1/6 Igneous Rocks

Magma

Molten rock material consisting of liquid rock and crystals.

Basaltic magma

Very hot (900–1200°C), very fluid (low viscosity), low SiO₂ (~50%), low volatile gases (“dry melt”), produces flowing/effusive eruptions.

Silicic magma

Less than 850°C, highly viscous, high SiO₂ (~65–77%), volatile gases may reach ~15% (H₂O, CO₂) (“wet melt”), produces explosive eruptions.

Intrusive

Igneous rock formed beneath Earth’s surface (plutonic).

Extrusive

Igneous rock formed on Earth’s surface (volcanic).

Most common intrusive rock

Granite.

Most common extrusive rock

Basalt.

Decompression melting

Melting caused by a decrease in pressure while the rock stays hot.

Flux melting

Melting caused by adding volatiles, which lowers a rock’s melting temperature.

Heat transfer melting

Complete melting caused by contact with hot magma.

Why different magma compositions form

(1) Source rock type (2) Amount of partial melting (3) Whether/how much assimilation occurred.

Common elements in igneous rocks

Si, O, Al, Ca, Na, K, Fe, Mg.

What controls magma physical properties

SiO₂ and water control density, viscosity, and extrusion style.

Intrusive rocks most likely found in

Continental crust (granite is common in continental crust).

Extrusive rocks most likely found in

Ocean crust (basalt is common in ocean crust).

higher levels of silica(SiO₂) can affect density, viscosity, and eruption style in what way?

Higher silica levels increase viscosity, but lower density, with explosive eruptions.

Lower levels of silica(SiO₂) can affect density, viscosity, and eruption style in what way?

Lower silica levels decrease viscosity but increase density, leading to less explosive eruptions.