How Does Magma Form? (Composition)

Magma Formation

Magma is a boiling liquid and semi-liquid rock. It usually consists of silicate liquid, iron, magnesium, calcium, potassium, etc. It also contains dissolved gases such as water vapor, carbon dioxide, and sulfur. Today we are discussing the magma formation. Let’s start!

What is Magma?

Magma is a wholly or partially molten rock stored under the earth’s surface. If this reaches the surface, it will be called or referred to as lava. Lava is also characterized as lava tube and lava flow, which we discussed in the previous post.

Origin: Most of the time, Magma is found in Earth’s uppermost mantle. The greatest quantities are produced in divergent and convergent plate boundaries or hot spots.

Composition of magma: Magma consists of three components. It consists of a liquid component, a salt component, and a gaseous component. The liquid component is mostly mobile ions of the eight most common elements found in Earth’s crust.

So silicon, oxygen, aluminum, potassium, calcium, sodium, iron, and magnesium are the most common elements in Earth’s crust. They are the ayahs that find in magma. The gaseous component of magma is mostly water vapor, carbon dioxide, or sulfur dioxide.

Types of magma: There are four types of magma.

  1. Ultramafic – Silicon dioxide 45%, Very high in Iron, Magnesium, and Calcium, and Low in Potassium, Sodium.
  2. Mafic or Basaltic – Silicon dioxide 45-55%, Very high in Iron, Magnesium, and Calcium and Low in Potassium, Sodium.
  3. Intermediate or andesitic – Silicon dioxide 55-65%, Iron, Magnesium.
  4. Felsic or Rhyolitic – Silicon dioxide 65-75%, Low in Iron, Magnesium & High in Potassium, Sodium.

Characteristic of magma: Viscosity is the resistance to flow. If magma is very molten, it will flow quickly and not be very viscous. But if you have magma that is not very hot, it will not flow very quickly.

The high percentage of silicon dioxide or sulfur dioxide in the magma contains higher viscosity. Lower temperature magmas have a higher viscosity, and higher temperature Magnus has lower viscosities.

Magma crystallization: Magma becomes rock through crystallization. When energy is removed, the atoms pack and turn from gaseous to liquid. Then it goes from a liquid state to a solid state. The ions and the atoms will pack when it cools or loses all their heat and energy. Then It creates crystals.

How Does Magma Form?

Magma is formed through a process known as magma generation or magmatism. It occurs beneath the Earth’s surface and involves the melting of rocks in the Earth’s mantle and crust. Several processes can lead to magma formation:

Decompression melting: This occurs when the pressure on a rock decreases, typically due to the upward movement of tectonic plates or mantle plumes. As the pressure decreases, the melting point of the rock decreases, causing it to melt and form magma.

Flux melting: When water or other volatile substances are introduced into the Earth’s mantle or crust, they can lower the melting point of rocks. This process is known as flux melting. Water-rich fluids are often released during subduction, where one tectonic plate moves beneath another. These fluids can cause the overlying rocks to melt and generate magma.

Heat transfer: Magma can also form through heat transfer from a nearby magma body or the Earth’s mantle. The heat raises the temperature of the surrounding rocks, causing them to melt and form magma.

Once magma forms, it is less dense than the surrounding rocks, so it rises toward the Earth’s surface. Then it moves upward, the magma can undergo further changes, such as crystallization, differentiation, and mixing with other magmas. When magma reaches the surface, it is called lava, and it can erupt through volcanic vents or fissures, resulting in volcanic activity.

Magma formation, at first, needs specific conditions like hot enough rock to rise above the melting point of the minerals. Depending on the type, it makes up that rock, usually between 800 to 1900 degrees Celsius. It will take more heat to make the rock melt at higher pressures.

Magma Formation
Magma Formation

The pressure tends to inhibit the melting of the asthenosphere. The asthenosphere moves up into this lower-pressure region. It’s still hot, which a temperature is 1300 Celsius or higher.

Magma can lose heat simply by being erupted onto the earth’s surface. Magma will contact the atmosphere or the oceans when it erupts onto the earth’s surface. In both cases, hot magma comes into contact with something quite cold.

It means lava will quickly lose heat to the atmosphere or the oceans through conduction and radiation. Magma will cool down very fast and solidify to give solid volcanic rock.

How does nature create magma? Nature makes magma in three different ways.

i) The first one results from decreased pressure without increasing temperature, which can melt decompression. So decompression melting can create magma.

ii) The other way data creates magma is by introducing water or sometimes impurities. It can sufficiently lower the melting temperature of hot mantle rocks to generate magma.

iii) The third-way nature creates magma is by heating crustal rocks above the melting temperature. When this heat is transferred to the rocks surrounding it, it causes them to melt and rise to the surface.

If magma crystallizes inside the earth without coming out on the surface, it is called magma and intrusive or plutonic igneous rock. If magma comes out and cools down on the surface, it creates volcanic or extrusive igneous rocks. Also, if a volcano erupts, all the molten rock that comes out is called lava, not magma.

How is magma formed? Magma can be formed by melting the earth’s crust or within the mantle. Crust and mantle are almost entirely solid, indicating that magma only forms in special places where pre-existing solid rocks melt. There are three conditions for forming magma.

1. Temperature: A rising magma from the mantle brings the heat with it and transfers heat to their surrounding rocks at shallower depths which may melt.

It will melt whenever the temperature is high enough for the rock to hit the melting point. But that melting point depends on how much pressure the rock is under. Different layers act differently if it’s under a lot of pressure and the melting point increases.

2. Pressure: Melting occurs due to a decrease in pressure. It is also called decompression melt. So the decrease in pressure affecting a hot mantle rock at a constant temperature permits melting, forming magma. When pressure is decreased, melting can occur because the bonds between particles can be broken down.

3. Volatiles: Adding volatiles like water decreases rock’s melting point. At convergence zones, the subducting plate heats, causing a release of water. It decreases the melting point of the surrounding rock. This rock melt generates magma.

There are 2 processes referred to for magma formation.

1. Decompression melting: Decompression melting creates magma by reducing pressure at a constant temperature. It occurs at divergent boundaries where tectonic plates separate.

2. Flux melting: Flux melting occurs after introducing volatile, breaking the rock’s chemical bond. Also, It occurs when water or carbon dioxide is added to rock. These compounds cause the rock to melt at lower temperatures.

Magma can also create when hot, liquid rock enters Earth’s cold crust. As the liquid rock solidifies, it loses heat to the surrounding crust.

Why does magma rise to the surface?

Magma is liquid, and magma rocks are solid salt materials denser than liquid materials. Therefore the liquid material will rise to the top, and the solid material will stay at the bottom. That’s why magma rises to the top. Magma is less dense than the solid rocks around it. That’s why it will rise to the top.

Read More: Why Do Volcanoes Erupt?


BOWEN, NORMAN. “MAGMAS. ” Geological Society of America Bulletin.
Greeley, Ronald; Schneid, Byron. “Magma Generation on Mars: Amounts, Rates, and Comparisons with Earth, Moon, and Venus.” Science.
Spera, Frank, “Physical Properties of Magma,” in Sigurdsson, Haraldur (editor-in-chief) (ed.), Encyclopedia of Volcanoes, Academic Press.
Foulger, G.R. Plates vs. Plumes: A Geological Controversy. Wiley–Blackwell.
Detrick, R. S.; Buhl, P.; Vera, E.; “Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise.”
Sparks, R. Stephen J.; Cashman, Katharine. “Dynamic Magma Systems: Implications for Forecasting Volcanic Activity.”

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher.I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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