As this blog is about volcanic and seismic activity, a word or two on what a volcano is might be helpful.
A volcano is defined by the Oxford English Dictionary as mountain or hill with a crater or vent through which rocks, rock fragments, lava, hot vapour and / or gases are or have been erupted through the Earth’s crust. However, said mountain or hill may be quite small, even just a depression or a rupture in the Earth’s surface.
The island of Vulcano is the source of the term, volcano, itself. Vulcano Island is located in the Tyrrhenian Sea, north of Sicily, made up of several active volcanoes, including calderas.
So what causes the lava and other matter to be erupted? What is the Earth’s crust? These are questions some of the questions we will look at in this blog.
Heat generated at the Earth’s core drives the geological processes which result in volcanic activity. For starters, we will look at the basics of the earth’s composition, magma and the source of the energy to enable eruptions to occur.
Basics of the Earth’s Composition
This is a very basic description of the Earth’s composition. The Earth is made up of three main parts: the core, the mantle and the crust. The core is very hot and temperatures decrease towards the Earth’s surface. Most of the Earth is solid, only the outer core is liquid. Evidence for this structure has been gleaned from seismic studies, notably how the different wave types generated by an earthquake pass through the Earth, geophysics and the study of rocks.
The radius of the Earth is around 6,378 km, in other words the centre of the Earth’s core can be found 6,738 km down. The core makes up around one third of the Earth’s mass. It is made up of an outer core which starts at around 2,900 km down and an inner core which starts at around 5,100 km down.
Material in the core is too dense to make its way to the surface, so there is some uncertainty over its composition. What we do know is inferred from geophysical studies of the Earth and the chemical analysis of meteorites. During the Earth’s formation, as rocks and fragments combined to form the planet, denser matter sunk towards the core under gravitational and other forces. Iron is the chief component of the core, with nickel at the inner core and a lighter element in the outer core (possibly, oxygen, sulphur, carbon, hydrogen or potassium). The iron in the core and the electrical currents in the molten outer core are the source of the Earth’s magnetic field.
Seismic studies have shown that the outer core is impermeable to earthquake shear waves (S waves) so acts like a liquid. Whether or not a layer is liquid or solid is down to the balance between temperature, pressure and chemical composition: while the inner core is around 4,700°C, immense pressure keeps the rock solid.
The mantle is composed of solid rocky materials that are less dense that the outer core; it makes up two thirds of the Earth’s mass. Density differences mean that the mantle is a distinct layer from the outer core. The most abundant elements in the mantle are silicon and oxygen, that form silicates. The mantle is made up of around 45% silica. Magnesium and iron are the third and fourth most abundant elements. Many other elements are to be found in the mantle, but these tend to be depleted near the boundary with the crust.
The composition of the mantle is inferred from xenoliths (small fragments of rock) contained in some basalt magmas and kimberlites. Whether or not these are representative of the mantle as a whole or just the fragments that have been erupted is open for debate.
The upper mantle is joined to the crust; the combined layer is referred to as the lithosphere. Below the lithosphere, also in the upper mantle, is the asthenosphere. The asthenosphere, being weaker than the lithosphere, enables lithospheric slabs to move around (plate tectonics). The asthenosphere moves at the rate of a few centimetres a year from a process called solid-state convection; hot mantle rises, transfers heat to the lithosphere and the resulting cooled mantle sinks. The heat in the lithosphere is dissipated through conduction or via rising magma.
The lithosphere is around 120 km thick. It’s boundary with the asthenosphere is defined by the temperature at which rocks become ductile, around 1,350°C.
The crust is a silicate rich brittle layer covering the mantle; it comprises less than 0.5% of the Earth’s mass. There are two types of crust: oceanic crust, c. 6 km to 11 km thick, mostly basalt, which makes up ocean floors; and, continental crust, c. 25 km to 90 km thick, composed of igneous rocks (granite and andesite), sedimentary rocks and metamorphic rocks, which, as the name suggests, make up the continents and the continental shelves. Igneous rocks are those resulting from volcanic processes. Sedimentary rocks are those made up of fragments produced by erosion or decay of rocks on the surface. Metamorphic rocks are sedimentary or igneous rocks altered by changes in temperature and / or pressure.
Magma is the molten rock from either the mantle or the crust, itself, that makes its way through the crust to where it may be erupted as lava at a volcano or volcanic fissure. Magma and lava are the same rock: it is magma until it is erupted; and, lava is the erupted matter.
The composition of the magma and how it is generated determine the eruptive style of the volcano: e.g. effusive or explosive.
The energy required for matter to be erupted is heat from the Earth’s core. The Earth’s core is made up of radioactive materials; their radioactive decay generates heat. Most heat today is generated from four long-lived radioactive isotopes: two uranium isotopes,235U and 238U; one thorium, 232Th; and, one potassium, 40K. Additional heat came from the decay of the shorter-lived aluminium isotope, 26Al earlier in the planet’s formation. Asteroid bombardment has also added kinetic energy.
So we know have the Earth’s crust, magma and heat. What happens next? Watch this space.
The Armchair Volcanologist
24 June 2020
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