Assam Higher Secondary Geology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Moho discontinuity

The boundary between Earth's crust and mantle, marked by an abrupt increase in seismic P-wave velocity to about 8 km/s; depth varies from ~5 km under oceans to ~70 km under mountains.

S-wave shadow zone

The region between 103 and 180 degrees from an earthquake epicentre where S-waves are not recorded, because shear waves cannot travel through the liquid outer core.

P-wave shadow zone

The band from about 103 to 143 degrees from an epicentre where direct P-waves are absent, caused by refraction at the mantle-core boundary; beyond 143 degrees P-waves reappear via the core.

Asthenosphere

The partially molten, plastic layer of the upper mantle beneath the rigid lithosphere, roughly 100 to 350 km deep, over which tectonic plates move and where seismic waves slow down.

Isostasy

The state of gravitational balance in which lighter crustal blocks float on the denser mantle; Airy's model varies root thickness, while Pratt's model varies crustal density at constant depth.

Continental drift

Alfred Wegener's hypothesis that continents once formed a supercontinent, Pangaea, and have since drifted apart, supported by matching coastlines, fossils, glacial deposits, and rock formations.

Sea-floor spreading

The process where new oceanic lithosphere forms at mid-ocean ridges as magma rises, cools, and moves laterally, producing symmetrical magnetic stripes and increasing crustal age away from the ridge.

Benioff zone

An inclined zone of earthquake foci dipping beneath a convergent margin, tracing the descending subducting slab from shallow to depths of about 700 km.

Transform boundary

A plate margin where two plates slide horizontally past each other, producing shallow-focus earthquakes and strike-slip faults but no volcanic activity, as along the San Andreas Fault.

Slab pull

The gravitational force exerted by a dense, cold subducting plate as it sinks into the mantle; it is generally considered the dominant driver of plate motion.

Mohs hardness scale

A relative scale from 1 (talc) to 10 (diamond) ranking minerals by scratch resistance; a mineral scratches those below it and is scratched by those above it.

Silicate tetrahedron

The basic structural unit of silicate minerals, a silicon atom surrounded by four oxygen atoms; its arrangement into chains, sheets, or frameworks determines mineral properties.

Bowen's reaction series

The sequence in which minerals crystallise from cooling magma, from olivine and calcium plagioclase at high temperature to quartz and muscovite at low temperature.

Phaneritic texture

An igneous texture in which individual mineral grains are large enough to be seen with the naked eye, indicating slow cooling of magma deep underground in intrusive bodies.

Aphanitic texture

An igneous texture with grains too small to see unaided, resulting from rapid cooling of lava at or near the surface, typical of basalt and andesite.

Porphyritic texture

An igneous texture containing large crystals set in a finer groundmass, recording two cooling stages: slow crystallisation at depth followed by rapid cooling near the surface.

Clastic sedimentary rock

Rock formed from cemented fragments of pre-existing rocks, classified by grain size: conglomerate (gravel), sandstone (sand), and shale (silt and clay).

Foliation

The parallel alignment of platy minerals such as mica in metamorphic rocks, produced by directed pressure and shown in slate, phyllite, schist, and gneiss.

Metamorphic grade

The intensity of metamorphism a rock has experienced, indicated by index minerals; low grade gives chlorite and slate, high grade gives garnet, kyanite, and gneiss.

Rock cycle

The continuous transformation of rock among igneous, sedimentary, and metamorphic types through melting, cooling, weathering, erosion, deposition, lithification, and metamorphism.

Lithification

The process by which loose sediment becomes solid rock through compaction by overlying weight and cementation by minerals such as calcite, silica, or iron oxide.

Igneous rock classification

Grouping of igneous rocks by silica content and texture: ultramafic peridotite, mafic gabbro and basalt, intermediate diorite and andesite, felsic granite and rhyolite.

Assam-Arakan basin

A Tertiary sedimentary basin in Northeast India containing the Barail and Tipam formations, which host major coal, limestone, and hydrocarbon deposits.

Shillong Plateau

A Precambrian granite and gneiss upland in Meghalaya, separated from the Himalaya by the Brahmaputra valley and bounded by the Dauki Fault to the south.

Magnetic anomaly stripes

Symmetrical bands of alternating normal and reversed polarity recorded in oceanic crust on either side of a mid-ocean ridge, confirming sea-floor spreading and dating the crust.

Epicentre

The point on Earth's surface directly above the focus of an earthquake, where seismic waves arrive first and shaking is usually strongest.

Focus (hypocentre)

The actual point within the Earth where an earthquake rupture begins and seismic energy is released, located at some depth below the epicentre.

Seismograph

An instrument that records ground motion from earthquakes, producing seismograms from which the time, location, depth, and magnitude of events are determined.

Richter magnitude

A logarithmic scale of earthquake size based on the amplitude of seismic waves recorded by a standard seismograph, calibrated for distance from the epicentre.

Active continental margin

A convergent plate margin where oceanic lithosphere subducts beneath a continent, producing a trench, volcanic arc, fold mountains, and deep-focus earthquakes.

Mid-ocean ridge

An underwater mountain chain along a divergent boundary where magma rises to form new oceanic crust, characterised by shallow earthquakes and basaltic volcanism.

Specific gravity

The ratio of a mineral's density to that of water; useful for identifying dense minerals such as barite, galena, and magnetite during hand-specimen examination.