Extrusive rock

Extrusive rock, also known as volcanic rock, is a type of igneous rock formed when magma erupts onto the Earth’s surface, cools, and solidifies rapidly. The quick cooling process prevents large crystals from developing, resulting in a fine-grained or glassy texture. Extrusive rocks are formed from lava flows, volcanic ash, and other volcanic materials expelled during eruptions. They play an important role in shaping the Earth’s surface and are key indicators of volcanic activity and plate tectonic processes.

Formation Process

Extrusive rocks originate from magma that reaches the surface through volcanic vents or fissures. When magma is expelled as lava, it cools quickly upon contact with the cooler atmosphere or water, solidifying within a short time. This rapid cooling leads to the development of small mineral crystals or sometimes none at all.
The process can be summarised in stages:

1. Magma Generation: Magma forms deep within the Earth’s crust or upper mantle due to melting of rocks caused by heat, pressure reduction, or addition of volatiles.
2. Magma Ascent: Buoyant magma rises through cracks or conduits toward the surface.
3. Eruption: The magma erupts as lava, pyroclastic material, or volcanic gases through volcanoes or fissures.
4. Cooling and Solidification: Upon exposure to air or water, the lava cools rapidly, forming solid rock.

The rapid cooling rate results in a fine-grained (aphanitic) texture, distinguishing extrusive rocks from intrusive (plutonic) rocks, which cool slowly below the surface and develop coarse crystals.

Characteristics of Extrusive Rocks

• Texture: Fine-grained or glassy due to rapid cooling. Some may have porphyritic texture, containing small crystals within a finer matrix.
• Crystals: Small or microscopic; sometimes absent in volcanic glass.
• Colour: Varies from light to dark depending on silica content.
• Formation Environment: Forms on or near the Earth’s surface.
• Occurrence: Commonly found around volcanoes, mid-ocean ridges, and fissure eruptions.

Types of Extrusive Rocks

Extrusive rocks are classified according to their mineral composition and texture, which reflect the chemical composition of the magma and the cooling environment.

1. Basalt:
   • Dark-coloured, fine-grained, mafic rock rich in iron and magnesium but low in silica.
   • Most abundant extrusive rock on Earth, forming the oceanic crust and large continental lava plateaus.
   • Example locations: Iceland, Deccan Plateau (India), Columbia River Basalt (USA).
2. Andesite:
   • Intermediate composition between basalt and rhyolite.
   • Common in volcanic arcs associated with subduction zones.
   • Found in the Andes Mountains, from which it gets its name.
3. Rhyolite:
   • Light-coloured, felsic rock rich in silica.
   • Often associated with explosive volcanic eruptions.
   • Forms volcanic domes and tuffs.
4. Dacite:
   • Intermediate to felsic composition with a higher silica content than andesite.
   • Found in continental volcanic regions such as the Cascade Range (USA).
5. Obsidian:
   • Volcanic glass formed by very rapid cooling of lava with high silica content.
   • Smooth, glassy texture; typically black or dark brown.
6. Pumice:
   • Light, porous volcanic rock formed when gas-rich lava cools quickly.
   • Floats on water due to its numerous air-filled cavities.
7. Scoria:
   • Dark, vesicular rock formed from basaltic magma containing gas bubbles.
   • Common around volcanic cones and lava flows.
8. Tuff:
   • Consolidated volcanic ash ejected during explosive eruptions.
   • Forms when ash particles compact and cement together.

Texture and Structure

The texture of extrusive rocks depends on cooling rate, viscosity, and gas content of the lava:

• Aphanitic Texture: Fine-grained; crystals too small to be seen with the naked eye.
• Porphyritic Texture: Larger crystals (phenocrysts) embedded in a fine-grained matrix, indicating two stages of cooling.
• Glassy Texture: Lava cooled so rapidly that crystals did not form (e.g., obsidian).
• Vesicular Texture: Numerous gas cavities (vesicles) formed by trapped volcanic gases (e.g., pumice, scoria).
• Amygdaloidal Texture: Vesicles later filled with secondary minerals like quartz or calcite.

Distribution and Occurrence

Extrusive rocks are widespread on Earth and form in various tectonic and volcanic settings:

• Mid-Ocean Ridges: Continuous eruptions of basaltic lava create new oceanic crust.
• Hotspots: Isolated volcanic centres such as Hawaii and Iceland produce vast quantities of basalt.
• Subduction Zones: Volcanoes along convergent boundaries (e.g., the Andes, Japan) produce andesitic and rhyolitic lavas.
• Continental Flood Basalts: Massive eruptions produce extensive plateaus, such as the Deccan Traps (India) and Siberian Traps (Russia).

Economic Importance

Extrusive rocks hold considerable economic and industrial significance:

• Construction Materials: Basalt and andesite are used as crushed stone, aggregate, and building materials.
• Industrial Uses: Pumice is used as an abrasive and in lightweight concrete; obsidian was historically used for cutting tools and ornaments.
• Soil Formation: Weathering of volcanic rocks produces fertile soils rich in minerals, supporting agriculture in volcanic regions.
• Mineral Deposits: Associated with valuable metal ores, such as copper, gold, and silver, found in volcanic regions.
• Geothermal Energy: Volcanic regions underlain by extrusive rocks are important sources of geothermal power.

Comparison with Intrusive Rocks

Examples of Extrusive Rock Landforms

Extrusive volcanic activity creates distinctive surface landforms, including:

• Lava Plateaus: Formed by successive basaltic lava flows (e.g., Deccan Plateau).
• Volcanic Cones: Built from alternating layers of lava and ash (e.g., Mount Fuji, Japan).
• Shield Volcanoes: Broad, gently sloping mountains formed by low-viscosity basaltic lava (e.g., Mauna Loa, Hawaii).
• Calderas: Large depressions formed after explosive eruptions and magma chamber collapse (e.g., Crater Lake, USA).