Thompson Island Provincial Park is a 145 hectare nature reserve established in 1985. Ontario Parks places it in Lake Superior south of Thunder Bay.
The official page says geologists will find the island interesting for Paleohelikian gabbro dikes and sills formed about one billion years ago.
Thompson Island is a compact Lake Superior nature reserve with a specific geological identity. Ontario Parks explains that the dikes and sills formed when pressurized volcanic lava deep within the earth's crust was pushed through vertical and horizontal bedrock crevices.
That lava or magma hardened into gabbro, a coarse-grained dark rock. The official page also notes raised cobble beaches and Arctic-alpine vegetation growing at its most southerly limit.
The park is most useful for visitors researching Lake Superior island geology, ancient igneous features, and edge-of-range plant communities. Any visit needs to respect the nature reserve classification and the challenge of island access.
Plan around geology research, responsible photography, island-access planning, Lake Superior weather checks, raised beach interpretation, Arctic-alpine plant awareness, map review, and low-impact observation where access is appropriate.
Because it is an island reserve, the water crossing can be the biggest planning factor.
That mix of billion-year-old rock, raised shorelines, and southern-limit vegetation makes the island sensitive as well as interesting.
Plan the island as a weather-dependent reserve, not a casual stop.
Confirm access, Lake Superior conditions, no-facility limitations, sensitive vegetation guidance, maps, alerts, weather, communications, and emergency planning through Ontario Parks before travelling.
Non-operating park in Ontario Parks locator.
