Volatile (astrogeology) facts for kids
Volatiles are special chemicals that can easily turn into a gas. Think of water boiling into steam – that's a volatile changing form! Things that don't easily turn into gas are called refractory substances.
On Earth, we often talk about volatiles found in hot, melted rock called magma. In space science, volatiles are studied in the outer layers or air of planets and moons. Common volatiles include nitrogen, carbon dioxide, ammonia, hydrogen, methane, sulfur dioxide, and water.
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Volatiles in Space
Scientists who study planets often sort volatiles into two groups: "gases" and "ices."
- Gases are volatiles with super low melting points, like hydrogen and helium.
- Ices are volatiles that melt at slightly warmer temperatures, above about 100 K (which is –173 °C or –280 °F).
It's a bit tricky because these "gases" and "ices" can actually be solid, liquid, or gas depending on the conditions. For example, Jupiter and Saturn are called gas giants, and Uranus and Neptune are ice giants. But inside these planets, most of their "gas" and "ice" is actually a very hot, thick liquid!
Far out in space, beyond Jupiter's path, comets become active when their water ice turns directly into gas. Even super-volatiles like carbon monoxide and carbon dioxide can make comets active very far from the Sun, up to 25.8 AU (3.86 billion km) away.
Volatiles in Volcanoes
In the study of rocks formed from magma, volatiles are the parts of magma (mostly water vapor and carbon dioxide) that affect how volcanoes erupt.
How Volatiles Affect Eruptions
When magma has a lot of volatiles and is thick (like felsic magma with lots of silica), it tends to cause explosive eruptions. Imagine shaking a soda bottle – the gas wants to escape quickly!
If magma has a lot of volatiles but is thin (like mafic magma with less silica), the gases can escape more easily. This leads to gentler eruptions and can even create lava fountains.
Some volcanic eruptions are explosive because water mixes with magma near the surface, releasing a lot of energy suddenly. But often, the eruption is caused by gases already mixed into the magma itself.
As magma rises closer to the surface, the pressure drops. This causes the dissolved gases to come out of the magma, forming bubbles. These bubbles grow and connect, creating a network. This makes the magma break apart into small drops or sprays, or even clots of liquid in the gas.
Most magma (95-99%) is liquid rock. But even a small amount of gas can create a huge volume when it expands at the surface. This is why gas is so important for making volcanoes erupt explosively. Magma deep inside the Earth's mantle and lower crust has a lot of volatiles.
Besides water and carbon dioxide, volcanoes also release other gases like hydrogen sulfide and sulfur dioxide. Sulfur dioxide is common in basaltic and rhyolite rocks. Volcanoes also release a lot of hydrogen chloride and hydrogen fluoride as volatiles.
How Volatiles Dissolve in Magma
Three main things affect how volatiles spread out in magma:
- The surrounding pressure.
- The type of magma.
- The temperature of the magma.
Pressure and magma type are the most important. To understand how magma behaves as it rises, we need to know about solubility. Solubility is how much of a substance can dissolve in another.
For example, water dissolves better in rhyolite magma than in basalt magma. Both types of magma lose water as they rise to the surface because the pressure drops.
Magma is undersaturated if it has less water than it can hold. Deep in the Earth, magma is usually undersaturated with water and carbon dioxide. Magma becomes saturated when it holds the maximum amount of water it can. If it keeps rising and more water tries to dissolve, it becomes supersaturated. When magma is supersaturated, the extra water can turn into bubbles or water vapor and be ejected.
Carbon dioxide is less soluble than water in magma, so it tends to form bubbles deeper down. This means carbon dioxide bubbles often start forming before the magma even reaches the main magma chamber. These carbon dioxide bubbles can then rise to the top of the chamber and leak out through cracks. During an eruption, magma usually loses more carbon dioxide than water. Overall, water is the main volatile that drives most eruptions.
How Bubbles Form (Nucleation)
Bubbles start to form when a volatile becomes saturated (meaning the magma can't hold any more of it). This process is called nucleation. The molecules of the volatile gather together to form tiny bubbles.
Sometimes, the surface tension of the liquid tries to shrink the bubbles back into the magma. However, bubbles form more easily in irregular spaces. This is why solid crystals, which are often found in magma chambers, are perfect places for bubbles to start forming. They are like tiny starting points for bubbles.
If there are no good places for bubbles to start, they might form very late, and the magma can become extremely supersaturated. If magma rises quickly to the surface, it becomes more out of balance and supersaturated. When magma rises, there's a race between adding new molecules to existing bubbles and creating brand new bubbles. Crystals inside the magma can really affect how bubbles grow and form.
See also
In Spanish: Volátiles para niños
