Igneous rocks

Magma and its origin

When you go down a mine, the temperature rises by about $30\text{ }^\circ\text{C}$ per kilometer, and lava flows come out at more than $1000\text{ }^\circ\text{C}$, yet seismic waves show that the mantle beneath the crust is almost entirely solid. Under what specific physical circumstances do rocks manage to melt and feed a volcano?

Hotspots: a fixed source under a moving plate

A hotspot is a plume of hot mantle that rises under a plate and melts by decompression. The plate keeps moving but the plume stays put, so a volcano is carried away from its supply and dies while a new one grows over the plume. The result is a chain of islands that get older with distance from the active volcano. Eruptions are not strictly limited to the youngest island: late (rejuvenated) volcanism and fractures called rift zones can feed eruptions on older islands too.

Xenoliths: samples of the mantle

As magma rises it can tear off pieces of the rock along its path and carry them to the surface. These foreign fragments, enclosed in the igneous rock, are xenoliths. Some basalts, such as those at San Carlos in Arizona, carry peridotite xenoliths torn from the upper mantle. They show that the upper mantle is mostly olivine and pyroxene: a direct sample of rock we cannot drill to.

Plutonic, hypabyssal and volcanic rocks

If you compare a slab of granite with a block of black basalt, the granite shows crystals of quartz and feldspar several millimeters across, visible to the naked eye, while the basalt has a dark groundmass in which no individual mineral grains can be made out. Why do two rocks of igneous origin display such radically different textures and crystal sizes?

Intrusive bodies and their contacts

A magma that stops underground does not just sit there: it heats the rock around it and is changed by it. Geologists read these contacts to work out what happened and in what order.

Magmatic differentiation: Bowen's reaction series

If you melt basalt in a crucible at $1200\text{ }^\circ\text{C}$ and let it cool slowly, the first minerals to precipitate are dense and rich in magnesium, but the remaining melt at $750\text{ }^\circ\text{C}$ becomes viscous and has a chemical composition very similar to that of granite. How can a single original magma give rise to such different types of rock as the temperature falls?

Settling and reaction rims

Olivine and pyroxene are denser than the melt, so in a magma chamber they can sink and pile up on the floor as ultramafic cumulates. The lighter, silica-richer liquid stays above. Separating crystals from liquid this way is fractional crystallization. If the crystals do not separate, they keep reacting with the melt, so the liquid composition does not change much and little felsic magma forms.

The two branches behave differently. In the continuous branch, plagioclase keeps the same crystal structure and only swaps calcium for sodium gradually. In the discontinuous branch, the structure changes at each step: an olivine crystal that stays in contact with the cooling, silica-richer melt reacts with it and gets a reaction rim of pyroxene. The rim forms only where the olivine stays in contact with the melt; if the crystal is completely sealed off from the melt, it cannot react and no rim forms.

Volcanism: viscosity and eruption styles

Gas drives the eruption. Magma holds dissolved gas, mostly water vapor, then CO₂ and SO₂. Under pressure the gas stays dissolved, like CO₂ in a sealed soda bottle. As magma rises, pressure drops and bubbles form. In fluid basalt the bubbles escape quietly, so the eruption is effusive. In viscous magma they get trapped, pressure builds, and the magma shatters explosively. More gas plus more viscosity means a more violent eruption.

Volcano shapes and hazards. A Hawaiian eruption builds a broad, gently sloping shield volcano such as Mauna Loa; the main hazards are lava flows that destroy property, and volcanic gas. A Plinian eruption builds a steep composite volcano (stratovolcano) like Mount St. Helens and sends an ash column tens of kilometers high; the hazards are ashfall, pyroclastic flows and lahars (volcanic mudflows). The Volcanic Explosivity Index (VEI, 0 to 8) rates the size of an eruption.

Lava, pyroclasts and gases

The products of a volcano tell you how its magma behaved. Here is what comes out of a vent.

Aa and pahoehoe. Basaltic lava forms two surface types. Pahoehoe is smooth and ropy: hot, fluid, slow-moving lava whose skin wrinkles as it moves. Aa is rough and jagged: cooler, more viscous lava, often flowing at a higher rate or on steeper slopes, whose crust breaks into sharp clinkers. Temperature, viscosity and flow speed control which one forms, and pahoehoe can turn into aa as it cools and loses gas downslope. Both occur at Kīlauea.

Lava shape follows viscosity. Fluid basalt (low silica) runs for tens of kilometers in long, thin flows. Rhyolite (high silica) is so viscous that it piles up near the vent as a dome or a short, thick flow, like the Mono Craters in California or the dome that grew inside the crater of Mount St. Helens.

Pyroclasts. Explosive eruptions shatter magma into ash (under 2 mm), lapilli (2 to 64 mm) and bombs (over 64 mm). Ash that falls and consolidates makes tuff; a hot cloud of ash and gas that races along the ground (a pyroclastic flow) leaves ignimbrite.

Lava structures

Once lava stops moving, it keeps telling its story.

Columnar jointing. As a thick basalt flow cools it shrinks, like mud cracking as it dries. The contraction builds tension and cracks open at fairly regular spacing, which gives mostly six-sided prisms. The cracks grow inward perpendicular to the cooling surface, so the prisms stand vertical in a flat flow that cooled from the top and bottom, and they tilt if the cooling surface is tilted. Devils Postpile in California is a famous example.

Pillow lavas. When lava erupts underwater, the cold water instantly chills its outside into a glassy skin. Lava pushes through the skin and forms a new rounded lobe, again and again, stacking pillow-shaped masses. Pillows are therefore evidence of an underwater eruption. When they appear in rock now high in a mountain range, such as the Coast Ranges of California, they show that the rock was once seafloor that was later uplifted. These slices of oceanic crust are called ophiolites.

Hotspots, magma plumbing and large igneous provinces

Same source, different volcano. Mantle melting makes basaltic magma. What happens next depends on the plumbing. If magma rises quickly through open conduits, it erupts as fluid basalt and builds a shield or a cinder cone. If it stalls in a crustal chamber for thousands of years, it cools, crystals settle out (differentiation), and the leftover liquid becomes silica-rich, viscous and gas-rich, so it feeds explosive eruptions and domes. A volcanic system is a network: mantle source, deep reservoirs, shallow chambers, conduits and dikes. Volcanoes on the same hotspot can have different plumbing because of crust thickness, magma supply rate and fractures.

Large igneous provinces (LIPs). Some eruptions are enormous. Flood basalts such as the Columbia River Basalt Group (Washington, Oregon and Idaho), the Deccan Traps and the Siberian Traps pour out hundreds of thousands to millions of cubic kilometers of fluid basalt from fissures in about a million years or less. The CO₂ and SO₂ they release can change climate and ocean chemistry, and some LIPs coincide with mass extinctions. Mid-ocean ridges also make huge volumes of basalt, but along about 65,000 km of ridge and over hundreds of millions of years, so the rate at any one place is low, and their gases are released into the deep ocean and balanced by the carbon cycle.

Back to the start: granite and basalt differ in their cooling rate (and in their composition). Kīlauea's lava flows are basaltic and fluid, whereas silica-rich magmas like the one at Mount St. Helens, being viscous, produce explosive eruptions.

Summary