GEOL201 - IGNEOUS PROCESSES AND VOLCANOES

Magma Formation

Magma forms primarily through partial melting of mantle rocks, commonly occurring in subduction zones, rift zones, and mid-ocean ridges.

Partial Melting

The process by which only some minerals in a rock melt while others remain solid, producing magma with a different composition from the source rock.

Decompression Melting

Melting occurs where pressure decreases faster than temperature, allowing melting without added heat

Flux Melting (Subduction Zone Melting)

Occurs when water and volatiles released from subducted plates lower the melting point of overlying mantle rocks, generating magma.

Country Rock

The pre-existing rock into which magma intrudes; may be altered by heat and chemical exchange.

Xenolith

A fragment of country rock that becomes enclosed within an intrusive igneous body, distinct from the host magma.

Subduction Zone

A tectonic region where an oceanic plate sinks beneath another plate into the mantle, generating magma and volcanic arcs.

Classification of Igneous Rocks

Based on two main factors: texture (crystal size) and composition (chemical and mineral makeup).

Slow cooling magma forms a 

coarse-grained texture

Crystalline Structure

A regular, repeating arrangement of atoms in a mineral that minimizes repulsion

Covalent Bonding

A strong chemical bond where atoms share electrons

Ionic Bonding

A bond formed when electrons are transferred between atoms, creating positive and negative ions

Metallic Bonding

A bond where electrons move freely among atoms, giving metals their conductivity and malleability.

Van der Waals Bonding

Weak intermolecular attractions present in soft minerals like graphite.

Polymorph

Minerals with the same chemical composition but different crystal structures

Graphite vs. Diamond

Both composed of carbon, but diamond’s strong covalent bonds make it extremely hard, while graphite’s layered structure and weak van der Waals bonds make it soft.

Two Most Abundant Elements in Earth’s Crust

Oxygen and silicon, which combine to form silicate minerals, the dominant component of Earth’s crust.

Silicate Minerals

Minerals containing silicon-oxygen tetrahedra as their basic building unit; form the majority of Earth’s crustal rocks.

Mafic Magma Composition

Low in silica and rich in magnesium and iron; forms dark-colored rocks like basalt and gabbro; typically less viscous.

Felsic Magma Composition

High in silica and lighter elements like sodium and potassium; forms light-colored rocks like granite and rhyolite; very viscous.

Intermediate (Andesitic) Magma Composition

Intermediate silica content; produces rocks like andesite; associated with subduction zones and explosive eruptions.

Magma Viscosity

The resistance of magma to flow; controlled by temperature, silica content, and dissolved gas content.

Viscosity and Gas Retention

High-viscosity magmas trap gases, leading to explosive eruptions; low-viscosity magmas allow gases to escape, producing gentle eruptions.

Bowen’s Reaction Series (Cooling Order)

predicts the sequence of mineral crystallization as magma cools: first olivine and pyroxene, followed by amphibole, biotite, feldspar, and quartz.

Intrusive vs. Extrusive Environments

Intrusive rocks form beneath Earth’s surface (slow cooling, large crystals); extrusive rocks form at or near the surface (rapid cooling, small crystals).

Plutonic Processes

Geological processes that occur deep below Earth’s surface, including magma intrusion and the formation of batholiths and sills.

Igneous Differentiation

The process by which magma evolves in composition during cooling through crystal settling, partial melting, or magma mixing.

Volcano

A vent or opening in Earth’s surface through which magma, gases, and ash erupt; often builds up landforms of volcanic rock.

Volcanism

The process by which molten material from within Earth reaches the surface, forming new crust and ejecta.

Magma Composition and Eruption Style

Magmas rich in silica and volatiles tend to erupt explosively; low-silica magmas produce fluid, less violent eruptions

Basaltic Magma

Low-silica, low-viscosity magma that flows easily and allows gases to escape; results in gentle, effusive eruptions.

Andesitic Magma

Medium-silica, medium-viscosity magma often produced at subduction zones; eruptions are moderately to highly explosive.

Rhyolitic Magma

High-silica, high-viscosity magma that traps gases and causes violent, catastrophic eruptions.

Viscosity (in Magma)

A measure of a magma’s resistance to flow; higher silica content increases viscosity and eruption explosiveness.

Gas Content in Magma

Determines eruption intensity; more trapped gas leads to more explosive volcanic behavior.

Effusive Eruption

A gentle volcanic eruption dominated by lava flows, common in basaltic volcanoes like those in Hawaii.

Explosive Eruption

A violent volcanic eruption driven by trapped gases in viscous magma; produces ash, pyroclastic flows, and large ejecta.

Pyroclastic Material

Fragmented volcanic debris ejected during explosive eruptions, including ash, lapilli, and volcanic bombs.

Pyroclastic Flow

A fast-moving, hot mixture of gas, ash, and volcanic rock that flows down volcano slopes, extremely destructive and deadly.

Ejecta

Any material (solid, liquid, or gas) expelled from a volcano during an eruption.

Composite Volcano (Stratovolcano)

Large, steep-sided volcano formed from alternating layers of lava and pyroclastic debris; often associated with subduction zones (e.g., Mt. St. Helens, Mt. Pinatubo).

Shield Volcano

Broad, gently sloping volcano built by fluid basaltic lava flows; forms over hotspots (e.g., Mauna Loa, Hawaii).

Cinder Cone Volcano

Small, steep-sided volcano built primarily of pyroclastic fragments; often basaltic in composition.

Lava Dome

A steep-sided dome of viscous lava that forms above a volcanic vent

Caldera

A large depression formed when a volcano’s summit collapses following a massive eruption, often forming crater lakes.

Pillow Basalt

Rounded, pillow-shaped lava formations created when basaltic lava erupts underwater at mid-ocean ridges or seafloor spreading centers.

Pahoehoe Lava

Smooth, ropy basaltic lava that forms during slow, gentle effusive eruptions.

Felsic Lava Flows

Thick and slow-moving lava flows associated with high-silica magmas; travel shorter distances than mafic flows.

Flank Eruption

An eruption occurring on the side of a volcano rather than its summit, typically basaltic and less viscous.

Creation of New Land

Volcanic eruptions can form new landmasses as lava solidifies; volcanic soil is also highly fertile.

Factors Controlling Eruption Strength

1) Amount of gas dissolved in the magma, 2) Magma’s viscosity (resistance to flow).

Mt. Pinatubo Plate Tectonic Setting

Located at a convergent subduction zone in the Philippines where the oceanic plate is subducting beneath the continental plate.

Mt. Pinatubo Eruption Indicators

Increased deep earthquakes, silica-rich emissions, and widespread seismic activity signaled magma movement before the 1991 eruption.

Lahar (Volcanic Mudflow)

A destructive mudflow of water, ash, and volcanic debris down a volcano’s slopes triggered by water mixing with ejecta

What conditions of temperature and pressure do blueschist facies metamorphic rocks indicate?

High pressure and low temperature

At what type of plate boundary do blueschist metamorphic rocks form?

Subduction zones

Which mineral is the first to crystallize in a cooling magma chamber?

Olivine

What causes partial melting at subduction zones?

Dewatering of the downgoing slab

What mechanism causes partial melting where a “slab window” exists?

Decompression

What produced the magma that now constitutes the “Morros” or “ 7 sisters” in the San Luis Obispo area?

A hot spot

What is the most likely composition of the magma produced by partial melting of mafic rocks?

intermediate

Where does hot spot lava/magma originate?

Below the asthenosphere

Fast cooling magma forms a

fine-grained or glass texture

What is a blueschist facies?

A metamorphic facies that forms under high pressure and low temperature, typical of subduction zones.

A facies is a

A set of metamorphic rocks formed under similar temperatures and pressures

Bacteria on the seafloor can live off cooling magma due to a process called

chemosynthesis

Two types of magma formed at hotspots are

mafic and felsic magma

Flood basalts are formed by

low viscosity lava that flows easily along Earth’s surface and deposited in layers

Batholith are

deep intrusive igneous rock

Partial melting occurs because of

varying mineral melting points; minerals rich in silica melt first, mafic materials melt at high temperatures

Gaps in subducting zones that allow hot asthenosphere to rise and melt rock and magma are called

slab windows

Seafloor spreading center are caused by

mantle rising to fill gaps, causing decompression melting

Explosive eruptions are caused by  

magma rich in silica

Composite volcanoes mainly form at

subduction zones

Pyroclastic flow occurs because

hot gas, ash, and ejecta rush down flanks at extremely high speeds

Calderas form when a

magma chamber in a composite volcano collapses, causing a depression

Underwater volcanic mountains formed by hotspots are called

seamounts

Pillow basalt forms at

seafloor spreading centers, through magma rapidly cooling underwater

Serpentine

a group of hydrated magnesium silicate minerals including lizardite and antigorite

Serpentinite

a rock made of serpentine minerals, usually dark green or bluish

Chrysotile is 

fibrous and asbestos in form

Lizardite is

soft and sheet like, and waxy

Antigorite is 

dense, tough, and wavy