to
dolostone. So a combination of quieter water setting and later alteration to dolostone
created rock with a very different appearance from that of the stairs and banisters.
BANISTERS
Northrop’s upper stairs and balconies have banisters composed of rock that is very
similar to that of the stairs. Apart from a slight color variation, the major difference is that
the banisters retained their polish, having seen much less wear and tear.
2ND
A polished surface allows textural details to stand out a bit more than on the stairs.
Composed almost exclusively of broken, abraded fragments of fossils, the rock’s original
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sediment was deposited in a fairly high-energy environment. The complete absence of
detrital material suggests a location well removed from any river mouth or lowland area.
Because carbonate shells dissolve more readily in cool water, it is also likely that this
rock formed in a warm water, possibly tropical setting.
3RD
At first it can be frustrating trying to pick out individual fossils, until one realizes that
every grain seen is actually a broken fragment of a fossil. Waves and currents broke up
the fossils to create coarse carbonate sand. Only a few shells were left unbroken. The
cross-sections of these scattered unbroken shells can be seen as elongated white shapes
just below the irregular dark line.
LABELS
The dark line is another example of a stylolite. Stylolites are the edges of thin threedimensional planes of insoluble material that formed as the rock’s calcite component
dissolved under pressure as the rock was buried.
OUTSIDE NORTHROP
Backdrops for concerts, protests and convocation, the benches, walls, and planters of
Northrop plaza also are evidence that Minnesota once had a very different topography.
Composed of igneous rock that formed as magma slowly cooled, these rocks originated
far below a long-vanished mountain chain that stretched across the state.
BENCHES
Even at a distance, the rock's coarsely crystalline texture is apparent.
2ND
Large crystals only form when magma cools very slowly. The most likely setting for such
slow cooling is deep within the Earth, where overlying layers of rock insulated the
magma.
3RD
Close up, it is relatively easy to identify which minerals are present in the rock. Most of
the rock consists of roughly equal amounts of pink potassium feldspar and white
plagioclase, with black amphibole and a small amount of gray to clear quartz making up
the rest. Equal amounts of the potassium and plagioclase feldspars, along with the limited
amount of quartz, identify this rock as quartz monzonite, a variety of granite.
WALLS
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At first glance, the texture of the rock appears to be fairly monotonous, but some
irregularities are present.
2ND
Small black areas are scattered through the crystalline mass. Although they initially
appear as blemishes, marks or graffiti, these black areas actually originated as small
fragments of rock that were caught up in the magma that formed the surrounding granite.
This type of inclusion is called a 'xenolith', a Greek word for 'strange’ or 'foreign' rock.
3RD
A xenolith is a fragment of rock that was incorporated into magma, as the magma rose
towards the Earth’s surface.
LABELS
Xenoliths are often some of the best clues we have as to the nature of the rocks deep
within the Earth. Although some xenoliths, like this one, come from relatively shallow
depths, others may be fragments of mantle rock that would otherwise never make it to the
Earth’s surface. Similar processes occur in volcanic systems. Diamonds are probably the
most well known examples of geological materials that make their way to near surface
environments as fragments of deep rock caught up within moving magma.
PLANTERS
A mosaic of colored crystals makes this a prized decorative stone, but also tells a
geologic story of Minnesota’s past plate tectonic processes.
2ND
Slow cooling magmas typically occur far below the Earth’s surface, where overlying rock
insulates the magma. Consequently, even the existence of this coarsely crystalline rock as
part of Minnesota’s present landscape illustrates the Earth’s dynamic nature.
As mountains are worn away, their root rock rises as the weight of overlying rock
decreases. Eventually deep-formed rocks can be exposed at the surface as the last vestige
of now-vanished mountains. While past mountain building shows that Minnesota was
once an active plate boundary, the rock’s composition provides crucial evidence for
which type of plate boundary was present.
3RD
Composed of feldspars, amphibole and quartz crystals, this rock originated as a silica-rich
magma. Large amounts of silica-rich magmas can form deep within continent collision
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zones. This magma cools to form the roots of continental mountain chains like the
Himalayan or Appalachian Mountains.
As Northrop’s igneous rock came from the Rockville area, the plaza’s walls, benches and
planters are part of Minnesota’s geologic legacy. Almost two billion years ago, a
continent collision zone dominated our landscape. Looking north from the plaza area, a
time traveler would see a line of snow-capped mountains stretching along the northwest
horizon. The rocks you see here came from the root of that mountain range, which
formed as micro-continents collided to build North America.