Metamorphic Rocks Sometimes the temperature or pressure becomes high enough to alter rock but not high enough to melt it back into magma. The result is a metamorphic rock. When high temperature and pressure combine and change the texture, mineral composition, or chemical composition of a rock without melting it, a metamorphic rock forms. During metamorphism, a rock remains solid while it changes form.

How are the high temperatures achieved? High temperatures are ultimately derived from the Earth’s internal heat. Deep burial Nearby igneous intrusions

How are the high pressures achieved? Deep burial Compression as a result of mountain building

Metamorphic Minerals Minerals in a rock change into new minerals that are more stable under the new temperature and pressure conditions. These mineral changes are called solid-state alterations.

Metamorphic Textures There are two metamorphic texture groups Fossils are rarely preserved in metamorphic rocks. Foliated Rocks These are metamorphic rocks where there are clear layers and bands of minerals. The high pressure during metamorphism causes minerals with flat or needle-like crystals to form with their long axes perpendicular to the applied pressure. Examples include: slate, phyllite, schist, gneiss

Metamorphic Textures Nonfoliated Rocks These are composed mainly of minerals that form with blocky crystal shapes. Examples include quartzite (metamorphism of sandstone) and marble (metamorphism of limestone)

Porphyroblasts These are large metamorphic mineral crystals that do not form from magma but rather from solid rock (garnet is an example)

Grades of Metamorphism Different grades of metamorphism result from varying amounts of temperature and pressure. Low grade – lower temperature and pressure. High grade – higher temperature and pressure. Depending on the type of metamorphism, different mineral types will form. This is helpful to scientists because it can help to explain the metamorphic history of the area.

Types of Metamorphism The minerals that form and the degree of change in the rocks provide information as to the type and grade of metamorphism that occurred.

Regional Metamorphism When high temperature and pressure affect large regions of the Earth’s crust, belts of regional metamorphism are created. Metamorphism can range from low grade to high grade Folding and deforming of the rock area will likely occur and the minerals and rock types will change.

Contact Metamorphism This occurs when molten material (ex: an igneous intrusion) comes into contact with solid rock. This results in high temperatures and moderate to low pressure. Temperature decreases with distance from the intrusion and therefore metamorphic effects also decrease.

Hydrothermal Metamorphism This occurs when very hot water reacts with rock and alters its chemical and mineral composition. As hot fluids migrate in and out of the rock during metamorphism, the original composition and texture of the rock can change. Valuable deposits of gold, copper, zinc, tungsten, and lead are formed in this manner.

Economic Importance of Rocks and Minerals Many economic mineral resources are produced as a result of metamorphic processes.

Metallic Mineral Resources Metallic resources are usually found in the form of metal ores although deposits of pure metals are occasionally discovered. Many metallic deposits are precipitated from hydrothermal solutions. Native gold, silver, and copper deposits tend to occur in hydrothermal quartz veins near igneous intrusions or in contact metamorphic zones. Most hydrothermal metal deposits are in the form of metal sulfides like galena (PbS) or Pyrite (FeS 2 ).

Nonmetallic Mineral Resources Metamorphism of ultrabasic igneous rocks produces talc and asbestos. Talc is used as a dusting powder and lubricant. Asbestos has been used for fireproofing and insulation but it has been shown to be a carcinogen.