Igneous Rocks


Lecture 6 - Igneous Rocks

Lisa Tauxe

Rocks - a brief overview

The solid Earth (the mantle and crust) is made of rock. You may have noticed that there are many kinds of rocks, from the soft sandy rocks that form the cliffs at Scripps beach to the hard rocks that form the mountains to the East of San Diego. Geologists have developed a way of classifying the various rocks and understand fairly well where they come from and where they go. There are three general types of rocks, those that form from melt (igneous rocks), those that are deposited from air or water (sedimentary rocks), and those that have formed by "cooking" or otherwise altering another rock (metamorphic rocks). Sedimentary rocks form by breaking down other kinds of rocks into small particles and washing or blowing them away; metamorphic rocks form from other rocks and igneous rocks form by melting other rocks. Thus rocks are always changing form and are redistributted as part of a giant cycle of renewal. This cycle is called the Rock Cycle.

  • Igneous Rocks: form by crystallizing melted material (magma). They can form either on the surface (extrusive igneous rocks), or deep in the crust (intrusive or plutonic igneous rocks). Volcanoes are places where magma erupts as lava or ash.
  • Check out this very cool page with lots of links.
  • Sedimentary rocks: form by lithification (turning into rock) of sediments (stuff that is deposited by wind or water), after burial. All rocks tend to break down into smaller grains when exposed to the surface. These smaller grains then get swept away by winds or rivers and land somewhere as sediment. This sediment will harden (become cemented) when it is buried and it turns into a sedimentary rock. Beach sand is not a sedimentary rock - but IS sediment. The stuff in the cliffs IS a sedimentary rock formed from the sediments.
  • Metamorphic rocks: form by altering the solid stat of any type of rock. This happens when the temperature, pressure or fluid environment change and a rock changes its form (e.g. limestone turns to marble).
  • Igneous rocks: a closer look

    Where (and why) do igneous rocks form?

    Rocks melt when the temperatures, pressures, and/or water content are sufficient to bring the rock to the melting tempertare (duh). This most occurs 1) when rocks have been brought up from deeper in the mantle for example at a RIDGE, 2) rocks with water in them are brought from the surface down into the mantle, for example at a SUBDUCTION ZONE, or there is some "HOT SPOT" arising from deep in the mantle unrelated to a plate boundary. About 90% of all melting occurs at plate boundaries (spreading or subducting). The rest is called "intra-plate" and is typified by Hawaii. For a summary of where melts form, see Figure 4.7 in your book.

    When a partial melt forms, it rises and collects in a magma chamber (see Figure 3.2 in your book). In the magma chamber, the melt continues to crystallize thus changing its chemistry. This is a process known as magmatic differentiation. As magmas cool, different minerals will crystallize out of the melt. By studying the crystallization of melts in the laboratory, this process is fairly well understood. If these minerals settle out of the melt to the floor of the magma chamber, (see Figure 4.11 in your book), the chemistry of the remaining melt changes from a more mafic to a more felsic melt; thus, if fractional crystallization is taken to the extreme granite can be gotten from what was originally a basaltic melt.

    The magma chamber may erupt from time to time. If the melt doesn't make it to the surface, it forms an intrusive rock. (see Figure 4.16 in book). Intrusive bodies can be big balloon shapes (plutons), sub-horizontal slabs (sills) or sub-vertical walls (dikes).

    If it does make it, it becomes an extrusive rock. Extrusives can flow out over the ground (lava flows) or be blasted into the air to form ash falls and pyroclastics.

    Classification of igneous rocks

    Igneous Rocks have a two-dimensional classification scheme based on chemistry, grain size and texture.

  • chemistry:
  • The key to chemical classification in igneous rocks is the amount of Silica (SiO2) in the magma. (Of course people who study this make a much bigger deal out of it! - see this web site). If magmas don't have much silica, their minerals are dominated by magnesium and iron (Fe) - hence the term MAFIC (MA- from the magnesium and FIC from the Fe), or even ULTRAMAFIC for the really silica poor varieties. Silica rich magmas have a mineral named feldspar in them (see book) and are called FELSIC as a result. You will also see the words "acidic" and "basic" used for felsic and mafic respectively and you should be aware that this has nothing to do with pH! One can often tell about how much silica is in a rock just by its color. The more silica, the lighter the color.

  • grain size:
  • The main control of grain size is how fast the rock cooled from the molten state. Slow cooling allows bigger crystals to form, and fast cooling makes smaller crystals and even glass (no crystals). So the second dimension of igneous rock classification is whether the rock was formed by cooling on the surface as an extrusive rock. or in the crust as an intusive rock. Magma can either be erupted (extruded) as ash to make pyroclastic rock or as lava to make volcanic rocks.


    SiO2 (wt. %) <4545 -52 52 - 57 57 - 63 63 - 68 >68
    Compositional or Chemical EquivalentUltrabasicbasicbasic to intermediateintermediateintermediate to acidic or silicicacidic or silicic
    Magma Typeultramaficmaficmafic to intermediateintermediateintermediate to felsicfelsic
    Extrusive Rock Namekomatiitebasaltbasaltic andesiteandesitedaciterhyolite
    Intrusive Rock Nameperidotitegabbrodioritediorite or quartz dioritegranodioritegranite
    Liquidus Temperature
    Mafic Mineral Content
    Water Content
    Ca/Na or Ca/K

  • Texture
  • Igneous textures are classified by the presence or absence of crystals, the size of the crystals, and the size and density of vesicles (holes). Check out this page for a nice summary of igneous textures.

    Extrusive rocks

  • Pyroclastic rocks:

    Pyroclastic rocks are classified by grain size from BOMBS (>64mm) to ash (<2mm). Lapilli are pea-like grains often in a finer matrix.

  • Here is a nice picture I found to illustrate the classification scheme of pyroclastics:

  • Volcanic Rocks:

    Volcanic rocks are mainly classified by the amount of silica. There are four main categories with increasing silica: basalt, andesite, dacite and rhyolite.

    Intrusive Rocks

    Intrusive rocks cool slower and have coarser grain sizes than their extrusive counterparts. The big four of intrusive rocks are with increasing silica: gabbro, diorite, granodiorite, and granite.

    A few other points

  • Viscosity: Increasing silica not only makes magmas lighter (in color), but makes them more viscous (stiffer), so the beautiful movies of flowing glowing lava flows from Hawaii are of basaltic lavas with little silica.
  • Melting temperature: Increasing silica lowers the melting temperature, so that granites melt at about half the temperature that basalts do.
  • Water: Adding water also lowers melting temperature and decreases viscosity. Also, water and other gases make bubbles in the magma, contributing to the explosive power of some eruptions and also leaving holes in the rocks (vesicles).
  • Melting: While we're on the subject of melting, it is very important to note that each mineral has a different melting temperature, so rocks do not melt all at once, but bit by bit. Thus most magmas have floating bits of crystals in them (phenocrysts) and are not 100% melted.

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    Lisa Tauxe