Which type of rock is formed from cooling and solidification of lava ?

Question

UnlockIAS · Accepted Answer

Correct — B, Igneous. Igneous is the class defined by exactly this process: rock that has solidified from a melt. The name says so — it is from the Latin ignis, 'fire'. The melt has two names depending on where it is. Below the surface it is magma; once it has erupted and is flowing in the open air it is lava. So a rock formed from cooling lava is not just igneous, it is the extrusive or volcanic sub-type of igneous rock, and the word 'lava' in the stem is what pins it there. That distinction shows up in the grain. Lava at the surface loses heat in hours or days, so its crystals have no time to grow and the rock comes out fine-grained (basalt, andesite, rhyolite), or with no crystals at all if it chills fast enough — obsidian is volcanic glass, and pumice is lava frothed by escaping gas until it is light enough to float. Magma cooling kilometres down takes thousands of years and grows crystals big enough to see, which is why granite and gabbro are visibly coarse. Basaltic magma is also hotter than silica-rich magma, erupting at roughly 1,100–1,250 °C against about 800–1,000 °C for rhyolitic melts, and the runnier basalt is what builds the great flood-basalt sheets. India's own Deccan Trap is precisely that — fissure-erupted basalt lava — and its weathering produces the black cotton soil (regur) of the Deccan. Igneous rocks are also, for the same reason, unstratified and fossil-free: nothing living survives 1,000 °C.

Why the other options are wrong — (a) Sedimentary rock is built at or near the Earth's surface out of material that already existed — pre-existing rock is weathered, the debris is transported by the hydrological system, deposited, then compacted and cemented. Its signatures are the opposite of a lava flow's: it occurs in layers, and it is the only one of the three classes that contains fossils. Sandstone, shale, limestone and coal are sedimentary; none of them ever passed through a melt.  (c) Metamorphic rock is pre-existing rock altered in the solid state by heat, pressure or chemically active fluids — the crucial point being that it does not melt. Melting would put it back in the igneous box. Limestone becomes marble, sandstone becomes quartzite, shale becomes slate, granite becomes gneiss, coal becomes graphite. Heat is involved, which is why the option tempts, but transformation without melting is a different process from solidification from lava.  (d) There is no rock class called 'crystal'. A crystal is a solid whose atoms are arranged in an ordered, repeating lattice — a property that a mineral grain has or lacks, not a statement about how a rock originated. Granite is full of visible crystals and is igneous; marble is crystalline and is metamorphic. The option works as a decoy only because cooling melts really do grow crystals, so the word feels adjacent to the right answer.

Concept: Rocks are classified by origin, into three classes and only three. Igneous rocks solidify from a molten silicate melt. Sedimentary rocks accumulate from debris or chemical precipitates at the Earth's surface and are then compacted and cemented. Metamorphic rocks are earlier rocks of either class recrystallised in the solid state under heat and pressure. The three are linked in the rock cycle: igneous rock weathers into sediment, sediment lithifies into sedimentary rock, burial converts it to metamorphic rock, and further heating melts it back to magma, which cools to igneous rock again. Igneous rocks are called primary rocks because they are the only ones that do not need an earlier rock to exist first — everything else is ultimately recycled from them. Within the igneous class the split is by depth of cooling: extrusive or volcanic rocks freeze at the surface from lava, hypabyssal rocks freeze in shallow dykes and sills, and intrusive or plutonic rocks freeze at depth from magma.

State PCS 2024 Q2 — Geography PYQ with Answer

State PCS 2024 Q2 — Geography PYQ with Answer