Geology
Some tourist guidebooks, such as the Michelin Guide to Montréal, state that Mount Royal is an extinct volcano. The mountain is not a traditional volcano as such. However, it is the deep extension of a vastly eroded ancient volcanic complex, which was probably active about 125 million years ago. The mountain was created when the North American Plate moved westward over the New England hotspot, along with the other mountains of the Monteregian Hills, by a process known as intrusion: The magma intruded into the sedimentary rocks underneath the area, producing at least eight igneous stocks. The main rock type is a gabbro composed of pyroxene, olivine and variable amounts of plagioclase. During and after the main stage of intrusion, the gabbros and surrounding rocks were intruded by a series of volcanic dikes and sills. Subsequently, the surrounding softer sedimentary rock was eroded, leaving behind the resistant igneous rock that forms the mountain.
The mineral montroyalite, discovered in Montréal, is named after the mountain that provided the definition sample.
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