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.

Author C.M. SCARFE

Links to Other Sites
Life of a Rock Star
This site tells the story of an extraordinary group of scientists who tramped, paddled and rolled across Canada in the nineteenth century to study the geology of Canada's varied terrain.

Strathcona Provincial Park
This nicely illustrated website traces the dynamic geological history of British Columbia’s first provincial park. Learn about rocks in the park, fossils, plate tectonics, volcanoes, glaciers, and more. From the BC Geological Survey.

Canadian Geophysical Union
The CGU, which began as a society dedicated to the scientific study of the solid earth, is now concerned with all aspects of the physical study of Earth and its space environment, including the Sun and solar system. Check out the online newsletters, information about the J. Tuzo Wilson Medal, and more.

Oldest rocks on Earth found in northern Canada
A news story about bedrock on the eastern shore of Canada's Hudson Bay that contains the oldest known rocks on Earth. From reuters.com.