Unit 3: Igneous Petrology

Table of Contents

Concepts of Igneous Petrology

Igneous petrology is the study of igneous rocks, which are rocks formed from the cooling and solidification of molten rock (magma or lava).

Origin and Nature of Magma

Magma is a complex, high-temperature fluid mixture of molten silicate rock, suspended solid crystals, and dissolved gases (volatiles). It originates from the partial melting of the Earth's crust and upper mantle.

Melting is caused by:

  1. Decompression Melting: Reduced pressure lowers the melting point (occurs at mid-ocean ridges).
  2. Flux Melting: Addition of volatiles (like H2O) lowers the melting point (occurs at subduction zones).
  3. Heat Transfer: Rising magma melts surrounding crustal rock.

Heat Flow and Geothermal Gradients

Heat flow is the movement of heat from the Earth's hot interior to the cooler surface. The Geothermal Gradient is the rate at which temperature increases with depth. On average, this is ~25-30°C per kilometer in the crust, but it varies.

Geothermal gradients through time: The Earth was much hotter in the past (e.g., Archean). This meant geothermal gradients were steeper, allowing for more extensive melting and the formation of unique rocks (like Komatiites) that do not form today.

Processes of Differentiation and Evolution of Magma

Magma evolution, or magmatic differentiation, describes all processes by which a single parent magma can generate a variety of different igneous rocks. This is why a single magma chamber can produce basalt, andesite, and rhyolite.

Key processes include:

Bowen's Reaction Principle and Reaction Series

Developed by N.L. Bowen, this is a model that describes the sequence in which minerals crystallize from a cooling mafic magma. It is the cornerstone of igneous petrology and explains fractional crystallization.

The series has two branches that merge:

1. Discontinuous Series (Mafic Minerals)

A sequence of reactions where one mineral reacts with the melt to form the *next* mineral. The crystal structure changes at each step.

(High Temp) Olivine → Pyroxene → Amphibole → Biotite Mica (Low Temp)

If olivine is removed from the melt, it cannot react to form pyroxene, and the melt's composition changes.

2. Continuous Series (Plagioclase Feldspar)

A continuous reaction where one mineral (plagioclase) changes its composition as it cools. It does not change its crystal structure.

(High Temp) Ca-rich Plagioclase → Na-rich Plagioclase (Low Temp)

3. Residual Melt (Lowest Temperature Crystallization)

After the main branches are complete, the remaining melt is highly evolved and rich in silica, K, and Na. These minerals crystallize last:

Potassium Feldspar (K-Feldspar) → Muscovite Mica → Quartz

Importance of Bowen's Series:

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 rocks. It is a diamond-shaped diagram based on the relative percentages of four mineral groups:

How it works: The percentages of Q, A, P, and F are recalculated to sum to 100%. The rock's name is then determined by its position in one of the fields.

Textures and Structures of Igneous Rocks

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

Textures (Grain Size)

Structures

These are large-scale features in the rock body, often seen in the field.

Mode of Occurrence and Forms of Igneous Rocks

This describes where and how igneous rocks form.

Extrusive (Volcanic) Forms

Form when lava cools on the Earth's surface.

Intrusive (Plutonic) Forms

Form when magma cools beneath the Earth's surface. They are classified by their relationship to the "country rock" (the pre-existing rock they intrude).