Summerland’s ‘Cannonballs’

When the widening of Highway 97 was initially studied, efforts were made to preserve a section of rock that contained numerous ‘cannonballs’. These volcanic rocks are quite unique. Only one other location, the Pacaya Volcano in Guatemala, has similar geology. Unfortunately the surrounding rock was unstable and these ‘cannonballs’ were removed.

Summerland cannonball

Cannonball on Highway 97

cannonball three

 

 

 

 

 

 

 

 

These ‘cannonballs’ are also called volcanic bombs or lava bombs. They range in size from ping-pong size to massive two-metre-diametre spheres. The Summerland ‘cannonballs’ often display a tear-drop shape. Two theories exist to explain the formation of these lava forms.

CANNONBALLS COLOUR TWO SIZES Cannonballs from Highway 97Professor P. W. Francis in 1993 proposed that the shape is created by “post-impact mechanical rounding processes while travelling at high speed down the slope”1 of the volcano. A second theory is that the magma was expelled into the air and promptly landed in a body of water. This would explain the tear-drop shape and the minimal amount of crystallization of the interior of these rocks.

For more information on the Summerland cannonballs and an interesting look into the rebuilding of Highway 97, please consult Okanagan History.2

1P. W. Francis. Cannonball Bomb, a New Kind of Volcanic Bomb from the Pacaya Volcano, Guatemala. Geological Society of America Bulletin. Jan. 1, 1993 Vol. 76, no. 1, pp. 3-38.

Source of photographs: David Gregory, Editor of the Seventy-Seventh Report of the Okanagan Historical Society, Okanagan History.

Source: This post is a footnote in the article Life is a Blast — the rebuilding of Highway 97 (pp. 128-134) in Okanagan History. The Seventy-seventh Report of the Okanagan Historical Society2. Kelowna, BC: Kettle Valley Graphics, 2013.

Copies of the OHS 77th Report are available for purchase from Bob Hayes, 250-763-8859. To access the OHS collection online go to UBC Library Digital Collections. A link to the Okanagan Historical Society’s index to these reports can be found on the UBC Library site, or the OHS website.