How Does Magma Form? (Composition)

Magma Formation

Deep beneath the Earth’s crust, in a world untouched by sunlight and beyond the reach of the deepest oceans, lies the fiery forge where magma is born. This molten rock is the lifeblood of our planet, a seething, roiling substance that shapes the Earth’s surface in both creation and destruction. But how does magma form in this hidden realm?

The answer lies in a complex ballet of pressure, temperature, and the chemical composition of the Earth’s mantle. 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.

In this post, we will descend into the depths of the Earth to uncover the processes that melt solid rock into liquid magma, fueling volcanoes, forming a new crust, and playing a pivotal role in the tectonic activity that continuously reshapes our planet. Join us on a journey to the heart of the Earth, where the intense heat and pressure hide secrets crucial to understanding our world’s fiery foundation.

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 are found 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 they cool or lose 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, with a temperature of 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 form 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 they’re 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 be created when hot, liquid rock enters Earth’s cold crust. As the liquid rock solidifies, it loses heat to the surrounding crust.

How does magma become igneous rock?

Magma becomes igneous rock through a process known as cooling and solidification. Igneous rocks are formed from the solidification of molten rock material. They are categorized based on where this solidification occurs: either beneath the Earth’s surface (intrusive or plutonic igneous rocks) or on the Earth’s surface (extrusive or volcanic igneous rocks). The journey from magma to igneous rock involves several stages:

  1. Cooling of Magma

Intrusive Igneous Rocks: When magma cools slowly beneath the Earth’s surface, it forms intrusive igneous rocks. The slow cooling process allows time for crystals to grow large, resulting in rocks with coarse-grained textures, such as granite.
Extrusive Igneous Rocks: When magma erupts onto the Earth’s surface (becoming lava), it cools rapidly. The quick cooling process allows only small crystals to form, sometimes so small that they can’t be seen without magnification, leading to a fine-grained texture. An example of extrusive igneous rock is basalt.

  1. Crystallization

As the magma cools, different minerals crystallize at different temperatures—a process known as fractional crystallization. The sequence in which minerals crystallize is predictable and is described by Bowen’s Reaction Series. Early-forming minerals may have different compositions from those crystallizing later, affecting the overall composition of the resulting igneous rock.

  1. Solidification

Eventually, the magma cools to a point where it solidifies completely. This transition marks the final transformation of magma into igneous rock. The speed of this process and the environment in which it occurs significantly influence the rock’s characteristics:

Slow Cooling: Results in larger crystals (coarse-grained texture).
Fast Cooling: Results in smaller crystals (fine-grained texture) or a glassy texture if the cooling is extremely rapid, as in the case of obsidian.

  1. Textural Variations

The texture of igneous rocks not only depends on the cooling rate but also the amount of gas (volatiles) in the magma. High gas content can lead to the formation of vesicular (bubble-rich) textures, as seen in pumice.

  1. Emplacement and Erosion

Intrusive igneous rocks may eventually be exposed at the surface due to the erosion of overlying material, joining their extrusive counterparts in shaping the Earth’s landscape.

In summary, the transition from magma to igneous rock is governed by the cooling and solidification process, with the resulting rock type (intrusive vs. extrusive) and texture (coarse-grained vs. fine-grained vs. glassy) being influenced by the cooling rate, location of solidification, and original magma composition.

Why does magma rise to the surface?

Magma rises to the Earth’s surface due to a combination of factors related to buoyancy, pressure, and the composition of the magma itself. Let’s explore these factors in detail:

  1. Buoyancy

Magma is generally less dense than the surrounding solid rock. This difference in density is primarily because magma contains molten rock, volatiles (such as water vapor, carbon dioxide, and other gases), and sometimes crystals. The presence of these gases especially can significantly reduce the density of the magma. Due to its lower density, magma behaves buoyantly and tends to rise through the Earth’s crust, much like a less dense liquid rises through a denser one.

  1. Pressure

As magma forms, it often does so in areas of the mantle and crust where temperatures and pressures are high enough to cause rock to melt partially. This melting can occur in various settings, such as above a subducting tectonic plate, beneath mid-ocean ridges, or under hotspots. Once formed, the magma is under tremendous pressure from the overlying rocks. This pressure drives the magma to seek paths of least resistance, which often leads it upward through cracks, faults, or porous rocks toward the Earth’s surface.

  1. Thermal Expansion

As rock heats up, it expands. When rock in the Earth’s mantle or lower crust melts to form magma, the molten material takes up more space than it did in its solid form due to thermal expansion. This expansion contributes to the magma’s buoyancy and its tendency to rise.

  1. Crustal Deformation and Tectonic Activity

Tectonic processes can create pathways for magma to ascend. For example, the movement of tectonic plates can form rifts, subduction zones, and other structural weaknesses in the Earth’s crust. Magma can exploit these weaknesses as channels to rise towards the surface. In subduction zones, water released from the subducting plate lowers the overlying mantle’s melting point, generating buoyant magma that rises through the overlying crust.

  1. Chemical Buoyancy

The composition of magma can also influence its buoyancy. Felsic magmas (rich in silica) are more viscous and less dense than mafic magmas (lower in silica). The high viscosity can slow the ascent of felsic magmas, but their buoyancy still drives them upwards. Mafic magma, being hotter and more fluid, can rise more quickly through the crust.

  1. Gas Expansion

As magma rises and pressure decreases, gases dissolved in the magma expand. This expansion increases the volume of the magma, reducing its density even further and accelerating its ascent. The process can become a positive feedback loop, where the ascent of magma allows more gas to exsolve (come out of solution), which drives the magma upward more forcefully.

In summary, magma rises to the Earth’s surface due to its buoyancy relative to the surrounding solid rock, the pressure from the overlying rocks, thermal expansion, pathways created by tectonic activity, its chemical composition, and the expansion of gases within it. These factors work together to drive magma from deep within the Earth toward the surface, leading to volcanic eruptions and the formation of igneous rocks.

The journey through the conditions that melt rock into molten magma illuminates not just the birth of this fiery fluid but also the dynamic processes that drive the Earth’s geology. From fueling the eruptions of volcanoes to creating new land through slow, persistent flows, magma is a fundamental change agent on our planet.

This exploration into its origins offers us a glimpse into the Earth’s inner workings, revealing a world of extreme conditions and transformative power. May this knowledge enrich your appreciation for the planet’s ever-changing nature and inspire wonder about the unseen forces that shape our world. So, the next time you witness a volcanic eruption, either in person or through a camera lens, remember the incredible journey from solid rock to the molten magma that makes such spectacular displays of Earth’s power possible.

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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