9. Volcanoes beyond Earth

‘Volcanoes beyond Earth’ highlights volcanoes on other planets. There are many more volcanoes on Venus than there are on Earth, and many remain active. In the absence of plate tectonics and the kind of tectonic forces that raise Earth-style mountain belts, and of streams, rivers, and shorelines, it is volcanism and volcanic products that dominate the surface of this planet. Fossil volcanism occurs in the Moon, Mercury, and Mars; Io, the hypervolcanic moon of Jupiter; and the ice volcanoes of the Solar System. There is potential for volcanism on exoplanets within distant planetary systems.

8. What have volcanoes done for us?

‘What have volcanoes done for us?’ outlines how humans have benefited from volcanoes. Volcanism has helped make the planet as it is today, by contributing to the chemical composition of the atmosphere and oceans. Volcanoes provide rich, fertile environments for wildlife and agriculture. Ultimately, volcanoes have been essential for the evolution of the biosphere, the emergence of humanity and human civilization, and for the flowering of culture. However, volcanoes across the world are increasingly being exploited for tourism, quarrying, deforestation, and urbanization. They also pose a hazard, one that increases as burgeoning human populations encroach ever closer. The chapter then looks at methods of monitoring and mitigating volcanic hazards.

7. Volcanoes, climate, and the biosphere

‘Volcanoes, climate, and the biosphere’ explores how volcanism has perturbed both climate and the complement of living organisms on Earth, both locally and globally. Volcanic outbursts, depending on their nature and scale, may cause global warming or global cooling. In the historical record, even geologically modest eruptions have had dramatic repercussions. Volcanoes can affect local weather. It is possible that climate change can, in turn, affect volcanism.

6. Hidden volcanoes

‘Hidden volcanoes’ highlights ancient volcanoes, which offer crucial perspectives that would not be clear from the study of modern volcanoes alone. Exhumed (uplifted and dissected) volcanoes provide vital glimpses deep inside a volcano, including how they work. They are windows showing what really happens when hot magma interacts with bedrock, ice-sheets, lakes, and deep oceans. Ancient volcanoes also reveal how volcanic processes vary with time. They have enabled us to discover new and awesome styles of eruption. Examples are described, from the highest mountain in Wales, Yr Wyddfa, or Snowdon, which was an explosive caldera volcano that emerged from shallow seas. Also described are ‘Large igneous provinces’, diamond-bearing volcanoes, and the most ancient volcanism on Earth.

5. Making and breaking volcanoes

‘Making and breaking volcanoes’ addresses how volcanoes are constructed and denuded and explains the shape of volcanoes and their internal architecture, including the differences between scoria cones, tuff rings, maars, and dome fields, shield volcanoes, and stratocones. Some volcanoes (‘monogenetic’ volcanoes) erupt just once, whereas others (‘polygenetic’ volcanoes) may continue erupting intermittently for millions of years. When sufficient magma is rapidly expelled from the shallow reservoirs beneath the volcano the overlying ground is left unsupported and collapses, creating a large topographic basin known as a caldera. As the caldera founders, its steep sides, formed so abruptly, are unstable and collapse inwards as a series of landslides. Tall volcanoes tend to collapse sideways in giant landslides, then grow and collapse again. Rain and meltwater also wears away volcanoes, forming lahars and floods, and choking drainage systems.

2. How do volcanoes explode?

‘How do volcanoes explode?’ examines how volcanoes explode, looking at the anatomy of an explosive eruption. It studies pumice-forming rhyolitic eruptions. One of the first things a volcanologist may do on arrival at a volcano is to figure out how the volcano has behaved in the past: what sort of eruptions, how large, and how frequent. Basaltic explosivity can be compared with rhyolitic and andesitic explosivity. Phoenix plumes rise from pyroclastic currents and contain mostly fine ash.

1. The making of magma

‘The making of magma’ discusses the formation of magma. Volcanism is driven by prodigious amounts of heat from deep inside the Earth that partly melts rock to form magma, which accumulates and evolves deep underground, and may drive powerful eruptions that expel ash and lava on to the surface, creating a volcano. This process can be studied by inferences based upon volcano behaviour, seismic information, chemical tracers in volcanic materials, experimentation, and other clues. From enormous, diffuse networks deep in the Earth, the magma rises and gathers together in the crust as a complex system of interconnected fractures, reservoirs, and conduits. These magma reservoirs can be detected by seismic imaging.

4. Lava

‘Lava’ focuses on lava. Within a kilometre or two of the ground surface, the rise of hot magma is accelerated by the growth of gas bubbles which increase the magma’s buoyancy. If the gases can readily escape from the magma, the magma may ascend more slowly without being ripped apart, and it eventually emerges from the volcano as a lava flow. There are many different types of lava flows: pahoehoe, a’a, block-lavas, inflated lavas, and lava domes, which decrepitate to form hazardous ‘block-and-ash flows’.

3. Volcanoes and water

‘Volcanoes and water’ assesses how water affects the behaviour of volcanoes. When rising hot magma encounters groundwater, as in a water-saturated sandstone, the pore water in the sandstone next to the magma flashes to steam, and the force of the expanding steam within the rock tears the sand grains apart from each other, and streams them away as it escapes. Peperite, ‘phreatic’ (steam) eruptions, phreatomagmatic eruptions, submarine eruptions, emergent Surtseyan eruptions, and ‘phreatoplinian’ eruptions are thought to be the most violent on Earth. Glacial volcanoes, and catastrophic watery ‘lahars’ accompany many eruptions.