Gneiss is an example of a metamorphic rock. Gneiss rock is the metamorphic version of granite. Gneiss is a high-grade foliated metamorphic rock. Granite is an igneous rock and gneiss is a metamorphic rock. GneissGranite gneiss is a metamorphic rock formed from granite. Gneiss, Schist, Quartzite. Gneiss is a banded foliated metamorphic rock. Gneiss is a metamorphic rock and its parent rock protolith could be a granite or schist.
Slate and gneiss are examples of metamorphic rock. Gneiss is a coarse grained foliated metamorphic rock. Log in. Metamorphic Rock. See Answer. Best Answer. Study guides. Q: What was gneiss before it was a metamorphic rock? Write your answer Related questions. Is gneiss an igneous rock or metamorphic rock?
What is gneiss rock? Is gneiss a metamorphic rock? Is Gneiss a sedimentary metamorphic or igneous rock? Is gneiss a igneous metamorphic or sedimentary rock? During the last few million years, it was raised and mixed into younger subducted rocks of the Franciscan complex. The body of eclogite is no more than meters across today. Gneiss "nice" is a rock of great variety with large mineral grains arranged in wide bands.
It means a type of rock texture, not a composition. This type of metamorphic was created by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. Nearly all traces of the original structures including fossils and fabric such as layering and ripple marks are wiped out as the minerals migrate and recrystallize.
The streaks contain minerals, like hornblende, that don't occur in sedimentary rocks. In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. You can see that unlike schist, which is more strongly aligned, gneiss doesn't fracture along the planes of the mineral streaks.
Thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. With still more metamorphism, gneisses can turn to migmatite and then totally recrystallize into granite. Despite its highly altered nature, gneiss can preserve chemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest Earth rocks known are gneisses from Acasta, in northern Canada, that are more than 4 billion years old.
Gneiss makes up the largest part of the Earth's lower crust. Pretty much everywhere on the continents, you will drill straight down and eventually strike gneiss. In German, the word means bright or sparkling. Greenschist forms by regional metamorphism under conditions of high pressure and fairly low temperature. It isn't always green or even a schist. Greenschist is the name of a metamorphic facies , a set of typical minerals that form under specific conditions—in this case relatively cool temperatures at high pressures.
These conditions are less than those of blueschist. Chlorite, epidote, actinolite, and serpentine the green minerals that give this facies its name , but whether they appear in any given greenschist-facies rock depends on what the rock originally was.
This greenschist specimen is from northern California, where seafloor sediment has been subducted beneath the North American plate, then thrust to the surface soon afterward as tectonic conditions changed. This specimen consists mostly of actinolite. The vaguely defined veins running vertically in this image may reflect the original bedding in the rocks from which it formed. These veins contain mainly biotite. Greenstone is a tough, dark altered basaltic rock that once was solid deep-sea lava.
It belongs to the greenschist regional metamorphic facies. In greenstone, the olivine and peridotite that made up the fresh basalt have been metamorphosed by high pressure and warm fluids into green minerals—epidote, actinolite or chlorite depending on the exact conditions. The white mineral is aragonite , an alternative crystal form of calcium carbonate its other form is calcite.
Rock of this kind is manufactured in subduction zones and is seldom brought to the surface unchanged. The dynamics of the Californian coastal region make it one such place. Greenstone belts are very common in Earth's oldest rocks, of Archean age. Exactly what they mean is still not settled, but they may not represent the kind of crustal rocks that we know today.
Hornfels is a tough, fine-grained rock that is made by contact metamorphism where magma bakes and recrystallizes the surrounding rocks. Note how it breaks across the original bedding. Marble is made by regional metamorphism of limestone or dolomite rock, causing their microscopic grains to combine into larger crystals.
This type of metamorphic rock consists of recrystallized calcite in limestone or dolomite in dolomite rock. In this hand specimen of Vermont marble, the crystals are small.
For fine marble of the sort used in buildings and sculpture, the crystals are even smaller. The color of marble can range from the purest white to black, ranging through the warmer colors in between depending on the other mineral impurities. Like other metamorphic rocks, marble has no fossils and any layering that appears in it probably does not correspond to the original bedding of the precursor limestone.
Like limestone, marble tends to dissolve in acidic fluids. It is quite durable in dry climates, as in the Mediterranean countries where ancient marble structures survive. Commercial stone dealers use different rules than geologists to distinguish limestone from marble.
Migmatite is the same material as gneiss but brought close to melting by regional metamorphism so that the veins and layers of minerals became warped and mixed. This type of metamorphic rock has been buried very deep and squeezed very hard. In many cases, the darker part of the rock consisting of biotite mica and hornblende has been intruded by veins of lighter rock consisting of quartz and feldspar.
With its curling light and dark veins, migmatite can be very picturesque. Yet even with this extreme degree of metamorphism, the minerals are arranged in layers and the rock is clearly classified as metamorphic.
If mixing is even stronger than this, a migmatite can be hard to distinguish from granite. Because it isn't clear that true melting is involved, even at this degree of metamorphism, geologists use the word anatexis loss of texture instead. Mylonite forms along deeply buried fault surface by crushing and stretching of rocks under such heat and pressure that the minerals deform in a plastic way monetization.
Phyllite is one step beyond slate in the chain of regional metamorphism. Unlike slate, phyllite has a definite sheen. Gneiss is an old German word meaning bright or sparkling. Other specimens - Click the thumbnails to enlarge. Introduction Features Igneous rocks Sedimentary rocks Metamorphic rocks.
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