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Magma and Lava

Magma and lava are both forms of melted rock. The difference between them is based upon their location. Magma is found below the Earth’s surface while lava is found on top of it. Still, their location actually affects their properties. That’s because it’s far colder above the crust, and there’s also much less pressure.

What is the Difference Between Magma and Lava?

As we’ve said, magma and lava are almost the same thing. Lava is magma that has reached the Earth’s surface. Even so, we see important differences between the two. That’s because magma is subjected to extreme heat and pressure.
Compared to magma, lava doesn’t last very long. It’s cooled by air above the crust. This causes it to rapidly harden into igneous rock.

Pumice, a rock made from lava. It looks like it could come from outer space!

Oppositely, magma will stay hot, having nothing to cool it down. This allows it to melt through solid rock. Lava can’t do this because it takes heat to melt something else. The lava would get cold and harden even faster. But, magma’s heat is always being restored. That’s because it’s very hot deep into the earth. This is especially true of the mantle, where most magma comes from.
Magma’s melting ability is an important part of the rock cycle. The asthenosphere, a layer of magma within the earth, acts like a 24/7 furnace. Tectonic plates are pushed into this furnace during a process known as subduction. Subduction melts the igneous and metamorphic rock which make up the plates. This forms new magma. At the same time, magma pops up at ocean ridges as lava. This creates new igneous rock.
A diagram explaining subduction.

A diagram displaying the process of subduction.

How hot is magma?

Most magma is between 1300 and 2400 degrees Fahrenheit. There are types of magma which are colder or hotter, but they are quite rare.

Properties Resulting from Pressure

Another thing we should note is that magma is under immense pressure. As we descend into the Earth’s layers, pressure increases. That’s because each layer is pushing down on the ones below it. This pressure is often strong enough to push matter together. It’s why, for example, the hot inner core can remain solid.

Is Magma a Liquid?

This is a tricky question. The answer is: Yes…sort of. Magma is a fluid. It flows instead of keeping shape. But, it flows very slowly, almost as if it’s still somewhat solid. 
To make sense of this, scientists sometimes refer to magma as a plastic instead of a liquid. By this, they mean that magma thins out (or flows) slowly. It’s kind of like a strip of plastic that’s being pulled apart.
Plastic film

Plastic film being stretched. Magma moves in a similar fashion.

What’s more, magma’s plasticity has huge consequences for our planet. Plate tectonics relies on magma in the asthenosphere. The plates drift over the ‘fluid’ magma. But, this movement is VERY slow. That’s because the fluid they’re drifting on isn’t a fluid at all. It’s more like a plastic. Contrast this with lava which flows around whole islands before it cools.

What is Magma? – Magma Composition

At the beginning, we said that magma is a kind of melted rock. But, that’s a simplification. For starters, not all magma contains the same sorts of rock. This matters a lot because it determines how plastic a magma is. Fortunately, we can classify magma composition by silica content.
A volcanic eruption.

A volcano erupting. How it erupts depends on what kind of magma is inside.

Silica is a mineral made-up of the elements silicon and oxygen. It’s found in many different kinds of rocks, and in different amounts. We use silica content as shorthand for which kinds of rock make-up a magma. The more silica magma contains, the slower it flows. Another way to say this is that the magma becomes more viscous.
Magmas that are more viscous have more explosive volcanic eruption types. That’s partially because it builds up inside of the volcano instead of flowing out smoothly. Additionally, viscous magmas contain lots of gas bubbles. It’s harder for gas to escape when it’s surrounded by silica.

Other Great Resources:

5 Differences Between Lava and Magma: https://differencecamp.com/lava-vs-magma-differences/
Magma Facts by Kiddle: https://kids.kiddle.co/Magma
Written by: Noah Louis-Ferdinand.