GeoE/EnvE 153: Earth Engineering Spring 2025 Lecture 7: Igneous Rocks and Volcanic Processes

Flashcards about Igneous Rocks and Volcanic Processes

Volcanism

The process by which magma and its associated gases rise through the Earth’s crust and are extruded onto the surface or into the air.

Volcano

A mountain formed from the magma and other solid material.

Lava

Magma after it reaches the surface

Magma’s viscosity

Directly related to its silica content. The more silica in a magma, the greater its viscosity.

Basaltic lavas

Low in silica, have low viscosity (are very fluid) and flow great distances

Dissolved gases

Increase the viscosity of the magma.

Factors affecting viscosity of magma

Chemical composition, especially silica, SiO2; temperature; and dissolved gas.

Divergent plate boundaries

Mid-ocean ridge, continental rift

Convergent plate boundaries

Ocean-ocean, ocean-continent

Eruptive Products

Material ejected from a volcano, including lava (liquids), pyroclasts (solids), and gas (water vapour, carbon dioxide, etc.)

Shield Volcanoes

Volcano produced by the accumulation of fluid mafic basaltic lava flows that results in broad, slightly domed structures with very little pyroclastic material

Composite Volcanoes

Volcano formed by the eruption of both lava and pyroclastic material, resulting in a cone with steep slopes and a layered internal structure

Cinder Cones

Volcano consisting of ejected lava fragments that cool and harden while in flight, usually a product of a single eruptive episode comprised of gas-rich mafic magma

Basaltic fissure eruptions

Tension and spreading of the continental crust, resulting in basalt plains that may be thousands of meters thick

Pyroclasts

Separation of gas from magma as it rises in a volcano, often occurring with explosive force, blasting particles of all sizes into the air

Tuff

Rocks formed from volcanic ash

Breccias

Sedimentary rocks formed from the angular coarse-grained pyroclastic material

Pyroclastic Material Size Classification

Dust, Ash (< 2mm)

Pyroclastic density currents

Common on composite volcanoes, have a high proportion of pyroclastic debris, have a temperature between 300°C and 800°C, move at velocity of several hundred m/sec, and follow valleys downwards.

Lahars

Pyroclastic debris comes into contact with water/snow/ice on the slope of the volcano, a high-density slurry is generated that moves downslope in valleys at ~ 10-20 m/sec

Magma

Rocks and minerals melt at high temperatures, this molten rock material

Igneous Rock

A rock formed by the solidification and crystallization of a cooling magma

Confining pressure

Increase in confining pressure increases melting temperature

Water Content

Increase in water content lowers melting temperature

Magma

Found under the Earth’s surface as hot molten (liquid) rock

Lava

Magma which has erupted through the crust and is now on the Earth’s Surface

The Role of Pressure

Pressure increases with depth

The Role of Volatiles

Water and other volatiles (gases) cause rock to melt at lower temperature.

How Magmas Evolve

As a magma cools, minerals crystallize in a systematic fashion based on their melting points

Bowen’s Reaction Series

Crystallization occurs in a predictable sequence

Magma Sources and Types

Mafic magmas produce basalt lavas

Magma Sources and Types

Intermediate magmas produce andesite lavas

Magma Sources and Types

Felsic magmas produce rhyolite lavas

Tabular

Thin in one dimension relative to the other dimensions

Massive

Very large and generally no distinct shape

Concordant

Parallel to the host (country) rock

Discordant

Cut across the layering of the host rock

Texture & Igneous Rock Identification

Overall appearance of the rock based on the size, shape and arrangement of its interlocking crystals

Glassy

Noncrystalline, very fine-grained; very rapid cooling

Aphanitic

Uniformly fine-grained; rapid cooling

Porphyritic

Large phenocrysts within fine-grained groundmass; two-stage cooling process

Vesicular

Numerous small holes on surface; gas escape during cooling

Phaneritic

Uniformly coarse-grained; slow, gradual cooling in subsurface

Aphanitic Texture

Fine-grained, rapid cooling

Phaneritic Texture

Coarse-grained, slow cooling

Porphyritic Texture

Large grains (phenocrysts), in a smaller grain matrix (groundmass)

Vesicular Texture

Containing voids (vesicles) left by gas bubbles that escape as lava solidifies

Glassy Texture

Unordered ions are “frozen” due to rapid cooling, before they are able to unite into an orderly crystalline structure

Pyroclastic Texture

Consist of fragments that are ejected during a violent volcanic eruption

Pegmatitic Texture

Exceptionally coarse-grained igneous rock (crystals >1 cm)