Metamorphic rocks

DATHAN C S

FormationPrimary/secondary rocks

Earth,s movement/ High temp and pressure

Partially/ wholly reconstituted/altered

Metamorphic rocks

Kinds of Metamorphism1) Thermal metamorphism (Contact metamorphism)

• Heat is dominant factor due to – Increase in depth– Intrusion of magma

A contact metamorphic rock made of interlayered calcite and serpentine from the Precambrian age

2) Dynamo thermal metamorphism(Regional metamorphism)

•Combination of Heat and pressure•More or less complete re-crystallization with new structures

Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah

Classification based on texture and structure of minerals

1. Foliated (Parellal structure)Rocks that contain mica and ferro magnesium minerals. Minerals flattened and arranged in parellal layers.

Ex:- Gneiss, Schist, Slate

2. Non foliated (Massive structure)Rocks that contain Quartz and feldspars.

Ex:- Anthracite, talc-schist

3. GranularRocks comprising mostly of equi-dimentional grains

Ex:- Quartzite, marble

Important metamorphic rocks

1. Gneiss

Typical folded strata in gneiss. Gneiss is a metamorphic rock, formed at high temps and pressures deep within earth’s crust. The high pressure produce the folds in the banding seen here. The lighter colored layers are formed by Quartz and feldspar and muscovite, biotite and hornblende are commonly abundant in the darker bands.

This sample, classified as gneiss, was identified as having come from Wyoming, from the North Laramie Mountains. One part of it was associated with an age of 2.6 billion years, and another part as 1.8 billion years.

This sample of gneiss rock shows layering of two different kinds of rock, leading to its classification as a metamorphic rock. The layering also shows bending, suggesting that it has been subject to pressures.This rock was included in a display of geologic ages at the Smithsonian Museum of Natural History. The suggested age was 2 billion years

2. SchistAn intermediate- to high-grade, foliated metamorphic rock. It

is highly variable in appearance, depending on the mineral content, which is a function of the precursor rock and specific temperature-pressure conditions. Schists typically have medium- to large-sized crystals, unlike the microcrystalline nature of slate & phyllite.

Graphite schist

Muscovite schist Biotite schist Garnet schist

Cholrite schist

Talc schist

3. Slate

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring and flagging because of its durability and attractive appearance.

4. Quartzite

Hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression

5. Marble

A non foliated crystalline rock, composed of calcite and dolomite

Pre-existing rocks and their equivalent metamorphosed rocks

Conglomerate (Sedimentary rock)

Gneiss

Pre-existing rocks and their equivalent metamorphosed rocks

Basalt (Igneous rock)

Schists

Sand stone(sedimentary rock)

Quartz-schist

Quartzite

Quartzite

Clayey Sandstone(sedimentary rock)

Mica-schist

slateShale

Mica-schistPhyllite

(sedimentary rock)

iron ores Haematite-- schist

Gabbro

Gabbro (Igneous rock)

Gneiss

Cholrite schist

Coal Graphite

Metamorphic rock textures• There are five basic metamorphic textures with typical rock types

Basic rocktexture

Metamorphic rocks Name of foliation

Slaty Slate and phyllite; slaty cleavage

schistose schist schistosity

gneissose gneiss gneissosity

granoblastic marbles and quartzite ----

hornfelsic hornfels and skarn ----

Some examples…..

Rocks metamorphosed at the deepest levels of the Earth's crust are brought up to the surface when mountain ranges are uplifted and eroded. The banded rocks in the cliffs beyond the mountain hut were originally lavas and sediments formed in the sea.

Metamorphic rocks can be formed from any type of older material. Here, the lighter brown rocks in the upper part of the cliff are calcareous schists formed from impure limestones, and the dark rocks below are mainly metamorphosed basalts. As Sifah, Oman.

Here in the Austrian Alps, the mountains in this view are built of granites, gneisses and schists formed in the Carboniferous and Permian periods, then metamorphosed again in the Tertiary period when the Alps were formed, and finally uplifted to the surface over the past 20 to 30 million years.

Huge thicknesses of sandstone, siltstone and mudstone, deposited in ocean basins, are folded, squeezed and metamorphosed into schists when continents collide. Now exposed in the Kuiseb Canyon, Namib Desert Park, Namibia.

This photograph shows the streaky banded texture typical of many gneisses. The light streaks are feldspar and quartz, the dark bands are mica and hornblende. Namaqualand, South Africa.

Pure marble is often white, but the presence of other mineral impurities gives coloured varieties. These marbles have bands and veins coloured yellow-green and grey-black by serpentine, mica and iron oxide. In this quarry the marble was being cut into large slabs for ornamental use. Ledmore, Assynt, NW Scotland.

Metamorphism causes new minerals to grow in a rock that was originally formed as a sediment. Here you can see rounded red garnet and a bladed grey mineral (kyanite) set in dark mica. Namaqualand, South Africa.

This is a metamorphosed basic igneous rock in which you can see large red garnet crystals, outlined by a rim of pale feldspar, set in dark pyroxene. Scourie, Sutherland, NW Scotland.

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