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. Plutonic rocks = 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.

Igneous Rock Textures Coarse-grained Fine-grained Porphyritic Glassy Vesicular Pyroclastic

lava magma

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)

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 overall size. They come in a variety of shapes and sizes: Discordant: cut across pre-existing fabric of rock layers Dikes are 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. Stocks are 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. Sills are 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.

Sill

Key Terminology Plutonic Intrusive Extrusive Volcanic Texture Phaneritic Aphanitic Porphyritic Glassy Vesicular Pyroclastic Magma Lava Bowen’s Reaction Series Assimilation Partial melting Fractional crystallization Discordant Concordant Dike Stock Batholith Sill Laccolith Lopolith

Pertinent Web Sites Ask a Geologist (USGS) Have a geological question? Here is your chance to ask it. Bob's Rock Shop An interesting site for hobbyists and collectors. Earth Resources Links (NAGT) An extensive listing of Earth resources links arranged by topic from the National Association of Geology Teachers (NAGT). Electronic Volcano (Dartmouth College) An excellent site with a wealth of information and numerous links to related sites. Geology Central One of the most extensive lists of geology related sites available online. Granite Page Everything you ever wanted to know about granites from Rob's Granite Page. Igneous Rock Classification A great site dealing with the classification and identification of igneous rocks hosted by James Madison University, Virginia. Igneous Rocks Links (NAGT) An extensive listing of igneous rocks links arranged by topic from the National Association of Geology Teachers (NAGT). Igneous Rocks Tour A good review of igneous rocks with photographs from California State University at Long Beach. Igneous Rocks (UBC) An overview of igneous rocks from the University of British Columbia. Mineral and Rock Description (National Park Service) Mineral and rock photographs and descriptions from the National Park Service. Online Rock Tutorial from W. H. Freeman Company An introduction to rock identification online tutorial.

Rock and Mineral Collecting (USGS) Selected references on rocks, minerals, and gemstones from the United States Geological Survey (USGS). Rock Classification and Identification A good tutorial on the classification and identification of common rocks from the Ministry of Energy and Mines, British Columbia, Canada. Rock Cycle (Mineralogical Society of America) The Mineralogical Society of America site offers a good, general description of the rock cycle directed toward elementary school students. Rock Cycle - University of British Columbia A general introduction to the rock cycle. Rockhounds Information Page The Rockhounds Information Page offers many links to mineral and rock related Websites. Rock Identification Tutorials Rock identification tutorials (see lab notes) from Texas A & M University. Rocks and Their Origins (UBC) An online study of rocks and their origins at the University of British Columbia. Rocks and the Rock Cycle Links (NAGT) An extensive listing of rocks and the rock cycle links arranged by topic from the National Association of Geology Teachers (NAGT).