When we observe a volcanic eruption, we most commonly use the terms lava and magma. However, many people don't know what volcanic magma is and how it differs from lava. To better understand volcanic phenomena, it's important to know the difference between magma and lava and how they interact in the context of an eruption.
For this reason, we are going to dedicate this article to telling you what the magma of volcanoes is, how it originates and what its characteristics are.
What is the magma of volcanoes
The word magma comes from the Greek and can be translated as "paste". Volcanic magma is a mixture of molten rock and volatile solid compounds that forms within the Earth. The substance is very unstable and may contain air bubbles and suspended crystals. Lava is easily found in volcanic chambers and can also be found in nearby rocks. Once the magma cools and crystallizes, it forms igneous rocks.
The compounds that make up magma range in temperature from 700°C to 1.300°C. These high temperatures can only be reached in Earth's subduction zones, that is, at mid-ocean ridges, continental regions, and other hotspots. The magma formation process is quite complex, which we will explain later. To learn more about these areas, you can read about the types of volcanoes and their geological classification.
magma types
Quite a few types of magma can be defined. However, the most common are three:
basalt magma
Basaltic magmas arise from the amalgamation of ultrabasic rocks, although their composition varies depending on the region of formation. They are low in silica (-50%) if they originate from mid-ocean ridges, and more alkaline and rich in sodium and potassium if they originate from the interior of tectonic plates. They are the most common type of magma and are fundamental in the formation of volcanic soils.
andesite magma
Andesite magma forms in subduction zones of the continental and oceanic crust and contains up to 60% silica and hydrated minerals such as hornblende and biotite. Andesite magma is the richest in water, but when it erupts, the water evaporates as steam. When this magma crystallized at depth, it formed diorite, and the water became part of the hornblende.
granite magma
This magma has the lowest melting point and can crystallize into large chunks of plutonic rock. It forms in orogenic belts like andesite, but from andesitic or basaltic magma that managed to penetrate and melt the sedimentary or igneous rocks of the Earth's crust . These rocks change the composition of the magma as they dissolve into it.
Where is it located
Magma originates in those regions of the Earth's crust and upper mantle where temperatures reach temperatures at which the rock-forming minerals begin to melt. However, the melting temperature also depends on other factors such as pressure or the presence/absence of water.
Therefore, increased pressure combined with a lack of water hinders melting, as occurs in the Earth's depths, for example. Conversely, the presence of water lowers the melting point of rock. Consequently, magma only forms and remains (unless it escapes) where conditions are favorable for its formation, such as the crust and upper mantle.
When a volcano erupts, magma is expelled as lava. Because lava crystallizes quickly, it forms fragments of volcanic glass, such as obsidian or pumice, instead of large crystals. This contrasts with the lava found in other, more fluid types of eruptions. For more information on these volcanic phenomena, you can consult the article on the relationship between earthquakes and volcanic eruptions to learn more about how earthquakes and eruptions are related.
How does magma form in volcanoes?
Magma gradually forms when the rocky material that makes up our planet melts. The rocks of our planet are made up of minerals with different melting points and different physical and chemical properties, which condition the formation of different types of magma. The high pressure inside the Earth determines the softening of the solid components.
As we have already noted, in liquid magma complexes the temperature is very high, between 700°C and 1.300°C. Generally, the composition of different types of magma is related to the environment in which it formed, usually subduction zones such as mid-ocean ridges, continental regions, and other critical points on Earth. This understanding is critical for comprehending volcanic activity in different regions of the world, which can range from volcanic activity in Indonesia to phenomena such as the possibility of eruptions in a warmer world.
There are two reasons why magma ceases to exist: one is the evolution of its energy through crystallization, and the other is that it is expelled as lava during volcanic eruptions. In both cases, upon solidification, it produces igneous rocks such as diorite, basalt, or granite.
Differences between magma and lava
The difference between magma and lava is location. When geologists talk about magma, they are referring to lava that is still trapped underground. If this molten rock reaches the surface and continues to flow as a liquid, it is called lava.
As it rises up the volcano, the magma or molten rock breaks the rock around it, creating small waves that are measured by seismographs and, depending on their strength, earthquakes can be felt on Earth. In the case of Cumbre Vieja, more than 25.000 earthquakes formed a more active swarm than usual, heralding a possible eruption. Subsequently, the National Geographic Institute reported more than 2.600 earthquakes in the region since the volcano woke up.
Magmas, or lavas, vary in their chemical composition, which gives them and the volcanoes that contain them different properties. In the case of Cumbre Vieja, the alternation between Strombolian and Hawaiian phases determines the development of its flow. The first lava fields that formed were of the Malpas type, where the lava fragmented and cooled rapidly. To learn more about this phenomenon, you can find more information about what lava is and its relationship to magma.
These lavas do not form tubes; to form tubes, hot lava is needed flowing at a constant and very gentle speed, melting and eroding the ground inside, making these tubes drains that reactivate the flow front.
As you can see, these confusions often occur when talking about volcanic eruptions and the news is booming. However, it is quite simple to know the terms and what are the differences between them. I hope that with this information you can learn more about what the magma of volcanoes is, its characteristics, origin and differences with lava.
