Magma, Volcanoes and Igneous Bodies
What Controls How a Volcano Erupts?
Not all volcanoes are the same. Some gently pour out thin lava flows that travel kilometres; others explode catastrophically, hurling ash and rock high into the atmosphere. The key variable is viscosity — how easily magma flows. Viscosity is controlled mainly by silica content and temperature. High-silica magmas (like those that feed andesite and rhyolite volcanoes) are sticky and trap dissolved gases; pressure builds until the eruption is explosive. Low-silica basaltic magmas are runny, allowing gases to escape gradually, producing quiet, passive eruptions.
Eruption Styles
A fissure eruption occurs when lava erupts through a linear crack in the crust rather than from a single point. The erupted lava is typically basaltic (low viscosity) and pours out in wide, flat sheets. Iceland is the best modern example, where the Mid-Atlantic Ridge intersects the island. Fissure eruptions produce broad lava plateaux and shield volcanoes with very gentle slopes.
A central vent eruption occurs when magma reaches the surface through a single pipe or chimney. The shape of the resulting volcano depends on magma viscosity. Basaltic magma builds a shield volcano with low, gently sloping sides because the lava flows far before solidifying. Higher-viscosity magma (andesitic) builds a steeper stratovolcano with more explosive eruptions and layers of ash and lava. Explosive eruptions can produce pyroclastic material — fragments of rock and solidified lava blown into the air.
High silica content = high viscosity = gases cannot escape = pressure builds = explosive eruption = steep-sided volcano. Low silica content = low viscosity = gases escape gradually = passive eruption = broad, gently sloping shield volcano.
Distinctive Lava Features
Basaltic lavas that cool slowly in thick flows often develop columnar jointing. As the lava cools and contracts, it cracks into regular polygonal columns, typically six-sided, perpendicular to the cooling surface. The Giant’s Causeway in Northern Ireland is the best-known British example. The columns are a direct record of the cooling geometry of the lava.
When basaltic lava erupts underwater, the outer surface quenches immediately in the cold seawater, forming rounded blobs known as pillow lava. Each new blob of lava breaks through the hardened skin of the previous one, producing a stack of overlapping pillows. Pillow lavas in the rock record are definitive evidence of submarine volcanic activity and are abundant in ancient ocean floor sequences preserved on land.
Columnar jointing — hexagonal columns in basalt, formed by contraction during cooling. Example: Giant’s Causeway, Northern Ireland. Pillow lava — rounded blobs formed when lava erupts underwater and quenches rapidly. Both are diagnostic field features used to interpret ancient volcanic environments.
Intrusive Igneous Bodies
Not all magma reaches the surface. Some cools and solidifies within the crust, forming intrusions. Three types are specified: sills, dykes and plutons. Each has a characteristic shape and field relationship that you need to recognise.
A sill is injected parallel to existing rock layers — it is concordant with the bedding. A sill can be recognised in the field because it lies parallel to the rock layers above and below it, and the rocks on both sides show baking (contact metamorphism) as evidence of heat. The Great Whin Sill of northern England is a well-known example, forming the resistant crag on which Hadrian’s Wall sits.
A dyke cuts across existing rock layers — it is discordant. Dykes are typically near-vertical and can be seen in coastal cliffs as dark sheets cross-cutting paler sedimentary beds. Because the dyke post-dates the rocks it cuts, cross-cutting relationships give relative age information: the dyke is always younger than the rocks it intrudes.
A pluton is a large, irregular igneous intrusion, typically of granitic composition. Plutons form deep in the crust where magma cools very slowly, producing the coarse grain size characteristic of granite. They often dome upwards, doming the overlying rocks. When erosion strips away the surrounding material over millions of years, the resistant granite is exposed at the surface. Dartmoor and Bodmin Moor in southwest England are parts of the Cornubian batholith — a chain of exposed granite plutons.
In the field, a pluton can be recognised by its lack of internal layering (unlike a sill), its irregular contact with surrounding country rock, and the metamorphic aureole — a zone of thermally altered rock surrounding the intrusion. The larger the pluton, the wider its aureole, because more heat was available over a longer cooling period.
The specification mentions a jelly lava flow simulation, which models how viscosity affects slope angle and flow shape. In the experiment, more viscous “lava” (thicker jelly) produces a steeper, shorter flow; less viscous “lava” (runnier jelly) spreads further and produces a shallower slope angle. This directly models the difference between a shield volcano built by low-viscosity basaltic lava and a steeper stratovolcano built by more viscous andesitic material. Exam questions may ask you to interpret the model or identify its limitations — for example, jelly does not reproduce temperature gradients or real gas behaviour.
Recognising intrusive and extrusive igneous bodies in the field and on geological maps is a core skill tested throughout the course. The field relationships — whether a body is concordant or discordant, whether it has a contact metamorphic aureole, and whether surrounding rocks are baked — are the primary evidence you will use when sequencing geological events in the next section on relative dating.