Metamorphism means, “change in form” and metamorphic rocks result after existing rock forms are transformed. The original rock is subjected to heat and pressure, which causes profound physical and/or chemical change. Metamorphism is the alteration of a parent rock into a metamorphic rock. Metamorphism happens when the protolith is subjected to heat and pressure because it is deep beneath the Earth’s surface.
(above) The North Cascades reveal many forms of metamorphic rocks. From skyline left to near right is Eldorado Peak composed of orthogneiss, Cascade Peak composed of schist, and Mixup Mt. composed of gneiss.
Parent Rock. The protolithic or original rock may be sedimentary rock or igneous rock or an older metamorphic rock. However, most metamorphic rocks come from three sedimentary parent rocks: Sandstone, Limestone, and Shale.
- Quartz Sandstone yields to metamorphic Quartzite
- Calcite Limestone yields to metamorphic Marble.
- Mineral clay Shale yields to metamorphic Slate, Phyllite, Schist, and Gneiss.
(above right) Detrital/Clastic Quartz Sandstone, parent rock of quartzite, originates on the continental shelf or near shore on beaches, islands, lakes, stream channels or dunes.
Changes. Pressure and temperature cause mineral, texture and rock changes.
- Mineral changes clay to chlorite to quartz, feldspar and mica.
- Texture changes bedding to slaty cleavage to schistosity to mineral banding.
- Rock changes Shale to Slate to Phyllite to Schist to Gneiss.
(above) Metamorphism changed the mineral, chlorite, into this crystal druse of transparent quartz crystals found in Hot Springs, AR.
Three Methods of Formation: Three major classes of metamorphic rock are based upon the formation mechanism: Contact, Barrovian, and Regional.
Heat. Contact Metamorphism happens when the protolith is subjected to heat from the intrusion of magma or hot molten rock from the Earth’s interior. The metamorphic result has a granular or hornfeisic texture. For example heat will change the mineral clay texture of shale into Hornfels with a granular texture.
(left) Once shale, until altered by heat, this hornfels has a granular texture. Hornfels have granular, platy or elongate crystals that are randomly oriented so no foliation is evident.
Pressure. Barrovian (Burial) Metamorphism happens when the protolith is subjected to pressure. The metamorphic result has a banded texture. For example pressure will change the mineral, chlorite, into the minerals of quartz, feldspar and mica.
(right) Once chlorite, until altered by pressure into feldspar, this Labradorite, part of the feldspars group of minerals, can produce a colorful play of light across cleavage planes.
Mountain-building. Regional or Dynamic Metamorphism happens when the protolith is subjected to temperature and pressure from tectonic processes. Typically in mountain-building regions the protolith is subjected to horizontal pressure, friction and distortion. The metamorphic result can have a slaty texture (Slate, Phyllite), a schistose texture (Schist), a gneissose texture (Gneiss), or a granoblastic texture (Granulite).
(left) The granoblastic texture in this basic granulite reveals slight grain flattening fabric and was created by tectonic horizontal pressure, friction and distortion.
Two Classifications of Texture: Metamorphic rocks can be classified by grain size and the degree of foliation. The two distinctive metamorphic textures are Foliation and Granular (non-foliated). Foliated means the minerals are layers. Non-foliated means the grains are equidimensional. Well-developed foliation is characteristic of most metamorphic rocks. Metamorphic rocks often break easily along foliation planes (the planes of layers).
Granular Texture. Granular rocks are usually composed of one mineral. For example quartz forms Quartzite and calcite forms Marble. Granular describes a metamorphic rock consisting of interlocking crystals or granules, that are almost entirely composed of one mineral. A granular texture is developed if a rock’s chemical composition is close to that of a particular mineral. This mineral will crystallize if the rock is subjected to high pressure and temperature. A granular texture is characteristic of some metamorphic rocks. Non-foliated textures include Marble, Soapstone, and Serpentine, as well as Quartzite and Hornfels.
(above left) This soapstone has a granulated, non-foliated crystalline texture and is composed of one mineral. It is a talc rich metamorphic rocks. Pure talcose soapstone is called steatite.
Foliation Texture. Foliation represents a distinct plane of weakness in the rock. Foliation is caused by the re-alignment of minerals when they are subjected to high pressure and temperature. Foliation occurs during the recrystallization process when planar patterns of strain form layers that reveal bands showing the colors of the minerals that formed within. Varieties of foliated textures are Schist, Gneiss, Slate, Phyllite and Mylonite.
(above right) Altered from slate, this phyllite shows bands of color from various minerals. It has fine-grained mica flakes in a preferred orientation whereas slate has extremely fine clay flakes that achieve a preferred orientation.
Three Foliation Layers: With foliation, new metamorphic minerals line up to produce distinct layering in the rock with three different looks: slaty cleavage (Slate), schistosity (Schist), and mineral banding (Gneiss).
(right) Altered from shale, this slate reveals a type of foliation known as “slaty cleavage”. Note the grains of mica, quartz, and ilmenite aligned with a preferred orientation.
Slaty Cleavage has clay minerals converted to chlorite that line up.- Schistocity has mica, amphibole, quartz, and feldspar completely intermixed.
- Mineral banding has bands of light colored quartz and feldspar.
(right) Altered from mudstone/shale into a higher degree of crystallization than slate, this schist example reveals the characteristic “scaly” schistose texture, caused by platy micas.
(right) Altered from sedimentary of felsic igneous rocks, and subjected to higher temperatures and pressures than schist, this gneiss example is foliated and displays mineral banding. The foliations are characterized by alternating darker and lighter colored sub-parallel bands, called “gneissic banding”.
Eight Common Metamorphic Rocks:
- Quartzite is coarse-grained and derives from sandstone, a clastic sedimentary rock.
- Marble has a uniform texture and derives from limestone or dolomite, both chemical sedimentary rocks.
- Slate is fine-grained and derives from shale, a clastic sedimentary rock.
- Phyllite is fine-grained and derives from Slate, a metamorphic rock.
- Schist is fine-grained and derives from Phyllite, a metamorphic rock.
- Gneiss is medium to coarse-grained and derives from Schist, a metamorphic rock.
- Mylonite is fine-grained and derives from complete pulverization of the parent rock.
Hornfels is fine-grained and derives from clay-rich rock altered by high heat.
(right) Mylonite, one of 8 common metamorphic rocks, has been put through the mill. It is formed during intense shearing encountered during folding and faulting, which nearly pulverizes the parent rock. It is fine grained, foliated and very hard.
Six Common Metamorphic Building Materials. For the purposes of masonry, the most commonly used building materials for building facings and paving are marble, quartzite, and gneiss. Schist can be used for paving. Slate and phyllite can be used for flooring and floor tiles.