Magma facts for kids

Magma is found deep inside Earth, in the mantle or crust.

Magma is the super hot, melted rock found deep inside Earth. It's the natural material that forms all igneous rocks, which are rocks made from cooled magma. People sometimes mistakenly call magma "lava", but lava is what magma becomes when it erupts from a volcano onto the Earth's surface.

Magma isn't just liquid rock; it can also contain tiny solid crystals and gas bubbles. We've found signs of magma not only on Earth but also on other planets and moons.

Magma forms when parts of Earth's mantle or crust melt. This happens in special places like where tectonic plates crash together (subduction zones), where they pull apart (rift zones and mid-ocean ridges), or over very hot spots deep underground. Once formed, magma slowly moves upwards through the crust. It can gather in large underground spaces called magma chambers.

While stored, magma can change its makeup. It might mix with other melted rocks or lose some of its gases. Eventually, magma can either erupt from a volcano as lava or cool down and harden underground. When it hardens underground, it forms different types of intrusions, like dikes or plutons. Scientists have even found magma directly during geothermal drilling projects a few times, like in Iceland and Hawaii!

How Magma Forms Deep Inside Earth

The inside of Earth is very hot, but rocks usually need special conditions to melt. Magma forms when rocks melt due to changes in pressure, the addition of water or carbon dioxide, or a big increase in temperature.

Melting from Pressure Changes

Imagine a rock deep inside Earth. It's hot, but the immense pressure keeps it solid. If this rock moves upwards, the pressure on it decreases. Even though it cools a little, the drop in pressure can be enough to let it melt. This is called decompression melting.

This process is super important for creating new oceanic crust at mid-ocean ridges. It also causes volcanoes in places far from plate boundaries, like in Europe or Africa.

Water and Carbon Dioxide's Role

Adding water to rocks can make them melt at much lower temperatures. Think of how salt melts ice faster. In subduction zones, where one plate slides under another, water is squeezed out of the sinking plate. This water then rises into the overlying mantle, causing the rocks there to melt and form magma.

Carbon dioxide can also help rocks melt, though it's less common than water. It can lower the melting temperature of rocks deep in the mantle. This can lead to special types of magma that form unique rocks like carbonatite.

Melting from Heat Increase

Sometimes, magma forms simply because the temperature gets high enough to melt the surrounding rock. This often happens in the continental crust. For example, if hot magma from the mantle pushes its way up, it can heat and melt the crust above it.

Also, in places where continental plates collide and the crust becomes very thick, the extra weight and heat can cause rocks to melt. The Tibetan Plateau, with its incredibly thick crust, is a good example where scientists believe parts of the crust have melted.

How Rocks Melt Gradually

Rocks are usually made of several different minerals. Each mineral has its own melting point. So, when a rock starts to melt, it doesn't all turn to liquid at once. Instead, it melts over a range of temperatures. The first part to melt is a mix of minerals that has the lowest melting point.

This "partial melting" means that the first magma formed can have a very different chemical makeup than the original solid rock. For example, a small amount of melting from the mantle can create very different magmas than a larger amount of melting.

Magma's Amazing Properties

Magma is a complex mixture of liquid rock, solid crystals, and gases. Its properties, like how easily it flows or how hot it is, depend on what it's made of.

What Magma is Made Of

Most magma is rich in a compound called silica. The amount of silica is a key factor in how magma behaves. Besides silica, magma also contains elements like oxygen, aluminium, calcium, magnesium, iron, sodium, and potassium.

Scientists classify silicate magmas into four main types based on their silica content:

• Felsic magmas: These have the most silica (over 63%).
• Intermediate magmas: These have a medium amount of silica (52% to 63%).
• Mafic magmas: These have less silica (45% to 52%).
• Ultramafic magmas: These have the least silica (under 45%).

Felsic Magma: Thick and Explosive

Felsic magmas are like thick, sticky syrup because they have so much silica. They are very viscous, meaning they don't flow easily. When felsic magma erupts, it often causes explosive volcanoes, throwing out ash and rock fragments. Sometimes, it can form thick, slow-moving lava flows or domes. These magmas can erupt at temperatures as low as 800°C (1,472°F).

Intermediate Magma: In Between

Intermediate magmas have a medium amount of silica. They are less sticky than felsic magmas but still thicker than mafic ones. They often form steep, cone-shaped volcanoes, like those in the Andes mountains. These magmas are usually hotter, ranging from 850°C to 1,100°C (1,562°F to 2,012°F). Their viscosity is similar to thick peanut butter.

Mafic Magma: Runny and Flowing

Mafic magmas have less silica and are much more fluid, like ketchup. They erupt at higher temperatures, usually between 1,100°C and 1,200°C (2,012°F to 2,192°F). Because they flow easily, mafic lavas create broad, gently sloping shield volcanoes or vast flood basalts. They can travel long distances from a volcano's vent.

Ultramafic Magma: Super Hot and Fluid

Ultramafic magmas are the hottest and most fluid type, with very little silica. They contain a lot of magnesium. Some, like komatiite, are thought to have erupted at incredibly high temperatures, around 1,600°C (2,912°F). At these temperatures, they flow almost like motor oil. However, Earth's mantle has cooled over time, so we don't see these types of eruptions today. Most ultramafic lavas are from billions of years ago.

Special Magma Types

Besides the main silicate types, there are some unusual magmas:

• Carbonatite lavas: These are very rare and are mostly made of carbonate minerals. The Ol Doinyo Lengai volcano in Tanzania is the only active one. These lavas are extremely fluid, almost like water, and surprisingly cool, erupting at temperatures between 491°C and 544°C (916°F and 1,011°F).
• Iron oxide magmas: These magmas are rich in iron and are linked to some iron ore deposits. They erupt at temperatures around 700°C to 800°C (1,292°F to 1,472°F).
• Sulfur lavas: These are formed from melted sulfur deposits and can flow at very low temperatures, sometimes as low as 113°C (235°F).

Gases in Magma

Magma contains dissolved gases, much like a soda contains dissolved carbon dioxide. The most common gas is water vapor, followed by carbon dioxide and sulfur dioxide. Other gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride.

The amount of gas magma can hold depends on pressure, its composition, and temperature. Magma deep underground, under great pressure, can hold a lot of water. But as magma rises and pressure drops, these gases form bubbles and escape. This is what makes volcanic eruptions explosive!

How Magma Flows (Viscosity)

This graph shows how magma's stickiness (viscosity) changes with its silica content and temperature.

Viscosity is how thick or sticky a liquid is. It's a super important property for magma. Magma's viscosity depends mostly on its silica content and temperature.

Magmas with more silica are much thicker and flow slowly. Think of honey versus water. Felsic magmas, with high silica, are very viscous. Mafic magmas, with less silica, are much more fluid. Hotter magmas are also less viscous than cooler ones.

Most magmas also contain solid crystals. This makes them behave like toothpaste. They won't flow until a certain amount of force is applied. This is why magma can move in "plugs" rather than flowing smoothly like water. If magma has too many crystals (about 60% or more), it becomes a "crystal mush" and acts more like a solid.

Magma's Temperature

Magma is incredibly hot! The temperatures of lava erupting from volcanoes are usually between 700°C and 1,400°C (1,292°F and 2,552°F). However, some rare carbonatite magmas can be as cool as 490°C (914°F), while ancient ultramafic magmas might have been as hot as 1,600°C (2,912°F).

Scientists have measured magma directly a few times during drilling. For example, in Hawaii, they found dacitic magma at a depth of 2,488 meters (8,163 feet) with a temperature of about 1,050°C (1,922°F).

How Heavy is Magma? (Density)

Magma's density (how heavy it is for its size) depends mainly on its composition, especially its iron content.

Magma is less dense than the solid rock around it. This is why it floats upwards through the Earth's crust. As magma rises and gases bubble out, its density decreases even more, helping it push towards the surface.

What Happens as Magma Changes?

Magma is rarely just a pure liquid. It's usually a mix of melted rock, solid crystals, and sometimes gas bubbles. As magma moves and cools, it changes.

Crystals Forming in Magma

As magma cools, different minerals start to form solid crystals at different temperatures. This is like how water freezes into ice, but with many different minerals. The first crystals to form are usually denser, so they can sink to the bottom of the magma chamber.

When crystals separate from the magma, the remaining liquid magma changes its chemical makeup. This process, called fractional crystallization, is how one type of magma can create many different kinds of igneous rocks. For example, a magma that starts out forming gabbro can eventually produce a melt that forms granite.

Original Magma Types

When rock first melts, the liquid is called a primary magma. This magma hasn't changed its composition yet. It's hard to find primary magmas because they usually start changing as soon as they form.

Sometimes, scientists identify a parental magma. This is a magma composition from which a whole series of different rocks found in an area could have formed through processes like fractional crystallization.

Magma's Journey to the Surface

Magma forms deep inside the Earth where it's hot enough to melt rock. Because it's less dense than the surrounding solid rock, magma slowly rises towards the surface.

Magma Underground: Intrusive Rocks

As magma moves through the crust, it can collect in magma chambers. If it cools and hardens underground before reaching the surface, it forms intrusive rocks. These rocks cool very slowly, allowing large crystals to grow. Examples include gabbro, diorite, and granite.

Magma on the Surface: Volcanoes and Lava

If magma reaches the Earth's surface during a volcanic eruption, it's called lava. Lava cools much faster than magma underground. This quick cooling means that crystals don't have much time to grow large. Sometimes, the lava cools so fast that it forms volcanic glass, like obsidian or pumice.

Before and during an eruption, gases dissolved in the magma escape. If a lot of gas escapes quickly, it can lead to an explosive volcanic eruption.

Using Magma for Energy

Magma is a huge source of heat, and scientists are exploring ways to use it for energy. The Iceland Deep Drilling Project (IDDP) is a great example. In 2009, while drilling for geothermal heat, they accidentally hit a pocket of magma 2,100 meters (6,890 feet) deep.

This was a rare event, so the IDDP decided to study it. They built a special steel case in the hole near the magma. The superheated steam and fluids from the magma were used to generate 36 megawatts of electricity. This made IDDP-1 the world's first "magma-enhanced geothermal system," showing a new way to harness Earth's powerful heat!

Images for kids

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  A single silica tetrahedron

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  Two silica tetrahedra joined by a bridging oxygen ion (tinted pink)

See also

In Spanish: Magma para niños