Most people will have some vague of early geography lessons when they were taught the different types of volcanoes, and the different types of rocks that make up the Earth. At the same time teachers may well have taught the difference between lava and magma, although the difference is such that it may have been overlooked.
The basic premise behind the difference between lava and magma simply relates to where it is found. As the British Geological Society puts it; Magma is “molten rock…beneath the Earth’s surface”, whilst Lava is “molten rock (e.g. basalt) erupted from a volcano.
For most audiences it is sufficient to say that magma is the molten rock found beneath the Earth’s crust, whilst lava is the molten rock that is found on the surface of the Earth. There are though subtle differences between magma and lava, and indeed the full wording of the British geological Society reads, “Magma: molten rock with dissolved volcanic gases, beneath the Earth’s surface.”
Although the composition of the centre of the earth is not known with any great deal of certainty, it is fairly save to say that that between the mantle of the Earth and the crust there are often pockets of trapped molten rock. This molten rock, magma, or melt as it is also known, ranges in temperature from anywhere between 700C (1300F) and 1300C (2370F). The wide range in temperature is due to the conditions in which the magma forms, dependent on material composition, including dissolved gases, and pressure. This also makes a difference as to the viscosity of the magma.
Non-viscous magma will flow quite quickly, and like all liquids will seek a path of least resistance, which will see it flow through cracks, and make its way up to the Earth’s surface. The heat of the magma is often sufficient to melt surrounding rock, increasing the volume of magma. Pools of magma will also often form in a magma chamber beneath volcanoes, and if it continues to flow through cracks nearer the surface will result in a volcanic eruption. Often though, magma will cool to such a degree before it reaches the surface that it solidifies into intrusive rock formations. This may sound like a good thing, but can create plugs and result in a build up of magma pressure that results in even more violent eruptions.
When the magma reaches the surface of the earth, it changes name to become lava, although like magma, the lava itself may have different degrees of viscosity. This again has a great deal to do with the temperature of the molten rock. Low viscosity lava has the potential to flow a number of miles quite quickly, resulting in long expanses of lava flows before it cools and solidifies. Highly viscous lava though will not flow very fast and will normally solidify close to the point of release.
Most lava flows are basalt magma, which has a temperature upon eruption of about 1200C (2200F), and the temperature is sufficient to destroy everything in its path as it flows. Few people though are killed by lava, and most able bodied people will have plenty of time to run and dodge it. The majority of deaths directly from volcanic eruptions comes from the clouds of poisonous gases, not the lava.
Additionally when magma reaches the surface the majority of dissolved gases are seen to fizz out of the molten rock, which is why there is a subtle difference in the composition of magma and lava.
In most cases it is sufficient to say that the difference between lava and magma relates to where it is found, above or beneath the Earth’s surface, but it is important to remember the subtle difference of its composition as an additional difference.