Mike Searle on the Mountain Ranges of Central Asia

Glossary

Alkaline rocks: Igneous rocks rich in alkali metals (sodium and potassium) relative to silica. These rocks are characteristic of specific tectonic settings, such as rift zones or hotspots.

Amphibolite facies: A set of minerals in metamorphic rocks that indicate that they have undergone moderate to high pressure and temperature, where amphibole (usually hornblende) and plagioclase are stable. The minerals in a metamorphic facies are a result of the rock's chemical composition and the attainment of chemical equilibrium.

Andesitic volcanics: Volcanic rocks with an intermediate composition between basalt and rhyolite that typically erupt above subduction zones and are characterized by intermediate silica content.

Active plate margin: A tectonic boundary at convergent margins or shear or transform margins. In a convergent margin setting, the overriding plate forms an active plate boundary with volcanism. The “ring of fire” bordering the western Pacific constitute active plate margins.

Andalusite: An aluminum silicate mineral that forms under low-pressure, high-temperature metamorphic conditions; kyanite and sillimanite are polymorphs of andalusite.

Barrovian sequence of metamorphism: A classic progression of metamorphic mineral assemblages with increasing pressure and temperature, typically seen in regional metamorphism.

Batholith: A large, deep-seated, intrusive igneous body, often composed of granitic rock, that covers an area greater than 100 square kilometers.

Biotite: A common mica group mineral, typically dark brown or black, found in a variety of igneous and metamorphic rocks.

Channel flow: The movement of ductile material, such as partially melted crust, through a low-viscosity channel, typically within orogenic belts.

Chlorite: A green, sheet silicate mineral that forms under low-grade metamorphic conditions.

Coesite: A high-pressure polymorph of quartz; polymorphs have the same elemental composition but different crystal structures.

Competent rock: A strong rock, resistant to ductile deformation. Such rocks are brittle and tend to joint and fault under stress.

Craton: A stable, ancient part of the continental lithosphere, often serving as the core of continents.

Dextral offset: A right-lateral displacement along a fault, where the opposite side moves to the right when viewed from either side.

Diorite: A coarse-grained, intermediate intrusive igneous rock composed mostly of plagioclase with smaller amounts of amphibole or pyroxene.

Eclogite: A dense, high-pressure metamorphic rock composed primarily of garnet and omphacite, formed in continental subduction zones.

Garnet: A group of silicate minerals commonly found in metamorphic rocks, notable for their cubic crystal structure and wide range of colors.

Gneiss: A foliated metamorphic rock with a banded appearance caused by the segregation of mineral layers.

Gorkha earthquake: A devastating earthquake that struck Nepal in 2015. Its epicenter was near the town of Gorkha.

Granite: A coarse-grained, felsic, intrusive igneous rock composed mostly of quartz, feldspar, and mica.

Granodiorite: A coarse-grained intrusive igneous rock intermediate in composition between granite and diorite, with more plagioclase than potassium feldspar.

Granulite facies: A high-grade metamorphic facies that characterizes rocks that have undergone regional metamorphism at temperatures of at least 700±50 degrees C and pressures of 2-15 kilobars.

Great Assam Earthquake: a series of earthquakes that occurred in the Assam region of India and the Tibetan Plateau in the 19th and 20th centuries. The largest of these occurred in 1897 and 1950.

Greenschist facies: A set of metamorphic conditions characterized by low pressure and temperature, with stable minerals such as chlorite, actinolite, and albite.

Hornblende: A dark, calcium-rich amphibole mineral common in igneous and metamorphic rocks.

I-type granite: Granites derived from the melting of igneous protoliths, often associated with convergent plate margins.

Kyanite: An aluminum silicate mineral that forms under high-pressure, low-temperature metamorphic conditions; polymorphic with andalusite and sillimanite.

Lamprophyre dyke: An intrusive igneous rock body, typically dark-colored, rich in mafic minerals, and associated with mantle-derived magmas.

Leucogranite: A light-colored, silica-rich intrusive igneous rock, mostly composed of quartz, feldspars, and muscovite, often formed during crustal melting.

Metamorphic facies: A set of mineral assemblages that indicate metamorphism within a specific range of pressure-temperature conditions.

Metamorphic isograd: A contour line marking the first appearance of a new metamorphic mineral, indicating a change in metamorphic grade.

Migmatite: A heterogenous rock formed under high-temperature conditions that induce partial melting.

Muscovite: A pale mica-group mineral, common in granitic and metamorphic rocks, known for its perfect cleavage.

Pluton: A large igneous body that has cooled from a magma at depth. Various sorts of pluton are defined by their overall shape and relationship to the surrounding rock. The largest (at least 100 square kilometers) are called batholiths.

Protolith: The original rock from which a metamorphic rock is formed.

S-type granite: Granites derived from the melting of sedimentary protoliths, often enriched in aluminum.

Silica undersaturated: Rocks with low silica content, often containing minerals like feldspathoids.

Sillimanite: An aluminum silicate mineral stable at high temperature and pressure, polymorphic with kyanite and andalusite.

Staurolite: A metamorphic mineral indicative of medium-grade metamorphic rocks.

Suture: A zone marking the collision and joining of two tectonic plates, often associated with ophiolite complexes and mountain ranges.

Syenite: A coarse-grained intrusive igneous rock, similar in composition to a granite, but with less than 5 percent quartz.

Syntaxis: A region where tectonic structures converge or bend sharply, commonly seen in mountain ranges.

Tethyan collision zone: The giant orogenic belt associated with the gradual closure of the Tethys Ocean that extends for over 15,000 km from the Atlas Mountains and the Alps in the west, to the Caucasus that border Europe and Asia, to the Himalaya and even further east to SE Asia.

Tourmaline: A boron silicate mineral found in igneous and metamorphic rocks, known for its prismatic crystals and diverse colors.

Ultrapotassic volcanics: Volcanic rocks with K2O greater than 3 percent by weight and a K2O/Na2O ratio of 2 or more.

Uranium-lead-monazite age: The age of a rock or mineral determined using uranium-lead dating of monazite, a mineral that often hosts some uranium within its crystal lattice.