Welcome to the third edition of “PEI: Permian to Present”. So far, we’ve looked at the Island’s famous red sandstone, learned why it is sometimes green-grey, and what it tells us about the environment in which it was formed and life at that time. But there’s one place on PEI where a different type of rock exists: Iron Rock in Malpeque Bay.
Iron Rock is unlike any other formation on PEI in that it is igneous, rather than sedimentary. Igneous rocks form from the cooling of hot, liquid rock. You can find igneous rocks scattered around the Island but these are ‘from away’, carried here by ice sheets during the last glaciation. Iron Rock is different in that it is a sill – igneous rock that forced its way between layers of sedimentary rock – and was here long before the last ice age.
I’ve sometimes heard Iron Rock called volcanic, but this is not true. Volcanic rocks are formed from volcanic eruptions and classed as ‘extrusive’. In these cases, lava cooled quickly once it hit the surface, and the resulting rocks are fine-grained (basalt, obsidian and pumice are examples). Iron Rock is ‘intrusive’, and its magma cooled underground and slowly into larger grains; granite is another example of intrusive igneous rock. In short, all volcanic rocks are igneous, but not all igneous rocks are volcanic (and liquid rock is called lava when it hits the surface, magma if underground).
Iron Rock is indeed rich in iron and other minerals, such as calcium and magnesium, and so is well-named. It’s younger than the surrounding sandstone – those sedimentary layers had to be there first for the igneous sill to force its way through. Iron Rock is about 247 million years old and records a time of increased geological activity preceding the separation of the supercontinent Pangea. In the tens of millions of years that followed, the Earth’s crust was stretched thin and eventually torn apart, creating the Atlantic Ocean.
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In the next edition of Permian to Present, we’ll explore PEI during the last Ice Age. But if you’re looking for more information on our pre-history, I’ve got some good news! Here are several fantastic, local resources I highly recommend:
Prehistoric Island Tours – geologist Laura MacNeil leads fantastic tours along one of PEI’s best fossil sites. You can give her tour Facebook page a follow.
PEI: Island at the Centre of the World – geoscientist John Calder has written the definitive book on PEI’s geological history, which is a must-read (and available locally at The Bookmark, Moonsnail Soapworks, and the PEI Museum & Heritage Foundation, as well as on Amazon). You can also follow the book Facebook page.
Prehistoric PEI – film-maker Will Beckett made a fascinating four-part documentary about PEI’s fossil history. Each runs about 10-15 minutes and shouldn’t be missed by anyone interested in our natural history. You can find Prehistoric PEI on YouTube.
The Last Billion Years – published by the Atlantic Geoscience Society, this easy-reading book takes a dive into the geological and fossil history of the Maritimes, all the way back to the Precambrian! It’s available from the Society, Amazon, or your favourite local bookstore may be able to order it in.
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