Geoscience · BEGINNER LESSON
MAGMA
Magma is molten rock underground. In crustal storage, it can be a mixture of melt, crystals, and dissolved or exsolved volatiles, and scientists measure those parts plus pressure and temperature.
What magma is and what scientists measure
Magma is molten rock below a planet’s surface. In the crust, it may sit in a magma reservoir, which is a region that contains melt and may also contain crystals and gas-rich fluid. Researchers measure how much melt is present, how many crystals are mixed in, how much volatile material such as water and carbon dioxide is dissolved or has separated into gas, and the pressure and temperature where the magma exists. Those measurements matter because magma can move, erupt, or freeze into solid rock.
[1] Architecture and dynamics of magma reservoirs - PMC - NIH [2] Geologists make magma in the lab [3] What Is a Volcano?
How magma changes underground
Magma does not stay chemically or physically fixed. As crystals grow, the remaining liquid can change by fractional crystallization, and different magmas can also mix. In crystal-rich mush, melt can be pushed out by compaction or by gas-driven filter pressing, creating more liquid-rich pockets. Volatiles are especially important because they affect density, phase changes, and viscosity, which is a measure of how easily a material flows.
[1] Architecture and dynamics of magma reservoirs - PMC - NIH
Why rising magma can erupt
As magma rises, pressure drops. Dissolved water can then exsolve, meaning it separates from the melt as a gas. One source describes this as boiling off during ascent, and it can happen strongly in subduction-zone volcanoes, where water released from a sinking plate helps generate magma. Loss of pressure and gas exsolution help drive eruptions by changing the magma’s physical behavior.
What happens if magma does not erupt
Magma is not guaranteed to reach the surface. Some reservoirs feed eruptions, but many freeze underground and form plutons, which are bodies of solid igneous rock. The source material also notes that a large share of magma supplied to the crust is emplaced underground, so solidification is a common end point. In some places, crystallizing sills can even heat surrounding crust enough to cause melting.
[1] Architecture and dynamics of magma reservoirs - PMC - NIH
Limits of what we know
Magma reservoirs can contain regions with very different properties, so one sample may not represent the whole system. The sources also show that researchers infer magma behavior from laboratory experiments, erupted rocks, and exposed ancient systems, because the underground environment is hard to observe directly. That means models are useful, but they simplify a complex and changing system.
[1] Architecture and dynamics of magma reservoirs - PMC - NIH [2] Geologists make magma in the lab
SEE THE IDEA
Magma underground: storage, rise, and outcomes
Rock becomes partially or wholly molten underground, often with added water that promotes melting.
The magma may sit as melt plus crystals and gas-rich fluid in a storage region.
Crystallization, compaction, mixing, and volatile separation alter composition and flow.
Lower pressure can make gas come out and help eruptions, or the magma can cool into pluton rock.
Magma evolves underground; pressure, crystals, and volatiles steer it toward eruption or solidification.
A simple path from melt generation to either eruption or freezing below ground.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.