1.5 - Metamorphic Rocks
Metamorphic rocks are recycled rocks that have been subjected to varying degrees of pressure and temperature. As a result of all this extreme heating and pressure, old minerals become unstable and new ones begin to form. In many cases, the resulting rocks display a preferred orientation of minerals, as new minerals form perpendicular to the maximum pressure exerted on the rock. Some important metamorphic rocks types are gneiss ('nice'), schist ('shist'), and slate. Metamorphic rocks are fairly uncommon at the earth’s surface so they usually do not contribute greatly to forming soils. Metamorphic rocks also weather slowly because of their hardness.
Distinguishing Features: Completely crystallized; crystals visible with the naked eye, but anisotropic pressure during crystallization leads to a preferred orientation of many newly formed minerals, which can lead to distinctive textures as shown above.
The two rock specimens shown below have similar mineral composition, but very different texture.
Main Minerals: Often similar to granite (feldspars, olivine, amphibole, pyroxenes), but notice the preferred orientation of the black biotite crystals in the specimen above.
Weathering Behavior: Gneiss generally weathers rather slowly, though some minerals might be affected by chemical weathering.
Impact on Soils: Since the mineral composition is often similar to granite and weathering rates are slow, gneiss tends to lead to acidic, poorly developed soils.
How it Forms: Gneiss is a metamorphic rock, which forms through recrystallization of pre-existing rocks under high temperature and pressure (high grade metamorphism). During metamorphosis the rock remains completely solid, and pressure is often anisotropic, which leads to preferred orientation of newly formed minerals.
Distinguishing Features: Often has a distinct foliation fabric (planar appearance), due to preferred alignment of newly formed minerals, such as muscovite. Novices sometimes confuse foliation with sedimentary layering, which is caused by changes in sediment particle size distribution and mineral composition. The images below show a specimen of schist displaying foliation, if the rock is polished and cut into a thin section, 30 mm thick, it is possible to observe the individual mineral grains.
The image below right shows that foliation is caused by stretched-out minerals. The specimen in the image above displays shiny, wrinkled surfaces due to the preferred alignment of muscovite crystals.
Main Minerals: Rather variable, depending on the initial mineral composition of the un-metamorphized rock.
Weathering Behavior: Many of the metamorphic minerals are affected by chemical weathering, crystal sizes can be very small, causing rapid weathering.
Impact on Soils: Depends on mineral composition.
How it Forms: Schist is a metamorphic rock, which forms through recrystallization of pre-existing rocks under fairly high temperature and pressure (medium grade metamorphism). During metamorphosis the rock remains completely solid, and pressure is often anisotropic, which leads to preferred orientation of newly formed minerals.
Distinguishing Features: Very fine grained rock, often easily split into thin plates, surfaces often shiny due to formation of preferentially-oriented metamorphic minerals (see schist). The boundary between slate and shale may be poorly defined as slate is the weakly metamorphosed equivalent of shale.
Main Minerals: clay minerals, quartz, all very small
Weathering Behavior: Small particle size favors chemical weathering, well developed cleavage planes are susceptible to physical weathering (e.g., due to frost wedging).
Impact on Soils: Depends on mineral composition, but fine grained nature of the rock can lead to high clay soils.
How it Forms: Slate is a fine grained metamorphic rock that has undergone relatively little change in mineral composition under moderately elevated temperature and pressure conditions. The original rock is usually sedimentary shale, and metamorphosis progressed just far enough to fuse the sedimentary particles together. This makes slate a much more durable rock than shale, which tends to crumble away easily. Since metamorphosis has not progressed very far, many of the original sedimentary features, such as bedding planes, are still visible, and slates are easily split into thin tablets. This makes slate an excellent (though expensive) material for roof shingles, chalkboards and pool tables.
Distinguishing Features: Fine-grained, white when pure, but often colored or swirled from impurities. Will react (fizz) when treated with acid, but reaction may be too slow to observe unless the marble is powdered, for example by scratching with a knife blade.
Main Minerals: calcite, dolomite
Weathering Behavior: Very slow to weather because of its hardness.
Impact on Soils: Will resist acidification similarly to limestone because of the calcium carbonate that dominates its composition.
How it Forms: Marble is a metamorphosed limestone. The heat and pressure of metamorphosis has resulted in the recrystallization of calcite and other carbonates in the parent limestone.