Why is gneiss banded?
The banding is usually due to the presence of differing proportions of minerals in the various bands; dark and light bands may alternate because of the separation of mafic (dark) and felsic (light) minerals. Banding can also be caused by differing grain sizes of the same minerals.
What is gneissic texture?
The “gneissic texture” refers to the segregation of light and dark minerals. It is indicative of high-grade metamorphism where the temperature is high enough, say 600-700 °C, so that enough ion migration occurs to segregate the minerals. Gneiss often forms from the metamorphism of granite or diorite.
What causes banding in rocks?
Flow banding is caused by friction of the viscous magma that is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or, if the magma is erupted, the surface of the earth across which the lava is flowing.
How is gneiss formed?
Gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist. It is formed by the metamorphosis of granite, or sedimentary rock. Gneiss displays distinct foliation, representing alternating layers composed of different minerals.
What rock type is Obsidian?
igneous rock
Rondi: Everyone, meet Obsidian , an igneous rock that from melted rock, or magma. Obsidian is an “extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an “intrusive” rock.
What type of rock has wavy banding layers?
Gneiss
Foliated (Banded) Metamorphic Rocks
| Characteristics | Minerals | Rock Name |
|---|---|---|
| Thick bands, wavy, semi-continuous layers of white quartz, feldspar, and mica Medium to coarse-grained Banded, coarsely crystalline Large, crystalline grains | Feldspar Quartz Mica Hornblende Garnet | Gneiss |
What type of rock is diorite?
Diorite
| Type | Igneous Rock |
|---|---|
| Texture | Phaneritic (Coarse-grained) |
| Origin | Intrusive/Plutonic |
| Chemical Composition | Intermediate |
| Color | Approximately half dark, half white minerals |
What is the difference between foliation and banding?
This parallel alignment causes the rock to split easily into thin layers or sheets. Foliation is common in aphanitic as well as phaneritic metamorphic rocks. Banding means that the rock consists of alternating, thin layers (typically 1 mm to 1 cm) of two different mineral compositions.
What does the word banding mean?
1. A thin strip of flexible material used to encircle and bind one object or to hold a number of objects together: a metal band around the bale of cotton. 2. A strip or stripe that contrasts with something else in color, texture, or material.
What rock turns into gneiss?
Gneiss is formed from another metamorphic rock called schist, which itself started out life as a sedimentary rock called shale. To form a gneiss you need to subject the original rock to very great pressures and allow time for new large crystals to grow slowly.
What makes a gneiss different from other rock bands?
Other bands contain platy or elongated minerals that show a preferred orientation that parallels the overall banding in the rock. It is this banded appearance and texture – rather than composition – that define a gneiss.
What are the uses of gneiss metamorphic rock?
Uses of Gneiss Gneiss usually does not split along planes of weakness like most other metamorphic rocks. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects. Some gneiss is durable enough to perform well as a dimension stone.
Which is formed at higher temperatures schist or gneiss?
Gneiss forms at higher temperatures and pressures than does schist. Gneiss is typically foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called gneissic banding.
Where does the word gneiss come from in English?
The word gneiss has been used in English since at least 1757. It is borrowed from the German word Gneis, formerly also spelled Gneiss, which is probably derived from the Middle High German noun gneist “spark” (so called because the rock glitters).