Igneous Rocks Dr. R. B. Schultz
Igneous Rocks Igneous rocks form from molten rock (magma) crystallizing below earth's surface or from volcanic activity. They commonly form at plate boundaries and are commonly exposed in mountainous areas. Igneous rocks form from crystallization of magma at depth (within the earth's crust) or at the surface (from volcanic eruptions) There are two (2) basic types or forms of igneous rocks: 1. Intrusive Igneous rocks = igneous rocks that form from cooling magma at depth 2. Extrusive igneous rocks = igneous rocks that form from volcanic activity (at or near surface) *Plutonic rocks are usually coarse-grained *Extrusive rocks are usually fine-grained
These samples represent igneous rocks which have formed at depth within the Earth, although each exhibit different textures.
Igneous rock textures depend on cooling history Intrusive textures: 1. Fine-grained texture (Aphanitic) -- due to fast cooling (at or near surface) 2. Coarse-grained texture (Phaneritic) -- due to slow cooling at depth 3. Porphyritic texture -- coarse crystals (phenocrysts) surrounded by fine-grained matrix (groundmass) forms due to initial slow cooling, then magma rising to (or close to) surface and the remaining magma cooling quickly Extrusive textures: 4. Glassy texture -- due to very rapid cooling -- magma cools so fast crystals don't have time to form. Obsidian (volcanic glass) forms this way. 5. Vesicular texture -- full of rounded holes (vesicles) -- forms due to escape of gas bubbles during cooling of lava. Pumice is a light-colored rock with this vesicular texture. 6. Pyroclastic texture- chunks of molten material that fuse together So, how does magma form? • Magma forms in the lower crust and mantle in which high temperature melts the rock and makes it into molten magma. • Rock composition, pressure, and water content influence the melting temperature of rocks also. • When magma reaches the surface through a volcanic eruption, it is referred to as lava.
Coarse-grained Fine-grained Porphyritic Glassy Vesicular Pyroclastic Igneous Rock Textures
Influences on rock melting temperature Composition -- if rock contains a mixture of minerals it will melt at a lower temperature compared to rocks that are mono-mineralic Pressure -- high-pressure environments raise the melting temperature of rocks Water content -- rocks that contain some water melt at lower temperatures than dry rocks Magma composition • The composition of the magma determines the composition of the rock that forms • Igneous rock classification scheme shows that rocks vary in silica content (e.g. quartz-rich rocks have higher silica content) • Gabbros and basalts are low in silica, with bulk compositions less than about 60% silica • Granites and rhyolites are high in silica (bulk compositions with higher than 60% silica)
Bowen's Reaction Series Professor Norman L. Bowen summarized results of experiments done early in the 1900’s on crystallization of granitic magmas. These experiments showed that there is a sequence of minerals that crystallize as the temperature of magma is lowered: Start with a collection of molten magma and progressively cool it. Minerals will crystallize (solidify) in a definite sequence. Bowen’s Reaction Series represents that sequence that has implications for other types of rocks as well, although it is only used to determine the crystallization sequence in a molten magma (intrusive igneous rocks).
Types of Igneous Intrusions Igneous intrusions are rock bodies that form from crystallization of magma at depth within earth's crust. They are categorized based on their shape and overallsize. They come in a variety of shapes and sizes: Discordant: cut across pre-existing fabric of rock layers Dikesare small igneous intrusions that cut across rocks into which the magma intrudes. They are commonly sheet-like, only a few meters wide, but possibly laterally extensive. Think of magma invading a vertical or near-vertical fracture in rock. Igneous rock would fill the crack due to crystallization of magma. One would call the rock body a dike. Stocksare fairly large (10’s of miles) igneous intrusions that cut across pre-existing rock layers. In size, they are on the order of an individual mountain peak. Batholiths are huge igneous intrusions made of many stocks. Their size is on the scale of an entire mountain range (100’s of miles). Concordant: follow pre-existing fabric of rock layers without interruption of layering. Sillsare also small igneous intrusions. They are sheets of rock that, unlike dikes, are parallel to pre-existing rocks. Think of magma invading sedimentary rocks by spreading out between rock layers. That magma would cool to form a sill. Laccoliths are rather large, mushroom-shaped intrusions that “puff up” in the center due to gases. Lopoliths are also large, inverted mushroom-shaped (spoon-shaped) intrusions that “sag down” in the middle because of dense rocks.
Key Terminology Plutonic Intrusive Extrusive Volcanic Texture Phaneritic AphaniticPorphyritic Glassy Vesicular Pyroclastic Magma Lava Bowen’s Reaction Series Assimilation Partial melting Fractional crystallization Discordant Concordant Dike Stock Batholith Sill Laccolith Lopolith