Sturgeon Lake Caldera

Sturgeon Lake Caldera is a large extinct caldera complex in Kenora District of Northwestern Ontario, Canada. It is one the world's best preserved mineralized Neoarchean caldera complexes, containing well-preserved mafic-intermediate pillow lavas, pillow breccias, hyaloclastite and peperites, submarine lava domes and dome-associated breccia deposits. The complex is some 2.7 billion years old with a minimum strike length of 30 km (19 mi).

• http://en.wikipedia.org/wiki/Sturgeon Lake Caldera