Mammal's Den Fireplace
Metamorphic rocks form from the alteration of pre-existing 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 oC.
• Mica Schist, like slate, is formed from clay-rich sedimentary rocks, but at much higher pressures – many miles of burial and about 600 oC. 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.)