Magma

Magma is the hot, molten material that comes from deep inside the Earth. It is made when rocks melt deep below the surface, forming a mix of liquid rock, tiny crystals, and gas bubbles. Magma is different from lava—lava is what we call magma when it reaches the surface through a volcano.

Magma forms in several places inside the Earth, such as where ocean plates dive under other plates, along rift valleys where continents pull apart, and at spots called hotspots. As magma moves upward, it can change, mixing with other melted rocks or losing gases. Sometimes it reaches the surface as lava, but other times it cools deep underground, creating different kinds of rocks and structures.

Scientists have only seen magma directly a few times, during drilling projects in places like Iceland and Hawaii. Studying magma helps us understand how volcanoes work and how new land forms on Earth.

Physical and chemical properties

Magma is made of liquid rock that often has tiny solid pieces floating in it. As magma moves closer to the Earth's surface, the pressure drops, and gases that were dissolved in the liquid start to bubble out. This means that magma near the surface contains solid pieces, liquid, and gas.

Most magma is rich in a substance called silica. Silica helps decide many of magma's properties, like how thick or thin it is and its temperature. Magmas can be grouped into four types based on how much silica they have: felsic, intermediate, mafic, and ultramafic.

Felsic magmas have the most silica and are very thick, like cool honey. They often erupt explosively. Intermediate magmas have a bit less silica and are thinner than felsic magmas. Mafic magmas have even less silica and are thinner still, flowing more easily. Ultramafic magmas have the least silica and are the thinnest, flowing very easily.

Origins

The temperature inside Earth changes with depth, called the geothermal gradient. This gradient happens because of a balance between heat from radioactive decay deep inside Earth and heat loss at the surface. On average, the temperature increases about 25°C for every kilometer deeper in the Earth's upper crust, but this can vary. In some places, like mid-ocean ridges or near mantle plumes, the gradient can be much steeper.

Rocks melt for several reasons: a drop in pressure, adding water, an increase in temperature, or a mix of these. For example, when rock rises from deep within Earth, the pressure drops, and the rock can begin to melt. Water also plays a big role—it lowers the temperature at which rocks melt. This is why we find magma in places where water is released from sinking ocean plates.

Decompression melting is the main way magma forms from the upper mantle. When solid rock rises, it cools slightly but not enough to stay solid, so it begins to melt. This process creates the ocean crust at mid-ocean ridges and causes volcanism in places like Europe and Africa.

Adding water to rock lowers its melting point a lot. For example, with water, certain rocks start to melt at about 800°C, but without water, they need temperatures close to 1,500°C. Water comes from the ocean floor when plates move, helping create magma that forms island arcs.

Increasing temperature is another common way magma forms in the continental crust. This can happen when hotter magma from deeper in Earth pushes upward or when the crust gets thicker and traps heat. For example, the Tibetan Plateau has very thick crust, and studies show melted rock deep below it.

When rocks melt, they do so over a range of temperatures because they are made of many minerals, each with its own melting point. The first melt that forms has a specific mix of minerals, called the eutectic. As more heat is added, the melt changes in composition. This process explains why different types of magma have varied chemical makeups.

Certain elements, like potassium and the rare-earth elements, don’t fit well into the structure of solid minerals and end up in the melt. These elements can become very concentrated in magma formed from only a small amount of melted rock.

Evolution of magmas

Most magmas are only fully melted for short periods. They are usually mixes of melted rock, crystals, and sometimes gas bubbles. These parts can separate as magmas change over time because they have different weights.

As magma cools, different minerals turn into solid pieces at different temperatures. This process is like the opposite of how the magma formed. When crystals form and stay in the melted rock, the overall mix stays the same. But if the crystals sink to the bottom, the remaining melted rock changes. This can make new types of rock from the original magma. For example, one kind of magma can change into a much hotter or cooler type of melted rock as it cools and crystals form.

Magmas can also change by mixing with other rocks or melting them. Sometimes, different magmas can mix together. In very rare cases, melted rock can split into two very different kinds of melted rock.

Migration and solidification

Magma forms deep inside the Earth in places called the mantle or crust where it is hot and pressurized enough to melt rock. Once it forms, magma moves upward because it is lighter than the surrounding rock. It can collect in special areas known as magma chambers, where it might stay for a while.

When magma cools down in these chambers, it turns into solid rocks. If it stays underground, it forms rocks like gabbro, diorite, and granite. If it erupts from a volcano, it becomes rocks such as basalt, andesite, and rhyolite. When magma erupts, it is called lava, which cools quickly and can form glassy rocks like obsidian, scoria, and pumice.

Main article: Volcanism

Use in energy production

The Iceland Deep Drilling Project tried to use heat from deep underground in Iceland to make energy. In 2009, while drilling, they found magma much closer to the surface than expected. They turned this discovery into a way to make electricity, producing 36 MW of power. This was the first time anyone had used magma to help create energy.