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 Page 1 of 5
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 sandstone and shale basalt 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 Page 2 of 5
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 Page 3 of 5
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: Shale is a sedimentary rock formed from clay, silt, or mud Shale turns to slate if buried about 2 miles deep and heated to about 500 o C. Mica Schist, like slate, is formed from clay-rich sedimentary rocks, but at much higher pressures many miles of burial and about 600 o 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.) Geology Page 4 of 5
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 (SiO 2 ) 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 Extrusive Silica Granite Rhyolite >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 (~1000 o 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 Page 5 of 5