Rainbow Gneiss EarthCache
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You will be looking at a road cut through an outcropping of Gneiss in the town of Morton, at the junction of Highways 19 & 71.
Logging Requirements:
1. The rock pictured below is at the exposed outcrop. What do you think caused the additional coloring on the surface of this rock?
2. Rub your hand along the gneiss and describe its texture. What does this tell you about the size of the crystals in the rock?
Rainbow Gneiss, also known as Morton Gneiss, is a coursely crystalline, foliated metamorphic rock from the Archean age, 3.5 Billion years old. It comes from the Rainbow Quarry in Morton, Minnesota. The primary minerals present in this rock are quartz(white specks), potassium feldspar(pink bands), plagioclase feldspar(gray bands) and biotite(black splotches). The texture and mineral assemblage of the Morton Gneiss indicates how the rock formed. The fact that it is a crystalline rock with large visible grains indicates that it originated as a granitic igneous rock that cooled slowly beneath the Earth's surface. The foliation, or alignment of the pink and gray mineral grains into bands, indicates that the original rock was subjected to great heat and pressure deep below the Earth's surface.
The Morton Gneiss has been actively quarried for use as a facing stone for buildings. It can be found adorning many buildings throughout the United States, including: the West Publishing building in St. Paul, the Banker's Life Insurance building in Des Moines, and the AIG building in New York, among others. It can also be found locally in Morton at the Liquor store on the main street.
Timeline:
- 3.5 Billion years ago the original layer of gray tonalite granite was created when tectonic movements, called the Mortonian event, caused a release of magma, that was comprised of quartz, mica, plagioclase feldpsar and small amounts of biotite, up to a level about 10 miles below the surface, where it could slowly cool and form into granite. At this point in time the rock was just a gray granite.
- 2.6 Billion years ago the rock underwent a high-grade metamorphism, due to an event called the Algoman orogeny, caused by a collision between the Minnesota River Valley terrane and the Superior province. The heat and pressure of this metamorphism caused the original gray granite to melt but also caused an intrusion of pink granitic magma. The pink granite and gray granite twisted and folded around each other like taffy creating the beautiful banded gneiss.
- 1.8 Billion years ago the gneiss underwent a low-grade metomorphism that reset the ages of the biotite in the rock. There were also some intrusions of gabbro, granite and basalt.
- 100 Million years ago there was a period of uplift that brought the bedrock close to the surface.
- 12,000 years ago the last glacier began receding and Lake Agassiz began draining into the River Warren. The river ran through this area and stripped away the topsoil and glacial debris to expose the Morton area gneiss outcroppings.
Sources:
The Age Of The Earth by G. B. Dalrymple
Earth Science Guy
Morton Gneiss Wiki
America's Volcanic Past
Geo Science World
Geochemical Journal
Stories In Stone by David B. William
U of W Rock Descriptions
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