Diabase is an igneous rock that forms mostly as shallow molten intrusions.
Photo by Roy Bassoo.
This igneous boulder of Precambrian age diabase was rounded and transported to the Almonte area from an unknown northern location by glacial action.
Photo courtesy Metcalfe Geoheritage Park Committee.
Look for small interlocking crystals of white feldspar and black irregular crystals of pyroxene and amphibole.
Display rock 22 formed from magma that intruded older geological formations. The smaller sized minerals indicate that the magma cooled quickly. When magma cools slowly the minerals have more time to grow larger. Some igneous rocks contain minerals that are up to one meter across.
Diabase, also known as dolerite in many other parts of the world, is a common igneous rock. It forms mostly as shallow molten intrusions, such as dikes, which cut across older rocks, and sills, which are intruded between rock layers, beds, and foliation.
Image from Chapter 1 of the book “Four Billion Years and Counting” published by the Canadian Federation of Earth Science and Nimbus (available at the Almonte branch of the Mississippi Mills Library. 557.1 FOU). Image downloaded from http://www.fbycbook.com/chapter-1. Graphic based on an original by Colman-Sadd & Scott (2003; Newfoundland and Labrador: Traveller’s Guide to the geology and guidebook to stops of interest; Geological Association of Canada).
Image from Chapter 1 of the book “Four Billion Years and Counting” published by the Canadian Federation of Earth Science and Nimbus (available at the Almonte branch of the Mississippi Mills Library. 557.1 FOU). Photograph by Wouter Bleeker, Geological Survey of Canada.
Dikes cut across older rocks and often occur as swarms of hundreds radiating from a single volcanic centre.
During the last ice age, this boulder was moved to the Almonte area from an unknown northern location by a glacier several kilometers thick.
Public domain image of the United States Geological Survey via Wikimedia Commons.
Diabase occurs far north of the Almonte area in the Canadian Shield rocks of the Grenville Province, as well as in the Superior Province.
Continental collisions during the Precambrian created the Grenville mountain chain about 1.1 billion years ago. Paleomagnetic research indicates that the diabase dike swarm occurred after the mountain building, and are therefore younger. The rocks of the much older Superior Province farther north also formed from the collision and fusing of continental fragments. Dikes of late Archean age, about 2.5 billion years old, are widely distributed across northern Ontario.
Display rock 22 may be the oldest sample on display at Metcalfe Geoheritage Park.