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FIREPLACE GEOLOGY
Dining Hall
The Dining Hall fireplace represents a
slice through a Cascade Mountain
volcano. Volcanoes are formed from
molten magma rising up from within
the Earth’s crust.
Magma that reaches the earth’s surface
can flow out as lava or can explosively
erupt into clouds of ash., cinders, and
hot gasses. The lava and ash pile up to
form the high Cascade volcanoes such
as Mount Rainier. Andesite (at the top
of the fireplace) is the most common
rock type in the Cascade volcanoes.
Much of the magma solidifies before it
reaches the surface. This is represented
by the light-colored ‘granite’ in the
middle of the fireplace.
The difference in appearance between
the andesite and granite is due more to
texture than composition. The
textures differ because the cooling rates differ. Slower cooling enables larger crystals
to grow in the granite.
The ‘country rock’ rock through which the magma flows will be metamorphosed
near the magma by intense heat and pressure. This is represented by the
metamorphic rock in the lower part of the fireplace.
Geology
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Great Hall
The Great Hall fireplace is made up of rocks representing the Olympic Mountains.
The Olympics are composed of ocean-bottom rocks that were scraped off the
oceanic plate as it slid beneath Western Washington.
The oldest Olympic rock is basalt that originally formed from lava erupting out of
crustal fissures into water at the bottom of the ocean.
The other Olympic rocks were sediments deposited on the ocean floor, buried, and
then hardened into sandstone and shale. Later, they were jammed against the basalt,
pushing it backward and upward until it now stands vertical at the eastern end of the
mountains
Olympics
basalt
sandstone
and shale
Juan de Fuca Plate
Diagrammatic cross section of the Olympics. As the Juan de Fuca plate moves under North
America, young ocean sediments are forced under previous slabs of sediment along curved thrust
faults – note the small arrows for the relative motion along these faults. This motion of the Juan
de Fuca plate generates large earthquakes along the Washington coast.
Basalt is typically dark
gray, but here we see
mostly orange rust stains
on the surface caused by
oxidation of iron-bearing
minerals in the basalt
The lighter colored rock
across most of the
fireplace is sandstone,
representing the
sedimentary rocks in the
Olympics.
Some of the Olympic rocks were dragged down deep enough to be partly metamorphosed by heat and pressure
– these are represented by the band of dark bluish-green altered basalt or serpentinite.
Birds Nest Lodge: Sedimentary Rock
Geology
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The fireplace is primarily comprised of horizontally layered sandstone. Pieces of
conglomerate and breccia are interspersed throughout the fireplace. (Breccia is a form of
conglomerate containing angular rock fragments that have not been transported far enough
to round off.) The fireplace also contains some very old fossils
Most sedimentary rocks are formed when eroded bits of older rocks are deposited and then
naturally cemented into a cohesive rock.
Very small particles (clay and silt) are deposited in quiet low-energy areas like lakes or the
deep ocean. When they harden into rock, they become shale. Larger particles (sand and
pebbles) are deposited in high-energy places like beaches and riverbeds. These harden into
sandstone and conglomerate.
Can you find these fossils in the fireplace?
The nautilus is a mollusk with tentacles, like an octopus or
squid. However, the nautilus has a shell in which to hide.
The shell is also partly gas-filled to control buoyancy, so it
can swim easily. This fossil nautilus is 500 million years old.
It lived before fish evolved and was the largest predator in
the ocean.
Trilobites were a form of sea
arthropod -- a bit like lobsters
or shrimp. The group lived
from the beginning of the
Paleozoic (570 million years ago) until the end (250 million
years ago) when they became extinct along with 90% of all
sea creatures that had been living at that time.
The first coral reefs were formed
about 400 million years ago. Like
the trilobites, the group was
decimated at the end of the
Paleozoic; however some corals
have survived down to the
present day. Coal is formed
from ancient plants that are
buried and subjected to heat and pressure for millions of years.
Thick mats of plant material accumulated in ancient swamps,
produced coal seams. This little piece of coal was probably a bit
of wood carried along and buried with the sand in an ancient
steam.
Geology
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Updated 3/18/2015
Mammal’s Den: Metamorphic Rocks
Metamorphic rocks form from the alteration of preexisting rock by high pressure and temperature. The
chemical composition of the rock remains essentially
the same, but the atoms are rearranged into different
mineral structures.
An example is carbon, which is changed by very high
heat and pressure from the mineral graphite (pencil
lead) into diamond.
High temperatures and pressures result from the deep
burial of rocks over geologic time. This fireplace
features a sequence of rocks representing a section
into the earth, with pressure and temperature
increasing downward.
From top to bottom there is:
Geology

Shale is a sedimentary rock formed from clay,
silt, or mud

Shale turns to slate if buried about 2 miles deep and
o
heated to about 500 C.

Mica Schist, like slate, is formed from clay-rich sedimentary rocks, but at
o
much higher pressures – many miles of burial and about 600 C. The bright
sheen of this rock is due to the shinny mica grains that have been aligned by
pressure.

Quartzite. The shelves are quartzite, which is formed from quartz-rich
sedimentary rocks over a wide range of high pressures and temperatures.

Gneiss forms many miles below the earth’s surface at temperatures nearly hot
enough to melt the rock. It is coarse-grained with irregular light and dark
bands reflecting fluid-like motion. (Note that there are other rock types, in
addition to gneiss, in the lower part of the fireplace.)
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Invertebrate Inn: Igneous Rocks
Igneous rocks are formed by the cooling of
hot molten material, called magma when it
intrudes below the surface of the Earth and
lava when it extrudes out across the Earth’s
surface.
Igneous rocks are classified according to grain
size and composition. The fireplace rocks are
all intrusive, and are coarse grained because
crystals had more time to grow in magma that
cooled slowly deep below the Earth’s surface.
Extrusive rocks are fine grained – grains too
small to see with the naked eye. The chemical
composition varies from rocks rich in silica
(SiO2) to those with a higher proportion of
other elements, typically manganese and iron.
Silica-rich rocks tend to be lighter in color.
Granite
Diorite
Gabbro
Intrusive
Granite
Extrusive
Rhyolite
Silica
>69%
Granodiorite
Dacite
62-69%
Diorite
Andesite
54-62%
Gabbro
Basalt
45-54%
When a lava flow cools enough to harden, it is still quite hot (~1000o C). With
further cooling, it contracts and breaks, often forming hexagonal columns. Three
pieces of basalt columns from Central Washington are arranged in front of the
fireplace.
Geology
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