Igneous Rock | The Canadian Encyclopedia


Igneous Rock

Early formed, dense crystals may separate from the magma, causing a change in the composition of the residual melt.
Mount Edziza
One of the composite and (at present) dormant volcanoes that form a belt from northern BC to the southern Yukon (photo by J.A. Kraulis).

Igneous Rock

 Igneous Rock, one of 3 rock classes, the others being SEDIMENTARY and METAMORPHIC rocks. Igneous rocks are the product of the solidification of magma, which is molten rock generated by partial melting caused by heat and pressure in the deeper parts of the Earth's crust or in the upper mantle. These hot magmas have lower densities than their source rocks and rise buoyantly to the surface. On the way, they cool and may crystallize partially or completely.

Early formed, dense crystals may separate from the magma, causing a change in the composition of the residual melt. This process of differentiation, along with compositional variations inherited from the original source material during partial melting, accounts for most of the diversity of igneous rocks. The coarse grain size of granites implies crystallization of a magma under conditions of slow cooling at depth; the finer grain size of basalts indicates more rapid cooling upon extrusion onto the Earth's surface. Glassy rocks imply conditions of rapid chilling with no time for crystal nucleation and growth.

Today, the floors of the oceans are composed largely of basalts; granites are a major component of the continental crust. Igneous activity has been present throughout the Earth's GEOLOGICAL HISTORY, and igneous rocks occur throughout the geological column in Canada (see GEOLOGICAL REGIONS). However, because internal temperatures were higher in the early stages of the Earth's history, the rate of magma production has probably declined over time.

Granites and basalts are well represented in the old rocks of the Precambrian SHIELD regions of Canada and in the younger, flanking mountainous regions of the Appalachians and the western CORDILLERA. Some of the best examples of the most recent volcanism in Canada can be found in BC: the Mt Garibaldi belt in the south; several centres in Wells Gray Provincial Park in central BC; and Mt Edziza and Level Mt in the north. Dates as recent as 1340 years ago have been recorded for the latest activity at Mt Edziza and 220 years ago for a flow at Aiyansh, near Terrace, BC. Some of these volcanic centres are possible sources of GEOTHERMAL ENERGY.