How does lava solidify?

Answer

Dear Brit,

Thank you for your interesting question!

Rocks from the Earth’s mantle (50-200 km deep below the Earth’s surface) or the Earth’s crust (a few tens of km deep) can indeed melt, for example by increasing its ambient temperature, or a decrease in the pressure under which it is located, or a supply of water in the rock at subduction troughs that lower the melting temperature of rock (eg the Mariana Trench or under the Andes).

This molten rock that is deep below the earth’s surface is called magma. That magma has a lower density than the surrounding mantle or crustal rock and therefore rises. During that ascent, the temperature and pressure drop from the surrounding rock. It is possible that at some point the magma will find a level where its density is equal to the surrounding rock, and it will then stop rising.

In this way we think that ‘magma chambers’ are formed in the Earth’s crust, down to a few kilometers below the surface. If the magma can stay there long enough, the drop in temperature and pressure will cause it to slowly cool down. The solidification then occurs through the slow growth of crystals of certain chemical composition in the magma.

If enough time goes into this process, crystals of a size that are easy to see with the naked eye can grow. During this process, gases are also released from the magma, which collect in the upper parts of the magma chamber and slowly but surely increase the pressure there. The formed crystals are heavier than the magma and will therefore slowly sink to the bottom of the magma chamber. This way you form a layered structure, in which the lower part is a mix of large crystals with a little magma, above that the still liquid magma, and finally the upper part with gases. Sometimes the magma chamber cools so slowly, and the gases are gently discharged to the surface. We are talking about a process that takes thousands or even tens of thousands of years. Usually, however, at some point the pressure becomes so great that cracks suddenly appear in the roof of the magma chamber, and magma continues to make its way to the surface to give rise to a volcanic eruption.

When magma comes to the surface during an eruption, we speak of lava. The hot lava has temperatures of 700-1200°C at the time of eruption, depending on its composition. When it comes into contact with the much colder air of the atmosphere (average 10-30°C), it will cool much faster than in a deep magma chamber. The outer part of the lava solidifies almost immediately and thus forms glass, as there is no time for crystals to grow. For example, the inner part of a lava flow solidifies in a few days, and will only have time to form microscopic crystals embedded in a glassy matrix. Because the cooling goes so fast, there is usually no time for gases to escape either. These will come out of solution from the magma, but the bubbles will not have time to move out. They thus remain trapped in the cooled rock as spherical small cavities. Just think of pumice made up of many of those bubbles, with a little cooled and solidified magma holding it all together.

Hopefully I was able to answer your question with this! Don’t hesitate to contact me again if you want more details or if something turned out to be unclear!

Many volcanic greetings!

Sam Poppe