Igneous Rocks: How They Form, Why Different Rocks Form & Bowen's Reaction Series

Igneous rocks are rocks formed when molten rock material cools and becomes solid.

But all molten material does not produce the same rock. The final rock mainly depends on three questions:

Rocks are not permanently fixed as one type. Over millions of years, the same material may pass through several geological processes and become a different type of rock.

However, a rock usually does not simply change its name directly. A specific physical or chemical process must act on it.

For example, granite exposed at the surface may break down into sediments. Those sediments may later become a sedimentary rock. If buried deeply, that sedimentary rock may be changed by heat and pressure into a metamorphic rock. If it melts completely, magma is produced; when that magma cools, a new igneous rock forms.

At some places inside the Earth, temperature becomes high enough for part of the rock to melt. This produces molten material called magma.

Magma is not normally a completely uniform liquid. It may contain molten material, mineral crystals and dissolved gases.

As magma loses heat, atoms and ions begin arranging themselves into organised mineral structures. This process is called crystallisation.

Igneous rocks are therefore commonly called primary rocks because they can form directly from molten Earth material.

This is the most important idea of this Post. A magma does not necessarily remain chemically unchanged while it cools. As temperature falls, different minerals crystallise at different stages. If some early crystals become separated from the remaining liquid, the composition of the leftover magma changes. This gradual change is called magmatic differentiation; when crystal separation is important, the process is commonly described as fractional crystallisation.

Step 1 — Start with hot parent magma

Imagine a hot magma containing silica, iron, magnesium, calcium, sodium, potassium and other elements.

Step 2 — High-temperature minerals crystallise first

At relatively high temperatures, minerals such as olivine, pyroxene and calcium-rich plagioclase become stable early. These minerals use comparatively large amounts of iron, magnesium and calcium from the melt.

Step 3 — Early crystals may separate from the liquid

If these early crystals settle, accumulate, stick to chamber walls or are otherwise removed from continued reaction with the liquid, they take some Fe–Mg–Ca with them.

Step 4 — The remaining magma becomes chemically different

The residual melt becomes relatively depleted in some iron, magnesium and calcium and relatively enriched in components such as silica, sodium and potassium. In this way an initially mafic melt can, under suitable conditions, evolve toward more intermediate and then more felsic compositions.

Step 5 — Now ask where each melt cools

Once the magma has a particular composition, its final rock name also depends strongly on whether it cools slowly underground or rapidly at the surface.

Therefore, mafic magma can give gabbro if it cools slowly underground and basalt if it cools rapidly at the surface. In the same way, intermediate magma gives the diorite–andesite pair, while felsic magma gives the granite–rhyolite pair.

Also remember that fractional crystallisation is not the only way felsic or intermediate magmas form. Partial melting of crustal rocks, magma mixing and assimilation may also contribute. For Geography examinations, the important point is to understand the basic differentiation mechanism without treating it as the only possible pathway.

A crystal grows when atoms and ions have enough time to move and arrange themselves into an ordered structure.

This explains one of the most important comparisons in geology:

Granite: slowly cooled underground → large visible crystals.

Rhyolite: broadly similar felsic composition but rapidly cooled near the surface → small crystals.

Why does silica affect viscosity?

Silica-rich melts can develop interconnected silica structures. These make it more difficult for the melt to flow freely.

Therefore:

Higher silica → generally greater viscosity → magma flows less easily.

Lower-silica basaltic magma → generally lower viscosity → flows more easily.

Eruption behaviour also depends on temperature, dissolved gases and other factors, so silica alone does not determine whether an eruption will be explosive.

These pairs are called broad compositional equivalents.

It does not mean that granite changes directly into rhyolite or gabbro directly into basalt.

It means that molten material of broadly similar composition can form:

Thus felsic magma may produce granite underground or rhyolite at the surface. Mafic magma may produce gabbro underground or basalt at the surface.

Memory ladder:

Ultramafic → Peridotite
Mafic → Gabbro / Basalt
Intermediate → Diorite / Andesite
Felsic → Granite / Rhyolite

In each pair, the first name is the common intrusive/coarse equivalent and the second is the common extrusive/fine equivalent.

Magma moving upward does not always reach the surface.

It may enter cracks, weaknesses, bedding planes or spaces between pre-existing rock layers. If it cools there, the solidified magma forms an intrusive igneous body.

Its final shape depends largely on the space into which magma entered.

Magma contains many chemical elements, but all minerals do not crystallise at the same temperature.

Some minerals become stable and crystallise while the magma is still very hot. Other minerals form only after the magma has cooled further.

Bowen's Reaction Series shows this general order of mineral crystallisation as magma cools.

A. Discontinuous Branch

Why is it called discontinuous?

Because the main mineral structure changes from one mineral group to another as temperature falls.

At very high temperature, olivine may crystallise first. If it remains in contact with the cooling melt under suitable conditions, reactions with the melt favour different lower-temperature minerals, broadly progressing toward pyroxene, amphibole and then biotite.

B. Continuous Branch

Why is it called continuous?

Because the mineral remains within the plagioclase feldspar family. Its chemical composition gradually changes instead of changing abruptly into a completely different mineral family.

At higher temperature, plagioclase is relatively calcium-rich. As cooling continues, progressively more sodium-rich plagioclase becomes stable.

C. What Happens at Still Lower Temperatures?

D. Most Important Question — Why Does the Remaining Magma Change?

Imagine a magma containing iron, magnesium, calcium, sodium, potassium, silica and other elements.

Early minerals such as olivine and pyroxene preferentially take significant amounts of iron and magnesium from the melt. Calcium-rich plagioclase also removes calcium.

Therefore, if these early crystals become separated from the remaining melt, the residual magma is no longer chemically identical to the original magma.

This is one of the basic ideas behind magmatic differentiation.

E. So Can Basaltic Magma Eventually Produce More Felsic Material?

Under suitable conditions, yes. If early mafic minerals crystallise and are effectively removed or separated from the melt, the remaining magma can become relatively more silica-rich and chemically evolved.

But this should not be simplified as:

The correct idea is:

For UPSC and State PSC Geography, understand the sequence, high-temperature versus low-temperature minerals, continuous versus discontinuous branches, and the changing composition of residual magma. Exact laboratory temperature ranges are usually unnecessary.

At the high-temperature end, minerals such as olivine, pyroxene and calcium-rich plagioclase are important. These minerals are strongly associated with mafic and ultramafic compositions.

At the lower-temperature end, potassium feldspar, muscovite and quartz become important. These minerals are strongly associated with felsic compositions.

This single relationship helps explain why basalt and gabbro commonly contain mafic minerals, while granite and rhyolite commonly contain quartz and feldspars.

Felsic rocks: generally lighter and generally less dense.

Mafic and ultramafic rocks: generally darker and generally denser.

But: colour alone is not a perfect test because rock colour can also be influenced by texture, alteration, weathering and the proportions of individual minerals.

Pegmatite: an exceptionally coarse-grained intrusive rock. Its very large crystals generally form during late stages of magma crystallisation where volatile-rich residual melts can allow rapid movement of ions and exceptional crystal growth.

How the Special Igneous Rocks Form

Detailed Deccan volcanism and Indian Rock Systems should be studied separately so that this post remains focused on basic igneous-rock concepts.

1. Igneous rocks are not always visibly crystalline. Obsidian has a glassy texture.

2. Fossils are generally absent from igneous rocks because they form from molten material.

3. Compact crystalline igneous rocks generally have low primary porosity, but joints, fractures and vesicles may create secondary spaces and permeability.

4. Vesicles are mainly caused by trapped gas bubbles, not simply by rapid cooling.

5. Porphyritic texture does not simply mean “medium-sized crystals.” It records large crystals embedded in a finer groundmass.

6. Granite and rhyolite are compositional equivalents; granite does not directly convert into rhyolite.

7. Bowen's Reaction Series describes mineral crystallisation and reaction during cooling; it is not merely a list of rock names.

Magma below ground; lava at surface.

Slow cooling → large crystals → commonly intrusive.

Rapid cooling → small crystals → commonly extrusive.

Extremely rapid cooling → glass.

Trapped gas bubbles → vesicular texture.

Two-stage cooling → porphyritic texture.

Granite ↔ Rhyolite = felsic equivalents.

Diorite ↔ Andesite = intermediate equivalents.

Gabbro ↔ Basalt = mafic equivalents.

Sill follows layers; dyke cuts across layers.

Bowen Series: different minerals crystallise at different stages as magma cools.

Discontinuous branch: Olivine → Pyroxene → Amphibole → Biotite.

Continuous branch: Ca-rich plagioclase → progressively Na-rich plagioclase.

Low-temperature end: potassium feldspar, muscovite and quartz become important.

Final memory rules:

• Temperature controls mineral crystallisation sequence; it does not by itself rename a rock.

• Composition decides the family: ultramafic / mafic / intermediate / felsic.

• Cooling place decides the common pair: Gabbro–Basalt, Diorite–Andesite, Granite–Rhyolite.

• Cooling speed mainly controls texture and crystal size.

• Fractional crystallisation can change the composition of the remaining melt because early crystals remove selected elements.

• A solid basalt does not simply cool into andesite or granite; magma evolution is the process being discussed.

Think Like UPSC

Whenever you see an igneous rock, ask these seven questions:

If these five questions are clear, most igneous-rock questions can be solved through logic rather than memorisation.