Types of Lava: Andesitic (Acidic) and Basaltic (Basic) Lava

At a Glance

• Basaltic lava Low silica, fluid, hot
• Andesitic lava Higher silica, viscous, cooler
• Eruption Basic quiet; acidic explosive
• Landform Shield or plateau versus steep cone
• Setting Hotspots, ridges versus subduction zones
• Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution

Where Does It Fit?

This topic follows the one on volcano types and explains the property that decides how a volcano behaves, namely the nature of its lava. It also serves the later topics on intrusive landforms, volcano distribution and the hazards of eruptions. Previous topic: Volcanoes: Types (Shield, Composite, Caldera, Flood Basalt, Mid-Ocean Ridge). Next topic: Intrusive Volcanic Landforms: Batholiths, Laccoliths, Lapoliths, Phacoliths, Sills, Dykes.

Why Does Magma Form?

The interior of the Earth is hot, but most of the mantle is solid because of the great pressure. Magma forms when the conditions change in one of three ways. First, decompression melting occurs when hot mantle rock rises and the pressure falls, as at divergent boundaries and under hotspots, so it melts without extra heat. Second, flux melting occurs when water released from a subducting plate lowers the melting point of the overlying mantle, as at subduction zones. Third, heat from rising magma can melt the surrounding crust. Magma is lighter than the surrounding rock, so it rises through cracks and collects in a magma chamber below the volcano, and it erupts when gas pressure builds up or new magma enters. Plate boundaries and hotspots therefore control where volcanoes occur, as discussed in Plate Tectonics: Major and Minor Lithospheric Plates.

What Are the Types of Lava?

The behaviour of an eruption depends on the chemistry of the lava, especially its silica content. Lava with more silica is thicker, or more viscous, and traps more gas.

On the surface, basaltic lava forms two main textures. Pahoehoe has a smooth, ropy surface, and aa lava has a rough, clinkery surface. Lava erupting under water cools quickly and forms rounded pillow lava. Lava flows are slow enough that people usually escape them, and the main killers in eruptions are ash fall, pyroclastic flows and gases.

What Is Pyroclastic Material and What Gases Are Released?

Explosive eruptions throw out solid fragments, called pyroclastic material or tephra. In order of size, these are volcanic dust and ash, cinders and lapilli (small stones), and bombs and blocks. A pyroclastic flow is a fast, hot cloud of gas and ash that races down the slope at speeds of over 100 km an hour and is the most lethal volcanic hazard, as at Pompeii in 79 CE. Volcanic gases contribute to the atmosphere: large eruptions inject sulphur dioxide into the stratosphere, where it forms aerosols that reflect sunlight and cool the planet for a year or two. The effects of eruptions are developed in the hazards of volcanoes, and the different styles of eruption in Volcanoes: Types (Shield, Composite, Caldera, Flood Basalt, Mid-Ocean Ridge).

As an analytical point, it is the gas content and viscosity together, and not the temperature alone, that determine how violent an eruption is, so a cooler acidic magma can be far more dangerous than a hotter basaltic one.

How Do Silica Content and Temperature Compare?

In the older classification used in textbooks, the lava is simply called acidic (rich in silica, light in colour) or basic (poor in silica, rich in iron and magnesium, dark in colour). The terms do not mean the lava is an acid; they refer to the silica content. Magma with a lot of dissolved water and gas becomes more explosive.

Where Does Each Type Occur?

• Basaltic lava: erupts at mid-ocean ridges, hotspots such as Hawaii and Iceland, and in flood basalt provinces like the Deccan Traps and the Columbia Plateau. It forms pillow lava under water.
• Andesitic lava: erupts at subduction zones, where wet oceanic crust melts, as in the Andes, Japan and Indonesia. The name comes from the Andes.
• Rhyolitic lava: comes from melting of continental crust, as at Yellowstone, and forms domes and giant calderas.

The shapes built by the two main types are described in Volcanoes: Types (Shield, Composite, Caldera, Flood Basalt, Mid-Ocean Ridge), and the intrusive equivalents in Intrusive Volcanic Landforms: Batholiths, Laccoliths, Lapoliths, Phacoliths, Sills, Dykes. As an analytical point, the plate tectonic setting of a volcano predicts its lava, and the lava predicts its hazard: the runny lavas threaten property but rarely life, while the sticky lavas cause deadly explosions, ash falls and pyroclastic flows.

Why Is This Topic Important?

• Foundation Basis of all volcano topics
• Lava Basic and acidic comparisons
• Hazards Pyroclastic flows

What Came Next?

The next topic describes the intrusive landforms that form when magma cools underground. Next topic: Intrusive Volcanic Landforms: Batholiths, Laccoliths, Lapoliths, Phacoliths, Sills, Dykes.

Key People and Terms

• Magma Molten rock below ground
• Lava Magma on the surface
• Tephra Solid volcanic fragments
• Pahoehoe Smooth ropy lava
• Aa Rough blocky lava
• Vent Opening of a volcano

Exam Corner

Mains Angle

Questions link lava chemistry to eruption style. These points add depth.

• Magma Origin by melting
• Lava Chemistry controls behaviour
• Hazards Ash, gas and flows