Volcanism – Causes and Lava Types

At a Glance

• Magma Molten rock below the surface
• Lava Magma that reaches the surface
• Causes Plate boundaries and hotspots
• Basic lava Low silica, runny; basalt
• Acidic lava High silica, thick; rhyolite
• Exam link UPSC Prelims, Geography: Mountains, Volcanoes, Earthquakes and Tsunami

Where Does It Fit?

This is the third topic of the category on mountains, volcanoes, earthquakes and tsunami. The previous topic mapped mountain ranges, and this one begins the study of volcanoes. The next topic covers the landforms made by volcanic activity. Previous topic: Significant Mountain Ranges of the World. Next topic: Volcanic Landforms – Extrusive and Intrusive.

What Is Volcanism?

In short: volcanism is the movement of molten rock from inside the Earth to the surface, where it erupts.

A volcano is a vent or fissure in the crust through which hot material from the interior is thrown out. Material that is molten rock beneath the surface is magma, and once it reaches the surface it is called lava. Alongside lava come gases, mainly water vapour with carbon dioxide, sulphur dioxide and hydrogen sulphide, and solid fragments called pyroclastic material, which range from fine ash to large volcanic bombs. The opening through which these emerge is the vent, and the bowl-shaped depression at the top is the crater. A volcano is called an active volcano if it has erupted in recent times and may erupt again, dormant if they are quiet but may erupt, and extinct if they are not expected to erupt. About 50 to 70 volcanoes erupt each year, and roughly 1,300 are considered potentially active on land.

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 and Types of Plate Boundaries.

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 Effects of Volcanoes, and the different styles of eruption in Types of Volcanic Eruptions.

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.

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 volcanic landforms, extrusive and intrusive. Next topic: Volcanic Landforms – Extrusive and Intrusive.

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