Thanks to Ron Walker, of the Canadian Coast Guard Base at Parry Sound, Ontario, for sharing these photos of the Nottawasaga Island Light.
Canadian Coast Guard photo
A large section of the outer wall of Nottawasaga Light fell sometime during 1 December 2004. The wall has had large cracks for years, probably caused by lightning strikes, and preservationsists were worried that damage like this might occur at any time. There was an electrical storm in the area not long before the collapse, so it's possible that another strike increased the damage beyond the wall's ability to stand.
Nottawasaga Island is a small island just off Collingwood on the south shore of the Georgian Bay, Lake Huron. The island is the site of one of the six great Imperial Towers, a chain of lighthouses built in 1858-59 to improve the safety of shipping around the Bruce Peninsula and to the developing ports of the southern Bay. The lighthouse is 94 feet tall and built of limestone.
In the summer of 2003 the Canadian Coast Guard announced that due to the structural deterioration of the building it would no longer send crews into the tower for maintenance of the light. Therefore, the light would simply be discontinued as soon as it failed. In August 2003, the lighthouse was added to the Lighthouse Digest Doomsday List of endangered lighthouses.
In the 2003 photo at right, the vague vertical streaks on the tower are cracks in the masonry; it is some of these cracks that led to the wall collapse in December 2004. Notice that the keeper's quarters and all other buildings of the light station have collapsed or been removed. A riprap berm gives the lighthouse some protection from wind-driven waves.
In the photo at left, the cracks are painfully obvious. Notice the irregular course of the cracks, which suggests that they were not caused by the stress of gravity. Their location high on the tower is also interesting; cracks caused by settling of a foundation usually start at the base of a building.
The cause of the cracking is not known for sure, but it is likely the cracks are caused by lightning strikes on the tower.
The Imperial Towers are double-walled, and there is a "dead" air space between the outer and inner walls. The cracking is only in the outer wall. During a lightning strike, the air in this dead space can become superheated, causing it to expand rapidly. Limestone is relatively porous but not porous enough: the sudden expansion could easily force open cracks in the outer wall similar to those seen in the photo.
The process is somewhat analogous to what happens when lightning strikes a tree: the rapid heating of the inner bark layer causes cracks to blow open in the outer bark. On a pine tree, which has its outer bark in plates, large sections of the bark are often blown off the tree.
The third photo is taken on one of the landings of the lighthouse. The alcove leading to the window penetrates through the inner wall to the outer wall. We see evidence here that the outer wall has indeed been blown outward.
Preliminary estimates are that it will cost at least $600,000 (Canadian) to repair the lighthouse. The Coast Guard does not have the money, but concerned residents of the Collingwood area and other Canadian lighthouse supporters are organizing to seek federal support as well as private funding for restoration.
Since the other five Imperial Towers share this double-walled design, they could all be vulnerable to similar damage. Clearly, it would be prudent to make sure that all these towers are equipped with well-designed lightning rods. This is standard operating procedure in lightning-prone areas of the U.S., but it hasn't always been standard practice at Canadian lighthouses.
All photos courtesy of the Canadian Coast Guard; not for commercial reproduction.
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Posted December 17, 2003. Checked and revised September 9, 2017. Site copyright 2017 Russ Rowlett and the University of North Carolina at Chapel Hill.