Unit 1: Igneous Petrology Fundamentals: Magma and Crystallization

Origin, Composition, and Types of Magma

What is Magma?

Magma is a complex high-temperature fluid substance found beneath the Earth's surface. It consists of molten rock (silicate melt), suspended solid crystals, and dissolved gases (volatiles).

Magma vs. Lava: Magma is molten rock *below* the Earth's surface. When magma erupts onto the surface, it is called lava.

Origin of Magma (Magmagenesis)

Magma forms by the partial melting of solid rock in the Earth's crust and upper mantle. Complete melting is rare. The three main ways to cause melting are:

1. Decompression Melting: Occurs when rock is moved to a shallower depth (e.g., at mid-ocean ridges), reducing the overlying pressure and lowering the melting point.
2. Flux Melting: Occurs when volatiles (like water or carbon dioxide) are added to hot, solid rock. These volatiles lower the melting point of the rock, causing it to melt (e.g., at subduction zones).
3. Heat Transfer Melting: Occurs when rising magma from the mantle brings heat into the crust, causing the surrounding crustal rocks to melt.

Composition of Magma

Magma is primarily a silicate melt, dominated by silicon (Si) and oxygen (O). Its properties are mainly controlled by:

• Silica Content (SiO₂): The most important factor. It controls viscosity.
• Volatiles: Dissolved gases (H₂O, CO₂, SO₂) that affect viscosity and eruptive style.
• Temperature: Typically ranges from 700°C to 1300°C.

Types of Magma

Magmas are classified based on their silica (SiO₂) content:

Mode of Occurrence

This describes where and how igneous rocks form. There are two main categories:

• Intrusive (Plutonic) Rocks: Form when magma cools and solidifies *beneath* the Earth's surface. Because it's insulated by surrounding rock, cooling is slow, allowing large crystals to grow.
• Extrusive (Volcanic) Rocks: Form when lava cools and solidifies *on* the Earth's surface. Cooling is rapid due to exposure to air or water, resulting in very small (or no) crystals.

Forms and Textures of Igneous Rocks

Forms of Igneous Rocks

These are the 3D bodies that igneous rocks form, classified by their relationship to the "country rock" (the pre-existing rock they intrude).

Intrusive (Plutonic) Forms

• Concordant: Intrusions that run parallel to the layers of the country rock.
  • Sill: A tabular (sheet-like) intrusion that is parallel to the rock layers.
  • Laccolith: A mushroom-shaped intrusion that pushes the overlying rock layers upward.
  • Lopolith: A massive, lens-shaped or saucer-shaped intrusion that is concave-up.
• Discordant: Intrusions that cut *across* the layers of the country rock.
  • Dike: A tabular (sheet-like) intrusion that cuts across rock layers.
  • Batholith: A very large (> 100 km² exposed area), irregularly shaped intrusion. They form the core of mountain ranges.
  • Stock: A smaller version of a batholith (< 100 km² exposed area).

Extrusive (Volcanic) Forms

• Lava Flows: Sheets of lava that flow over the surface. Common types include Pahoehoe (ropy, smooth surface) and Aa (blocky, sharp surface).
• Pyroclastic Deposits: Fragmental material ejected during an explosive eruption (e.g., ash, lapilli, bombs). These form rocks like tuff and ignimbrite.

Textures of Igneous Rocks

Texture refers to the size, shape, and arrangement of mineral crystals in a rock. It tells us about the cooling history.

• Phaneritic (Coarse-grained): Crystals are large enough to be seen with the naked eye. Indicates slow cooling (intrusive).
• Aphanitic (Fine-grained): Crystals are too small to be seen with the naked eye. Indicates rapid cooling (extrusive).
• Porphyritic: A mixed texture with large crystals (called phenocrysts) embedded in a fine-grained matrix (called groundmass). Indicates a two-stage cooling history (slow cooling first, then rapid cooling).
• Glassy: No crystals formed. Indicates instantaneous cooling (e.g., obsidian).
• Vesicular: Contains bubbles (vesicles) formed by escaping gases (e.g., pumice, scoria).
• Pyroclastic: Composed of broken fragments of rock, glass, and minerals (e.g., tuff).

Bowen's Reaction Principle and Crystallization of Magma

The Principle

Developed by N.L. Bowen in the early 20th century, this principle describes the sequence in which minerals crystallize from a cooling mafic magma. It's not a rigid path, but a model for how magma evolves.

The series is split into two branches that merge:

Discontinuous Series (Mafic Minerals)

As the magma cools, one mineral forms, reacts with the remaining melt, and forms the *next* mineral in the sequence. The crystal structure completely changes at each step.

1. Olivine (Highest Temp: ~1200°C)
2. Pyroxene
3. Amphibole
4. Biotite Mica (Lowest Temp)

Continuous Series (Plagioclase Feldspar)

This branch involves only plagioclase feldspar. At high temperatures, it is rich in Calcium (Ca). As it cools, it continuously reacts with the melt to become progressively richer in Sodium (Na).

Anorthite (Ca-rich) → Bytownite → Labradorite → Andesine → Oligoclase → Albite (Na-rich)

Residual Melt (Felsic Minerals)

After the two branches are complete, the remaining melt is rich in silica, potassium, and water. These crystallize at the lowest temperatures:

1. Potassium Feldspar (K-Feldspar)
2. Muscovite Mica
3. Quartz (Lowest Temp: ~700°C)

IUGS Classification of Igneous Rocks

The IUGS (International Union of Geological Sciences) provides a standardized system for classifying igneous rocks based on their modal mineralogy (the volume percentage of minerals present).

The QAPF Diagram

This is the most common IUGS classification, used for plutonic and volcanic rocks that are not ultramafic (i.e., < 90% mafic minerals) and not > 50% carbonate.

It is a diamond-shaped diagram (two triangles) based on the relative percentages of four mineral groups:

• Q = Quartz
• A = Alkali Feldspar (K-Feldspar, Albite)
• P = Plagioclase Feldspar
• F = Feldspathoids (Silica-poor minerals like nepheline; these cannot co-exist with Quartz)

How it works: The percentages of Q, A, P, and F are recalculated to sum to 100%. A rock cannot have both Q and F, so it will plot on either the top (QAP) triangle or the bottom (FAP) triangle. The rock's name is then determined by its position in one of the 15 fields.

Examples of QAPF fields:

• Granite: Plots in the QAP triangle, rich in Q and A.
• Granodiorite: Plots in the QAP triangle, balanced in A and P.
• Diorite: Plots in the QAP triangle, very rich in P, low Q.
• Syenite: Plots near the A-P line, very low Q.
• Foid Syenite: (Foid = Feldspathoid) Plots in the FAP triangle, rich in A and F.