METAMORPHIC ROCKS

CLASSIFICATION OF ROCKS

Classification of Rocks

Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary.

Igneous rocks crystallize from magma. Metamorphic rocks form by the deformation and/or

recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry.

Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock.

ROCK CYCLE

METAMORPHIC ROCKS IN THE ROCK CYCLE

Metamorphic rocks are formed where a parent rock is subjected to changes in pressure, temperature or chemistry (such as addition of fluids).

The rock cycle picture shows several areas where metamorphism is common. In subduction zones: as the oceanic plate descends into the mantle, both the sediments and the basalt floor are subjected to high pressure and low to high temperature conditions. Fluids may play an important role chemically change the rocks' composition.

Adjacent igneous intrusions - contact metamorphism, where cooler (country) rocks are altered by contact with a hot igneous intrusion, is another common type of metamophism. This type of metamorphism commonly produces a

At spreading centers (mid-ocean ridge): Fluids play an important role in hydrothermal alteration associated with magma emplacement on the sea

floor at mid-ocean ridges.

Metamorphism

Metamorphic rocks: form by recrystallization of either igneous or

sedimentary rocks. This happens when the temperature, pressure or fluid environment change and a rock changes its form (e.g. limestone turns to marble).

The range of temperatures for metamophism is 150C up to the melting temperature.

The type of rock formed is controlled by the parent rock and the pressure/temperature conditions.

Metamorphism causes growth of new minerals, rotation or deformation of mineral grains, and recrystallization of minerals.

Metamorphism

This whole mountainside shows rocks that have been bent or warped by heat and pressure.

Use your imagination, thinking of rocks like playdough, and figure out what happened to bend these rocks.

Metamorphic Pairs and Processes

Gneiss is Metamorphosed Granite

Granite is the rock to the left. It has no patternto the distribution of minerals. In the two rocksabove, there are parallel lines or bands that showthat heat and pressure have metamorphosed thegranite.

Metamorphism Results from Pressure

This rock was conglomerate. What happened to it?

In what direction was it oriented?

Tell the story of this rock.

Any Igneous or Sedimentary Rock can be Metamorphised

Quartzite is metamorphised sandstone. Remember, Quartz is the chief mineral in sandstone.

Marble is metamorphised limestone. Streaks are usually the result of impurities, or even fossils in the limestone.

Unpolished Marble

Quartzite

Marble

At the top right is unpolishedwhite marble. When polishedlike the other two, it showslines and stripes from stretched out fossils and impurities.

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