This article is about the process that forms volcanoes and other igneous formations. For the 18th century geological theory, see Plutonism
Non-viscous lava during an effusive eruption of Kīlauea
Magma from the mantle or lower crust rises through its crust towards the surface. If magma reaches the surface, its behavior depends on the viscosity
of the molten
. Viscous (thick) magma produces volcanoes characterised by explosive eruptions
, while non-viscous (runny) magma produce volcanoes characterised by effusive eruptions
pouring large amounts of lava
onto the surface.
In some cases, rising magma can cool and solidify without reaching the surface. Instead, the cooled and solidified igneous
mass crystallises within the crust to form an igneous intrusion
. As magma cools the chemicals in the crystals formed are effectively removed from the main mix of the magma (by a process known as fractional crystallization
), so the chemical content of the remaining magma evolves as it solidifies slowly. Fresh unevolved magma injections can remobilise more evolved magmas, allowing eruptions from more viscous magmas.
Driving forces of volcanism
Three types of plate boundary.
Aspects of volcanism
Cross-section through a stratovolcano (vertical scale is exaggerated)
- Large magma chamber
- Conduit (pipe)
- Layers of ash emitted by the volcano
- Layers of lava emitted by the volcano
- Parasitic cone
- Lava flow
- Ash cloud
Volcanoes are places where magma reaches the planet's surface. The type of volcano depends on the location of the eruption and the consistency of the magma.
Types of Intrusion:
- small Dike
- Pipe/Volcanic neck
Earthquakes are generally associated with plate tectonic activity, but some earthquakes are generated as a result of volcanic activity 
(though that itself is ultimately driven by the same forces).
The amount of gas and ash emitted by volcanic eruptions has a significant effect on the Earth's climate
. Large eruptions correlate well with some significant climate change
When magma cools it solidifies and forms rocks. The type of rock formed depends on the chemical composition of the magma and how rapidly it cools. Magma that reaches the surface to become lava cools rapidly, resulting in rocks with small crystals such as basalt
. Some of this magma may cool extremely rapidly and will form volcanic glass
(rocks without crystals) such as obsidian
. Magma trapped below ground in thin intrusions cools more slowly than exposed magma and produces rocks with medium-sized crystals. Magma that remains trapped in large quantities below ground cools most slowly resulting in rocks with larger crystals, such as granite
Existing rocks that come into contact with magma may be melted and assimilated into the magma. Other rocks adjacent to the magma may be altered by contact metamorphism
as they are affected by the heat and escaping or externally-circulating hydrothermal
Volcanism on other bodies
Volcanism is not confined only to Earth, but is thought to be found on any body having a solid crust and fluid mantle. Evidence of volcanism should still be found on any body that has had volcanism at some point in its history. Volcanoes have indeed been clearly observed on other bodies in the Solar System
– on some, such as Mars
, in the shape of mountains that are unmistakably old volcanoes (most notably Olympus Mons
), but on Io
actual ongoing eruptions have been observed. It can be surmised that volcanism exists on planets and moons of this type in other planetary systems
as well. In 2014, scientists found 70 lava flows which formed on the Moon in the last 100 million years.
The internal structure of the inner planets.
- ^ "Cooling Planets: Some Background: What is volcanism?" (PDF). The Lunar and Planetary Institute, Department of Education and Public Outreach. 2006. p. 4. Archived (PDF) from the original on 2013-08-01. Retrieved 2012-10-14.
- ^ Watson, John; Watson, Kathie (January 7, 1998). "Volcanoes and Earthquakes". United States Geological Survey. Archived from the original on March 26, 2009. Retrieved May 9, 2009.
- ^ a b Nemzer, J. "Geothermal heating and cooling". Archived from the original on 2012-11-01. Retrieved 2012-11-03.
- ^ Robock, Alan (2000). "Volcanic eruptions and climate". Reviews of geophysics 38 (2): 191-219. doi:10.1029/1998RG000054
- ^ "Recent volcanic eruptions on the moon". sciencemag.org. 12 October 2014. Retrieved 6 May 2018.
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Last edited on 22 February 2021, at 16:43
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