|
Morton Gneiss Stop #2 Tim Donahue and John Woodside ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Intro Named for the location of outcrops primarily found near Morton, Minnesota, the gneiss is a hybrid rock formed by the strewing out of basic inclusions in a granitic magma. It can be characterized by a highly contorted structure and displays a great variety of textures and colors (GSA, 46). Visible banding is present in gneiss and is usually due to the presence of differing proportions of minerals, but may also be the result of differing grain sizes of the same minerals. Gneiss is the result of the complex interaction of an original rock composition, pressure and temperature of metamorphism, and the addition or loss of chemical components. Location We visited the Morton Gneiss rock quarry in Morton, MN. The rock quarry is located near US Route 71 and 19. Located on the side of US19W is a hotel and a gas station, the quarry can be found over the hill behind the buildings. This area is the Northwest corner of Morton, MN. Formation Three principal rock types recognized: -Basic or Mafic inclusion -Gray Tonalitic Gneiss -Pink granitic gneiss ranging from quartz monzonite to leucogranite.
Figure 2A
Composition (Essential minerals) Quartz Micas (Also present) Feldspar Amphibole Physical Properties of Gneiss Colors Pink, Black, Silver, White Texture The Morton gneiss has a semischistose texture. Most parts of the rock are granoblastic, but separate folia or elongate groups of minerals impart a crude cleavage and a semischistose aspect. (Compton Hardness 6.5-8 Weathering Pattern
Figure 2B Located at this particular site is a quarry, so obviously the greatest alteration to the surface of the site is the result of human economic activity. The top of the outcrop, however, has large areas of mostly unaltered exposures. The surface of this is exposure is polished and smooth. The large scratches etched permanently into the surface suggest glacial movement over this outcrop. The Morton gneiss is known for being very durable both physically and chemically, so it is unlikely that surface winds could have done this, especially since the scratches are so deep. A large glacial lake existed during the last great glacial retreat. The resulting deluge from its release could easily have provided enough energy to erode the surface. The patterns of the striations are more consistent with glacial activity. History The Morton Gneiss formed in a period known as the Archean Era, which took place approximately 4-2.5 billion years ago. The Earth’s crust at this time had finally cooled enough for rocks and continental plates to begin formation. The rocks that formed during this time period were highly altered through subsequent metamorphic process, which is why the Morton Gneiss appears the way it does. There are two major periods of deformation recognized within the Morton Gneiss. The 1st recognized deformation occurred approximately 2.6 billion years ago involved the amphibolites and the tonalitic gneisses and was developed prior to the granitic phases. The second major deformation occurred approximately 1.85 billion years ago and was responsible for the highly twisted and contorted structure of the Morton Gneiss. Intrusive Bodies in Morton Gneiss Exposed aplite dike Aplite dikes are generally straight, but some may be offset due to faulting in the rock in which the dike cuts through. The aplite dike the WSU students recognized ran in a regular, straight fashion, and was roughly one foot in laterally exposed thickness. These dikes represent the last Archean igneous activity in this geologic area. Exposed Pegmatite Intrusion Pegmatite intrusions consist of very coarse-grained rock, having a grain size of 3 cm or larger, with crystals occasionally reaching a meter or so in length. In some areas, crystals greater than 10 meters have been recorded, but these are very rare. What we (WSU) did at this site
Figure 2C The quarry at which Winona State University observed the Morton gneiss was a very convenient site. The human economic activity at the quarry resulted in large sections of rock being separated and exposed from its natural position. Despite the environmental concerns, the quarry allows us to see both the naturally weathered surface and observe a fresh surface. We gathered samples from piles of freshly broken pieces for our rock record, as well as gauging the basic composition of this specific outcrop of the Morton Gneiss. We experienced the durability and strength of this rock firsthand as we tried to sample the formation. After initial, close up observation, we observed the structure of the rock as a whole. We observed the pattern of minerals and fold, and discussed how the joints interacted with the formation and deformation of this outcrop. We then climbed to the top to observe the weathering patterns, as well as the visible aplite dike. Commercial Uses
Figure 2D The Morton gneiss is a highly deformed rock formation. Not only does 3.8 billion years of partial melt and deformation make it a visually pleasing rock, but through time it developed into a strong and durable rock, thus making it appealing to the construction industry. The strength and aesthetic properties make it useful for ornamental exterior projects. The stone is also useful indoors for countertops and floors, but the weight of the stone and the fact that stone is not popular in homes limits its interior usefulness. The chemical properties of gneiss make it an economic aggregate as well. The minerals contained in the Morton gneiss are not very susceptible to chemical weathering; for example quartz, a significant component of the Morton gneiss, is one of the most stable minerals. References: 1.-http://www.gov.nl.ca/mines&en/geosurvey/dimension/wkshop.pdf 2.- http://www.geocities.com/RainForest/Canopy/1080/gneiss.htm 3- S.S. Goldich, JL Wooden, GA Ankenbauer, TM Levy, and RU Suda; Origin of the Morton Gneiss, southwestern Minnesota: Part 1, Lithology Geological Society of America 1980 4- R.R. Compton; Geology in the Field John Wiley & Sons, Inc., 1985 5- D.G.A. Whitten, J.R.V Brooks The Penguin Dictionary of Geology Hazel Watson & Viney Ltd, 1975 6- Figures 2A, 2B, 2C, 2D used with permission and taken by Dr. Toby Dogwiler, PhD.
|
