Science Facts

How Does Magma Form? – Composition & Characteristic

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

Origin: Magma, most of the time, is found from Earth’s uppermost mantle. The greatest quantities are produced divergent and convergent plate boundaries or in 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 found 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, Calcium & Low in Potassium, Sodium.
  2. Mafic or Basaltic – Silicon dioxide 45-55%, Very high in Iron, Magnesium, Calcium & 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 it’s not 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 taken away, the atoms will pack and turn from gaseous to liquid. Then it from a liquid state to a solid state. When it cools or loses all its heat and energy, the ions and the atoms will pack. Then It creates crystals.

How does magma form?

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

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 really hot, which temperature is 1300 Celsius or higher.

Magma can lose heat is simply by being erupted onto the surface of the earth. When magma gets erupted onto the surface of the earth, it’s going to come in contact with either the atmosphere or the oceans. In both cases, hot magma coming into contact with something quite cold.

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

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

i) The first one results from a decrease in pressure without increasing temperature that can produce decompression melting. So decompression melting can create magma.

ii) The other way that data creates magma is through the introduction of water or sometimes impurities. It can lower the melting temperature of hot mantle rocks sufficiently 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 causes the rocks to melt and rise to the surface.

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

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

Whenever the temperature is high enough for the rock to hit the melting point, it’s going to melt. But that melting point depends on how much pressure the rock is under. If it’s under a lot of pressure and the melting point goes higher different layers act differently.

2. Pressure: Melting occurs due to a decrease in pressure. This 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: By adding volatiles, such as water, decrease the melting point of rock. At convergence zones, the subducting plate heats, causing a release in 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 is the process of creating 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, which breaks the chemical bond in rocks. 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 intrudes into Earth’s cold crust. As the liquid rock solidifies, it loses its heat to the surrounding crust.

Why do magmas rise to the surface?

Magma is liquid, and magma rocks are solid salt materials that are denser than liquid materials. Therefore the liquid material will rise to the top, and the solid material will stay in 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.”

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