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Geosciences LibreTexts

5.2: Igneous Rock Identification

  • Page ID
    6083
  • Introduction

    Rocks are made of minerals. Rocks can be a mixture of different kinds of minerals, a mixture of many grains of the same kind of mineral, or a mixture of different grains of rocks. When you split a rock into very small pieces, the pieces are different from each other. For example, when you break granite apart, you get small pieces of quartz (clear), feldspar (pink or white), and mica (black). When you split a mineral into pieces, you still have pieces of the same mineral. If you break a big chunk of quartz into smaller pieces, you still have pieces of quartz.

    There are three basic rock types: Igneous, sedimentary, and metamorphic.

    Igneous Rocks

    Igneous rocks (fiery rocks) are made when molten material inside or outside the earth cools and becomes solid. This melted rock is called magma when it is inside the earth. When magma finds its way to the surface through cracks or volcanoes, it is called lava. When lava cools on top of the earth’s surface, it forms extrusive, or volcanic, igneous rock because it was extruded, or pushed, out onto the surface. Because it cools quickly, it only has time to make very tiny crystals. Extrusive, or volcanic, igneous rocks look dull and do not sparkle much because they are fine grained.

    If the magma stays inside the earth and takes thousands of years to cool, it has time to make big crystals. These crystals make a coarse-grained igneous rock called plutonic, or intrusive, igneous rock because the magma was intruded into cracks deep under the earth’s surface. These coarse-grained crystals make the rock look sugary as the flat crystal faces reflect the light in hundreds of little sparkles. The name of the igneous rock depends on what minerals are present. If there are lots of light-colored minerals and the rock is coarse grained, it is granite. If there are mostly dark-colored minerals and the rock is fine grained, it is basalt.

    Igneous Rock Charts

    Igneous – Phaneritic (coarse-grained) or aphanitic (fine-grained [smaller than pencil point])

    Igneous Rock Chart
    Texture V Color> Light colored: Pink, white, Gray, Green, Lavender Medium to Dark Colored: Purple, Greenish Dark Gray to Black Dark Green to Black
    “>Minerals > 10 – 30% orthoclase (potassium-feldspar)
    10 – 40% quartz
    0-33% Na plagioclase
    8-15% amphibole
    and biotite
    55-70% plagioclase feldspar
    15-40% biotite & amphibole
    25-70% Ca plagioclase
    25-75% dark mafic minerals (pyroxene, amphibole, olivine)
    0-5% Ca plagioclase
    65-100% olivine
    0-25% pyroxene
    0-10% ore minerals (magnetite, ilmenite, chromite)
    Composition “>– Felsic (sialic) Intermediate Mafic Ultramafic
    Fine = aphanitic “>Extrusive volcanic Rhyolite Andesite Basalt Komatite
    Coarse = phaneritic “>Intrusive plutonic Granite Diorite Gabbro Peridotite
    Environment “>– Subduction zones Subduction zones Mid-ocean ridges, hot spots Mid-ocean ridges, mantle

     

    Examples of Igneous Rocks
    Grain Color Rock Name
    course light colored Granite
    course medium colored Diorite
    course black (dark) colored Gabbro
    fine light colored (pastel) Rhyolite
    fine medium colored Andesite
    fine black (dark) colored Basalt
    ash particles fine-grained ash Tuff
    glass light colored, frothy Pumice
    glass dark colored, glass Obsidian

    Intrusive Igneous Rocks

    How Intrusive Igneous Rocks are Formed:

    1. While molten, they intrude (push into) other rocks.
    2. They cool below the earth’s surface, generally deep.
    3. They cool gradually and slowly.
    4. Individual crystals have time and space to grow large.
    5. Feldspar (pink to white blocky mineral) crystallizes at high temperatures early in the cooling process and, thus, has straight crystal sides.
    6. Quartz (clear, glassy mineral) crystallizes later and fills spaces.

    Characteristics:

    1. Coarse-grained – The grains, or crystals, are big enough to tell what mineral each one is. The crystals are usually larger than 1 millimeter (larger than the point of a pen or pencil). They are commonly 1/8 to 1 inch long.
    2. The edges of the crystals interlock. The crystals are shiny, flat surfaces which fit together like jigsaw puzzle pieces, with straight edges on some crystals.
    3. The minerals are very hard and will scratch glass. If the rock has been weathered by the elements, it may be crumbly.
    4. The texture is generally uniform (the same in all directions).

    Extrusive Igneous Rocks

    How Extrusive Igneous Rocks are Formed:

    1. While molten, they flow out onto the surface or flow into cracks near the Earth’s surface.
    2. They cool quickly with no time for large crystals to grow.
    3. They are, thus, very fine grained.
    4. Mineral composition can only be identified with a microscope unless phenocrysts are present (see below).
    5. Field identification of hand specimens is based on light or dark color, unless phenocrysts are present.

    Characteristics:

    1. Fine-grained – The grains are too fine to identify the minerals without a microscope.
    2. Some fine‑grained igneous rocks have rectangular crystals in them.
    3. Very hard! Specimens will scratch glass, although it will be crumbly if it has been out in the weather for a long time.
    4. Some volcanic rocks have vesicles, which are holes caused by gas bubbles when the lava was molten.

    Porphyritic igneous rocks have coarse crystals in a fine background. Crystals are two to three times size of the matrix, and less than 10% of rock is crystals.

    • Porphyritic phaneritic: Smaller crystals surround larger crystals (phenocrysts).
    • Porphyritic aphanitic: Massive, structureless ground mass surrounds crystals (phenocrysts).

    Matrix, or ground mass, is the fine‑grained background.

    Phenocrysts are the crystals surrounded by the matrix; these are usually large, straight-sided, and glassy minerals unless they have been weathered.

    Porphyritic is used as an adjective to modify the name of any fine-grained igneous rock that has less than 50% phenocrysts in it. Example: Porphyritic basalt, porphyritic andesite. Porphyry is used as a noun after the rock name if more than 50% of the rock is made of phenocrysts. Example: Basalt porphyry.

    Vesicular volcanic rocks: Vesicular is the adjective used to modify the name of any fine-grained igneous (volcanic) rock which has vesicles (holes from gas bubbles), making a cinder-like or clinker-like appearance. Example: Vesicular basalt.

    Scoria: Basalt with over 50% vesicles. It looks like cinders. It is irregularly porous with rough surfaces and sharp edges and is often red brown.

    Volcanic Glass

    How Volcanic Glass Rocks are Formed:

    Formed by sudden cooling with no time to form even tiny crystals. This rapid crystallization produces glass with a random arrangement of atoms, therefore, these rocks are slightly less dense. The volcanic glasses are usually from a more viscous (harder to flow) lava of rhyolite chemical composition.

    Vitrophyre is a glassy rock containing crystals (phenocrysts).

    Obsidian is a black glass with conchoidal (circular) fracture.

    Pumice is a light gray, glass froth with many, many holes from gas bubbles. If there are enough holes that are not connected, pumice will float. It is very light because of all the holes.

    Igneous – Glassy Chart
    Texture Composition Characteristics Rock Name
    Glassy ? not applicable Massive, black glass Obsidian
    Glassy ? not applicable Frothy, gray glass of sub parallel glass fibers with many squashed air bubbles – may float Pumice
    Glassy ? not applicable Gray glass, rounded spherical structures Perlite

    Volcanic Fragmental Rocks

    How Volcanic Fragmental Rocks are Formed:

    Formed by explosive eruptions of solid fragments and ash.

    Tuff: A light colored volcanic ash, sometimes with glass and pumice fragments in it. Some tuffs are light in weight if they were not compacted. Some are welded tuffs and look like rhyolite.

    Agglomerate: Contains volcanic fragments larger than 2 centimeters (about 1 inch in diameter) that were blown out of a volcanic vent. They are larger than ash particles, but have the same origin.

    Igneous – Pyroclastic (Fragmental) Chart
    Texture Composition Characteristics Rock Name
    Pyroclastic Volcanic ash, pumice fragments, some rock fragments or glass Light-colored volcanic ash, sometimes with glass and pumice fragments Tuff
    Pyroclastic Volcanic ash, pumice fragments, some rock fragments or glass Fine-grained or gritty, light in weight if not compacted; light color Ash fall tuff
    Pyroclastic Volcanic ash, pumice fragments, some rock fragments or glass Particles or grains are fused or welded, with flow lines Ash fall tuff
    Pyroclastic Round pebbles and bombs that were blown out of a volcanic vent, with ash Volcanic fragments larger than 2 millimeters (about size of pencil point) Agglomerate
    Pyroclastic Volcanic bombs, pebbles, ash, pumice fragments, some rock fragments, or glass Sharp, angular volcanic fragments larger than 2 centimeters (1 inch diameter) mixed with others Volcanic breccia

    Characteristics of Important Minerals in Igneous Rocks

    • Quartz: Occurs as irregular, glassy grains, commonly clear to smoky with no cleavage.
    • Muscovite: Brass or clear gray-colored flakes associated with quartz or K-feldspar. Perfect cleavage in 1 direction (layers)
    • Orthoclase (K-feldspar): Porcelain luster; commonly colored pink, white, or gray. Cleavage in 2 directions at right angles may be detected by a reflection of light when specimen is rotated.
    • Plagioclase: Usually gray or white in granite, dark-bluish color in gabbro. Striations common. 2 cleavage directions at right angle may be detected.
    • Biotite: Small black flakes with perfect cleavage in 1 direction (layers), reflects light.
    • Amphibole (Hornblende): Long, black crystals in a light-colored matrix. Cleavage at 60 and 120 degrees.
    • Pyroxene (Augite): Short, dull, greenish-black minerals in darker rocks. Cleavage in two directions at 90 degrees.
    • Olivine: Glassy, light-green to dark green grains.