CE11 - Lecture 3 - Magma and Igneous Rocks

Magma

Extremely hot liquid and semi-liquid rock located beneath Earth's surface, often containing suspended crystals and gas bubbles.

Igneous Rocks

Rocks formed by the cooling and solidification of magma.

Mafic Magma

Magma with low silica (45–52%) and high iron, magnesium, and calcium; low viscosity; erupts at ~1100°C; forms dark rocks like basalt and gabbro.

Felsic Magma

High-silica magma (>63%), very viscous, erupts explosively at lower temperatures (~800°C); forms light-colored rocks like granite and rhyolite.

Intermediate Magma

Magma with 52–63% silica and moderate ferromagnesian content; erupts at 850–1100°C; forms rocks like andesite and diorite.

Ultramafic Magma

Very low silica content (<45%) with very high ferromagnesian content; erupts at very high temperatures (~1600°C); rare and ancient.

Intrusive Igneous Rocks

Formed from magma that cools slowly beneath Earth’s surface; also called plutonic rocks (e.g., granite, gabbro).

Extrusive Igneous Rocks

Formed from lava that cools quickly at or above Earth’s surface; also called volcanic rocks (e.g., basalt, rhyolite).

Bowen's Reaction Series

A model describing the sequence of mineral crystallization from cooling magma, with two branches—discontinuous and continuous.

Discontinuous Branch

In Bowen’s Series, minerals like olivine, pyroxene, amphibole, and biotite form and transform into each other with decreasing temperature.

Continuous Branch

In Bowen’s Series, plagioclase feldspar changes gradually from calcium-rich (anorthite) to sodium-rich (albite) as temperature decreases.

Pluton

A body of intrusive igneous rock; forms when magma cools slowly beneath the surface.

Batholith

A massive pluton with an exposed surface area greater than 100 km².

Stock

A smaller pluton with a surface exposure of less than 100 km².

Sill

A sheet-like pluton that forms parallel to existing rock layers.

Dyke (or Dike)

A sheet-like pluton that cuts across existing rock layers.

Laccolith

A sill that causes the overlying rock to dome upward, forming a blister-like shape.

Pipe

A cylindrical conduit through which magma travels from deeper to shallower levels.

Vesicular Texture

A texture in volcanic rocks characterized by holes or cavities formed by gas bubbles during cooling.

Ferromagnesian Silicates

Dark-colored minerals rich in iron and magnesium found mainly in mafic rocks.

Silica (SiO₂) Content

Determines the viscosity and eruption style of magma; higher silica = more viscous and explosive magma.

Viscosity

The resistance of magma to flow; influenced by silica content—higher silica = higher viscosity.

Quenching

Rapid cooling of magma that prevents further mineral formation, used in experiments like Bowen’s.

Gabbro

Coarse-grained, dark-colored intrusive rock formed from mafic magma.

Basalt

Fine-grained, dark-colored extrusive rock formed from mafic lava.

Diorite

Coarse-grained intrusive rock formed from intermediate magma.

Andesite

Fine-grained extrusive rock formed from intermediate magma.

Granite

Light-colored, coarse-grained intrusive rock formed from felsic magma.

Rhyolite

Light-colored, fine-grained extrusive rock formed from felsic magma.

Joint

A fracture in rock where there has been no significant movement; affects engineering stability.

Sheet Joint

A type of joint in granite formed by pressure release, common in plutonic rock outcrops.

Differential Settlement

Uneven settling of a structure’s foundation, often due to rocks with varying weathering grades.

Columnar Jointing

A geological structure where volcanic rocks cool and contract into polygonal columns, often basalt.

Pyroclastic Sediments

Volcanic fragments ejected during eruptions, often found beneath volcanic rock layers.

Rock Bolts / Anchoring

Engineering supports used to stabilize rocks with open joints during excavation.

Weathering (in Plutonic Rocks)

Process that reduces strength and increases permeability, affecting suitability for construction.

Engineering Use of Extrusive Rocks

Used as aggregates in concrete, base course for highways, railroad ballast, and dam rock fill.

Intrusive vs. Extrusive Texture

Intrusive rocks are coarse and rough due to slow cooling; extrusive rocks are fine and smooth due to fast cooling.

Reaction (in Bowen's Series)

Process where early-formed minerals chemically react with remaining magma to form new minerals.

Anorthite

Calcium-rich endmember of plagioclase feldspar in Bowen’s continuous series.

Albite

Sodium-rich endmember of plagioclase feldspar in Bowen’s continuous series.