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Igneous Rocks: Textures, Composition, and Environments

← Builds on Rock Definition and the Rock Cycle

Terms you will need

  • aphaniticAphanitic is a texture in igneous rocks where the crystals are too small to see with the naked eye, usually because the rock cooled quickly.
  • glassyGlassy is a texture in igneous rocks where there are no crystals because the rock cooled so quickly that atoms couldn't form crystals.
  • vesicularVesicular is a texture in igneous rocks where there are many small holes left from gas bubbles that were trapped as the rock cooled.
  • porphyriticPorphyritic is a texture in igneous rocks where there are large crystals (phenocrysts) in a matrix of smaller crystals, indicating two different cooling rates.
  • pyroclasticPyroclastic refers to igneous rocks formed from fragments of volcanic material that were ejected during an eruption and then cooled.
  • phaneriticPhaneritic is a texture in igneous rocks where the crystals are large enough to see with the naked eye, usually because the rock cooled slowly underground.

How this topic runs

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Igneous rocks are a fundamental component of Earth's crust, formed from the solidification of magma or lava. Understanding the textures, compositions, and environments in which these rocks form is essential for interpreting Earth's geological history and processes. Igneous rocks provide critical insights into the conditions and events that have shaped our planet over billions of years.

Figures (10)

Basic types of intrusions: LaccolithSmall dikeBatholithDikeSillVolcanic neck, pipeLopolith
Basic classification scheme for igneous rocks based on their mineral composition. If the approximate volume fractions of minerals in the rock are known, the rock name and silica content can be read off the diagram. This is not an exact method, because the classification of igneous rocks also depends on other components, yet in most cases it is a good first guess.
QAPF diagram
Total alkali versus silica classification scheme (TAS) as proposed in Le Maitre's 2002 Igneous Rocks – A classification and glossary of terms[22] Blue area is roughly where alkaline rocks plot; yellow area is where subalkaline rocks plot.
AFM ternary diagram showing the relative proportions of Na2O + K2O (A for Alkali earth metals), FeO + Fe2O3 (F), and MgO (M) with arrows showing the path of chemical variation in tholeiitic and calc-alkaline series magmas
Schematic diagrams showing the principles behind fractional crystallisation in a magma. While cooling, the magma evolves in composition because different minerals crystallize from the melt. 1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclase crystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock forms.
Banded gneiss with a dike of granite orthogneiss
Geologic provinces of the world (USGS).mw-parser-output .col-begin{border-collapse:collapse;padding:0;color:inherit;width:100%;border:0;margin:0}.mw-parser-output .col-begin-small{font-size:90%}.mw-parser-output .col-break{vertical-align:top;text-align:left}.mw-parser-output .col-break-2{width:50%}.mw-parser-output .col-break-3{width:33.3%}.mw-parser-output .col-break-4{width:25%}.mw-parser-output .col-break-5{width:20%}@media(max-width:720px){.mw-parser-output .col-begin,.mw-parser-output .col-begin>tbody,.mw-parser-output .col-begin>tbody>tr,.mw-parser-output .col-begin>tbody>tr>td{display:block!important;width:100%!important}.mw-parser-output .col-break{padding-left:0!important}} .mw-parser-output .legend{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .legend-color{display:inline-block;min-width:1.25em;height:1.25em;line-height:1.25;margin:1px 0;text-align:center;border:1px solid black;background-color:transparent;color:black}.mw-parser-output .legend-text{} Shield Platform Orogen Basin Large igneous province Extended crust Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma
Volcanic eruptions of lava are major sources of igneous rocks. (Mayon volcano in the Philippines, erupting in 2009)
Natural columns of igneous rock separated from each other by columnar joints, in Madeira

Igneous rocks form when magma cools and solidifies. The texture of an igneous rock depends on how quickly the magma cools. If it cools slowly underground, large crystals form, creating a coarse-grained texture.

If it cools quickly at the surface, like lava, the crystals are tiny or absent, making a fine-grained or glassy texture. Igneous rocks are classified by their mineral composition, such as feldspar, quartz, and mica. Common types include granite, which is coarse-grained, and basalt, which is fine-grained.

The environment where the rock forms also matters—underground or on the surface. Understanding these features helps geologists interpret Earth's history and processes.

Key Points

  • Aphanitic is a texture in igneous rocks characterized by very fine-grained crystals that are too small to be seen with the naked eye, typically formed by rapid cooling of magma at the Earth's surface.
  • Glassy is a texture in igneous rocks where the rock solidifies so quickly that no crystals form, resulting in a non-crystalline, amorphous structure.
  • Vesicular is a texture in igneous rocks characterized by numerous small cavities or vesicles formed by gas bubbles trapped in the rock as it solidified.
  • Porphyritic is a texture in igneous rocks where large, well-formed crystals (phenocrysts) are embedded in a finer-grained matrix (groundmass), indicating a two-stage cooling process.
  • Pyroclastic is a texture in igneous rocks formed from the consolidation of fragments ejected during a volcanic eruption, including ash, pumice, and volcanic bombs.
  • Phaneritic is a texture in igneous rocks characterized by large, visible crystals that form when magma cools slowly beneath the Earth's surface.

Terms

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