7 Igneous rocks

Formation

• Igneous rocks are the first rocks to form on the crust; all other rocks come from them, so they are called the "mother rocks" or primary rocks.
• They form by cooling and crystallization of molten magma (inside the Earth) or lava (on the surface) as its temperature falls.
• Magma is a viscous liquid containing the eight main elements of the silicate minerals as ions, plus gases (mainly water vapour), held under high pressure in the asthenosphere.

Bowen's reaction series

• Bowen's experiments show that, as the magma cools, crystallization starts with minerals rich in iron, magnesium and calcium.
• Then the magma becomes richer in sodium and potassium, and its silicon content rises in the last stages.
• Six groups of minerals crystallize: olivine (first), pyroxene, amphibole, feldspar (plagioclase and orthoclase), mica (biotite and muscovite) and quartz (last).

Classification by mineral composition

• Ultrabasic (ultramafic): silica less than $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">45</span>}\mathrm{\%}$; the first rocks to crystallize; dark, rich in olivine and pyroxene, with a little calcium-rich plagioclase. Examples: plutonic peridotite and volcanic komatiite.
• Basic (mafic): silica $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">45</span>}\mathrm{\%}$ to $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">55</span>}\mathrm{\%}$; crystallize above $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">110</span>}{\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">0</span>}}^{\circ }$C; rich in iron, magnesium and calcium minerals, so black. Examples: plutonic gabbro, dolerite (porphyritic texture) and basalt, the most famous and used for road paving.
• Intermediate: silica $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">55</span>}\mathrm{\%}$ to $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">66</span>}\mathrm{\%}$; plagioclase feldspar, amphibole, mica, quartz and some potash feldspar; crystallize at moderate temperature, colours between light and dark. Examples: diorite (coarse), microdiorite (porphyritic) and volcanic andesite, named after the Andes.
• Acidic (felsic): silica more than $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">68</span>}\mathrm{\%}$ with sodium and potassium feldspars, mica, quartz (about $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">25</span>}\mathrm{\%}$) and amphibole; light pink, crystallize below $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">80</span>}{\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">0</span>}}^{\circ }$C. Examples: granite (coarse, used in construction and polished), microgranite (porphyritic), rhyolite (fine volcanic), obsidian (glassy) and pumice (frothy, light).

Classification by place of crystallization and texture

• Plutonic rocks: slow cooling deep in the Earth lets ions gather at crystallization centres, giving coarse texture with a few large crystals visible to the eye (granite, diorite, gabbro, peridotite).
• Intruded rocks: magma that rises but does not reach the surface cools between the surrounding rocks; slow cooling in depth gives large crystals and faster cooling nearer the surface gives small crystals. The texture is porphyritic (large crystals in a mass of small ones), as in dolerite, microdiorite and microgranite.
• Extruded (volcanic) rocks: lava cools very fast on or near the surface, so crystals have no time to form: glassy texture (obsidian), fine texture of microscopic crystals (rhyolite), or frothy texture from gas bubbles (pumice). Basalt, andesite and komatiite have fine texture.
• Equivalent igneous rocks have the same chemical and mineral composition but differ in place of formation, texture and grain size, for example granite (plutonic, coarse), microgranite (intruded, porphyritic) and rhyolite (volcanic, microcrystalline).

Shapes of igneous bodies in nature

• Intruded (plutonic) bodies:
• Batholith: the largest intruded bodies, extending hundreds of kilometres and several kilometres thick.
• Domes: magma rises in a narrow slot and collects instead of spreading; a normal dome from viscous magma that lifts the layers above is a laccolith (causing an anticline), while an inverted dome from fluid magma that bends the layers down is a lopolith (causing a syncline).
• Dykes: magma intruded so that it cuts across the surrounding rock.
• Sills: magma intruded parallel to the bedding planes, without cutting them.
• Extruded (volcanic) bodies:
• Lava flows: lava solidified on the surface in rope-like or pillow shapes.
• Pyroclastics (broken volcanic neck material): volcanic breccia with sharp-edged fragments around the volcano, and volcanic ash, very fine grains carried by wind for long distances.
• Volcanic bombs: oval massive pieces of lava that solidify near the surface.