EPS 101 Spring 2003– Adrian Brearley - Lectures 7-9

Igneous Rocks: Part 1

Mt. St Helens in Repose after 1981 Eruption

Keywords and concepts

General Terminology

Intrusive igneous rocks – igneous rock that has forced its way into the surrounding rock (sedimentary, metamorphic or igneous) as magma and crystallized slowly at depth.

Extrusive igneous rock – igneous rock formed by the eruption of lava onto the surface of Earth. Cooled rapidly.

Country rock – any rock into which an intrusive igneous rock has forced its way.

Lava – magma erupted onto the surface of the Earth that cools and crystallizes into an extrusive igneous rock.

Pyroclastic rocks – rocks consisting of broken fragments of lava or glass that are produced during violent volcanic eruptions.

Pyroclasts – fragments of crystals, glass and lava that make up pyroclastic rocks.

Volcanic ash – extremely small pyroclastic fragments, usually glass.

Tuff – volcanic rock formed by the accumulation and lithification of pyroclastic materials

Pumice – glassy volcanic rock type characterized by abundant vesicles (voids or holes). Voids are produced by the expansion of trapped gases during an eruption.

Obsidian – wholly glassy volcanic rock – very rich in SiO₂.

Porphyry – volcanic rock with a texture consisting of large crystals (phenocrysts) formed within a magma chamber surrounded by a fine-grained matrix of minerals that crystallized after eruption during rapid cooling.

Classification of igneous rocks - chemical and mineralogical characteristics

Felsic Rocks – dominant by feldspar and silica (quartz), light colored rocks, poor in Fe and Mg, rich in Si, K, Na, Al. Main minerals are quartz, orthoclase feldspar (KAlSi₃O₈), sodium-rich plagioclase feldspar, examples, granite (intrusive) and rhyolite (extrusive).

Intermediate Igneous Rocks – Compositions midway between mafic and felsic igneous rocks, i.e. neither rich or poor in silica. Contain plagioclase feldspar, pyroxene, amphibole, biotite mica. Examples, granodiorite and diorite (intrusive), dacite and andesite (extrusive).

Mafic Igneous Rocks – igneous rocks with compositions rich in magnesium and Fe (iron). Contain mafic minerals such as olivine and pyroxene as well as plagioclase feldspar. Example, gabbro (intrusive), basalt (extrusive).

Ultramafic Igneous Rocks – igneous rocks consisting primarily of mafic minerals with less than 10% feldspar. Rarely found as extrusive rocks. Example, peridotite (intrusive)– rock containing mostly olivine with minor pyroxene and amphibole.

Volcanic eruptions at Stromboli volcano in Italy - photos by Marco Fulle

Magmas

Viscosity – measure of liquid’s resistance to flow. Higher viscosity, less easily the liquid flows. Basaltic magmas are much less viscous than felsic magmas (e.g. rhyolitic).

Partial Melting – rocks undergo progressive melting as temperature rises, rather than melting at a single temperature. Partial melting is when only a portion of the rock has melted and crystals are still present.

Magma chamber – large volume usually within the crust consisting of molten rock (magma). Formed when rising magmas push aside or intrude into surrounding rock.

Magmatic differentiation – process by which a compositionally uniform magma changes its composition by the crystallization and removal of minerals at different temperatures as the magma cools.

Continuous Reaction Series – process observed within the plagioclase feldspars series during crystallization from a magma. The feldspars progressively and continuously change composition from Ca-rich to Na-rich to maintain equilibrium with the melt as cooling takes place.

Discontinuous Reaction Series – Process by which early formed high temperature minerals (e.g. olivine) reacted with the magma to produce new minerals as cooling proceeds. Sequence of reaction with decreasing T is olivine, pyroxene, amphibole, mica.

Fractional crystallization – process of crystallization during cooling where new minerals are separated from the melt when they crystallize (i.e. by sinking) and hence don’t react with the remaining melt. The melt changes composition progressively during cooling as the crystals are removed.

Bowen’s Reaction Series – simplified magmatic differentiation scheme combining both continuous and discontinuous reaction series.

Forms of magmatic intrusion

Plutons – large igneous bodies that form deep in the Earth’s crust. From 1 km³ to hundreds of cubic kilometers.

Batholith – the largest plutons – huge, irregular masses of coarse-grained igneous rock that cover at least 100 km³. Often consist of granite.

Stock – smaller pluton.

Discordant intrusions – plutons that cut across layers of country rock they intrude – batholiths and stocks are both types of discordant plutons.

Sill - tabular, sheet-like body formed by injection of magma between parallel layers of preexisting bedded rock Centimeters to hundreds of meters in thickness.

Concordant intrusion – intrusion whose boundary lies parallel to bedding layers (e.g. a sill).

Dyke – tabular igneous bodies that cut across layers of bedding in country rock, rather than between them. Major route of magma transport in the Earth’s crust.

Veins – deposits of minerals within a rock that are foreign to the host rock. A few millimeters to several meters across.

Pegmatite – veins of coarse-grained granite that crystallized from a late stage water-rich magma. Can be rich in water-bearing minerals that contain rare elements such as lithium and beryllium.

Hydrothermal veins – veins produced by the crystallization of minerals from water-rich solutions at much lower temperatures (250-350° C) than crystallization of magmas.