Assam Higher Secondary Geology key terms

A naturally occurring, inorganic solid with a definite chemical composition and an ordered internal atomic arrangement, usually crystalline. Rocks are made of one or more of these substances.

A coherent aggregate of one or more minerals or mineral-like materials forming part of the Earth's crust. It may be igneous, sedimentary, or metamorphic in origin.

A relative hardness scale from 1 (talc) to 10 (diamond) in which each mineral scratches those below it. Used for quick field identification of minerals.

The tendency of a mineral to break along flat planes that follow weak zones in its crystal structure. Described by number of directions and quality, such as perfect or poor.

The way a mineral breaks when it does not split along cleavage planes. Common patterns include conchoidal (curved shell-like), uneven, fibrous, and hackly.

The appearance of a mineral's surface in reflected light, such as metallic, vitreous (glassy), pearly, silky, or earthy. It is an important diagnostic property in hand specimen study.

The colour of the fine powder a mineral leaves when rubbed on an unglazed porcelain plate. It is often more reliable than the bulk colour for identification.

A mineral built around the silicon-oxygen tetrahedron, either isolated or linked into chains, sheets, or frameworks. Silicates form most of the Earth's crust and mantle.

A group of framework silicate minerals containing aluminium and potassium, sodium, or calcium. They are the most abundant minerals in the crust and a major component of granite.

A hard, glassy mineral composed of silicon dioxide, with no cleavage and a conchoidal fracture. It is common in granites, sandstones, and veins.

A group of sheet silicate minerals that split into thin, flexible, shiny flakes. Biotite is dark and iron-rich; muscovite is pale and potassium-rich.

Rock formed by the cooling and crystallisation of molten material called magma or lava. Slow cooling underground gives coarse grains; rapid cooling at the surface gives fine grains.

Hot, molten rock material generated within the Earth, containing dissolved gases and suspended crystals. When it reaches the surface it is called lava.

Magma that has erupted onto the Earth's surface or ocean floor. It cools quickly, producing fine-grained or glassy volcanic rocks such as basalt.

An igneous rock that solidified deep underground from slowly cooling magma, giving large, visible crystals. Granite is the classic example.

An igneous rock formed at or near the surface from rapidly cooling lava or pyroclastic material. It is typically fine-grained or glassy, as in basalt.

The size, shape, and arrangement of grains in a rock, reflecting how it formed. Terms include coarse, fine, glassy, porphyritic, and foliated.

Rock formed at or near the surface from deposited sediment or from chemical precipitation, later compacted and cemented. Sandstone, shale, and limestone are common types.

The breakdown of rocks in place by physical, chemical, or biological processes. It produces loose material that erosion can then remove.

The removal and transport of weathered rock material by agents such as running water, wind, ice, or gravity. It is distinct from weathering, which only breaks material down.

The laying down of transported sediment when the carrying agent loses energy, for example where a river slows or a glacier melts. Deposited layers may later become sedimentary rock.

The process by which loose sediment is converted into solid rock, mainly through compaction by overlying weight and cementation by minerals precipitated from pore fluids.

The squeezing of sediment by the weight of overlying material, which reduces pore space and expels water. It is one of the main steps in turning sediment into rock.

Rock changed in the solid state by heat, pressure, or chemically active fluids, without melting. Examples include slate from shale and marble from limestone.

A layered or banded structure in metamorphic rocks caused by parallel alignment of platy minerals or by segregation of light and dark minerals under directed pressure.

Metamorphism caused by heat from an intruding magma body affecting the surrounding rocks over a limited area. It typically produces non-foliated rocks such as hornfels.

Metamorphism affecting large areas, usually caused by heat and directed pressure during mountain building at convergent plate boundaries. It commonly produces foliated rocks.

The theory that the Earth's rigid outer shell is divided into plates that move over the weaker mantle, causing earthquakes, volcanoes, and mountain belts at their boundaries.

A convergent boundary where one lithospheric plate sinks beneath another into the mantle. It produces deep ocean trenches, volcanic arcs, and frequent earthquakes.

An underwater mountain chain where two plates pull apart and new oceanic crust forms from upwelling magma. Sea-floor spreading there creates new lithosphere.

The shaking of the ground caused by a sudden release of stored elastic energy, usually along a fault. It is recorded by seismographs and measured by magnitude scales.

A fracture in rock along which the two sides have moved relative to each other. Movement may be vertical, horizontal, or oblique depending on the stresses involved.

An elastic wave travelling through the Earth, generated by an earthquake or explosion. P-waves are compressional, S-waves are transverse, and surface waves travel along the ground.

The point inside the Earth where an earthquake rupture begins and seismic energy is first released. Its vertical projection on the surface is the epicentre.

The point on the Earth's surface directly above an earthquake's focus. Shaking is usually strongest near it, so it is used in locating and reporting quakes.

A bend in layered rock produced by compressional forces, commonly during mountain building. An upward arch is an anticline; a downward trough is a syncline.

The loose, weathered surface layer of the Earth in which plants grow, made of mineral particles, organic matter, water, and air. Its character depends on parent rock and climate.