Basalt

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

Basalt is a dark, fine-grained igneous rock. It forms when runny, low-silica (mafic) lava cools quickly on the Earth's surface. It matters because basalt makes up the ocean floor, shield volcanoes and huge lava plateaus such as the Deccan Traps. When it weathers, it becomes the black soil (regur) that India's cotton belt depends on.

Explanation

How basalt forms (cause → process → result)

• Cause: lava with little silica
• Silica (SiO₂, the compound found in quartz and sand) makes lava thick and sticky.
• Basalt lava has little silica. It has more iron (Fe) and magnesium (Mg).

• So the lava is thin and fluid, with low viscosity (resistance to flow).

• Process: fast cooling on the surface

• The lava comes out on the surface and meets air or water.

• It loses heat quickly, so its mineral crystals have no time to grow.

• Result: a fine-grained rock

• Basalts are extrusive igneous rocks that cooled quickly, so they have a fine-grained structure [3].
• The grains are too small to see easily with the naked eye.
• In rocks that cool fast, glass may form between the tiny crystals [3].

Where basalt fits among the rock types

• By origin: igneous. Igneous rock forms from molten rock ("igneous" comes from the Latin ignis, fire). Igneous rocks are primary rocks, because the Earth's first crust was igneous.
• By where it cooled: extrusive (volcanic). It cools on the surface as lava. It does not cool deep inside the crust as magma.
• By chemistry: mafic (basic).
• Mafic = magnesium + ferrum (Latin for iron).

• It is dark (black or dark green) and denser than felsic rocks such as granite.

• Its plutonic twin is gabbro. Gabbro has the same mafic make-up, but it cooled slowly deep underground. So gabbro is coarse-grained and basalt is fine-grained.

Basalt lava and volcano type

• Runny lava lets gas escape easily.
• Pressure does not build up → eruptions are quiet and flowing, not explosive.

• The lava spreads far before it hardens.

• Two main landforms result:

• Shield volcanoes: wide, gently sloping cones, e.g. Hawaii.

• Flood basalts: very wide, thick sheets of lava that build plateaus, e.g. the Deccan Traps.

• Fissure eruption (lava pouring out of long cracks in the crust, not from one crater):

• The lava spreads in thin sheets, one flow on top of another.
• Over time this builds a thick plateau with step-like edges.
• "Trap" comes from the Swedish trappa, meaning stairs. The name refers to this step-like look.

Columnar jointing: a basalt feature

• A thick lava flow cools and shrinks.
• Cracks spread through it in a regular pattern.
• This leaves tall columns, usually six-sided (hexagonal).

Where it occurs

• Ocean floor (world): new ocean crust forms as basalt at mid-ocean ridges, e.g. the Mid-Atlantic Ridge.
• Shield volcanoes: Mauna Loa and Kilauea in Hawaii (USA).
• Flood basalt provinces:
• Deccan Traps (India): they cover large parts of Maharashtra, plus parts of Gujarat, Madhya Pradesh, Karnataka and Telangana. They formed near the end of the Cretaceous period.
• Columbia River Plateau (USA).

• Siberian Traps (Russia).

• Columnar basalt:

• St Mary's Island, Udupi district (Karnataka coast, Arabian Sea) is a GSI National Geological Monument [2].

• Giant's Causeway (Northern Ireland, UK) shows the same feature.

• Landscape in India: the flat-topped plateaus and step-like slopes of the Western Ghats in Maharashtra are made of Deccan basalt.

Don't confuse with

• Granite: also igneous, but felsic and plutonic. It is light-coloured and coarse-grained, and it makes batholiths and old continental shields. Basalt is dark, fine-grained, mafic and extrusive.
• Gabbro: has the same mafic make-up as basalt, but it cooled slowly underground, so it is coarse-grained. Gabbro is the plutonic version of basalt.
• Obsidian and pumice: also extrusive, but they form from thick, silica-rich lava. Obsidian is volcanic glass with no crystals. Pumice is frothy, full of holes and floats on water. Basalt has fine crystals.
• Peninsular Gneiss (Lalbagh, Bengaluru): also a Karnataka GSI monument, but it is a metamorphic rock formed from old granitic rock. It is not a volcanic rock like the St Mary's Island basalt.

Prelims Hooks

• Basalt = extrusive = volcanic = fast cooling = fine-grained = mafic. The answer to "which rock forms the ocean floor and the Deccan Traps?" is basalt, not granite.
• Felsic vs mafic: felsic (more than about 65% silica) → light colour, viscous lava, explosive composite volcanoes (e.g. Fuji). Mafic basalt → dark colour, fluid lava, shield volcanoes (Hawaii) and flood basalts (Deccan).
• Feature–location match: Columnar basalt – St Mary's Island, Udupi, Karnataka (GSI National Geological Monument) [2]; Peninsular Gneiss – Lalbagh, Bengaluru. Both are in Karnataka.
• Flood basalt provinces: Deccan Traps (India), Columbia River Plateau (USA), Siberian Traps (Russia).
• Deccan Traps: formed by fissure eruption near the end of the Cretaceous. "Trap" = Swedish trappa (stairs). Basalt weathers into regur (black soil), which suits cotton.
• Trap: gabbro and basalt have the same chemistry. They differ only in cooling rate and therefore grain size.

Mains Points

• Rock type shapes land use (GS-I):
• Deccan basalt weathers into black cotton soil → it supports the cotton belt of Maharashtra and nearby states.
• Layered lava flows → flat-topped plateaus and step-like slopes of the Western Ghats in Maharashtra.

• In contrast, the granite and gneiss of the Karnataka plateau give bare domes and thin red soils. This shows how rock type controls soils and farming.

• Silica controls volcanic hazard (GS-I/GS-III disasters):

• Basalt eruptions (Hawaii, Iceland, the ancient Deccan) are fluid and mostly non-explosive. Their lava spreads over wide areas.
• Felsic volcanoes of the Pacific Ring of Fire (Japan, Indonesia) trap gas and explode. They are far more dangerous to people.

• So knowing the lava type helps in planning disaster risk.

• Geoheritage protection:

• GSI has notified 26 National Geological Monuments (PIB, 2016) [2].
• The columnar basalt of St Mary's Island is a tourist spot, so it faces pressure from tourism.
• This supports the case for a national geoheritage law.

Related concepts

• Igneous rock
• Felsic and mafic rocks
• Granite
• Kimberlite

Read more

• Igneous rocks: from magma to granite and basalt