The total surface area is about 36 km². The nearby 'Leadman Group' of islands one km east is often considered part of the Slate Islands.
The cooling effect of Lake Superior make the Slate Islands a particularly harsh habitat for its latitude. As a result, islands harbour arctic and alpine plant species such as Dryas Drumondii and Alpine Bistort, the latter of which is an Inuit delicacy eaten with seal oil. These arctic disjunts are reminders of ice ages and associated tundra conditions in this area in the past.
The islands are home to woodland caribou which have been studied extensively from 1974 to today by Dr. A.T. (Tom) Bergerud. The caribou are a classic example of island biogeography in action; the islands are notable for species that are absent but present on the adjacent mainland (red squirrel, moose, white-tailed deer, and grouse). No ungulates were present on the islands until the caribou arrived in the early 1900s. And, no predators of caribou were (or are currently) present. Caribou reached the highest population density in the world on the islands before the 1990s, with the herd estimated at 650 animals. After a food shortage and die-off in 1990, the numbers were reduced to about 100. Wolves reached the archipelago in the early 1990s preying heavily on the caribou but for reasons not entirely known they disappeared a few years later. Other mammals found on the islands include beaver, muskrat, snowshoe hare, short-tailed weasel, red-backed vole, and red fox.
The waters surrounding the Slate Islands have been protected from commercial fishing to preserve one of the last native stocks of lake trout in Lake Superior. The Islands have been a source of lake trout brood stock used at the Dorion Fish Hatchery, and fingerlings are planted back to Lake Superior to restore the fishery.
Human sites have been found on the islands dating to about 1000 A.D.
A lighthouse was built on Patterson Island, the largest island, in 1903 to help ships locate the harbour at the nearby town of Jackfish, Ontario. The island is named after William Patterson, a former lieutenant-governor of Saskatchewan. Later, a fishing station was built on McColl Island.
The original forests on the islands were modified by logging and forest fires. Up until the 1940s, the islands were used to stockpile boomed logs from the mainland Lake Superior north shore for export on lake freighters to pulp mills in the United States.
In 1985, the Slate Islands were protected as an Ontario Natural Environment Provincial Park. There are no facilities and the islands can only be accessed via boat or airplane. The islands remoteness is enforced by almost 9 km of open, wild, Lake Superior water and its distance from any large communities. It is frequented by naturalists, fishing parties, sailors exploring this Great Lake, and recently by an increasing number of sea kayaking parties.
The Slate Islands mark the centre of a large meteorite impact crater. The original crater rim is estimated at about 32 km (19.9 miles) in diameter, but this and most of the crater has subsequently eroded away, leaving the islands which are interpreted as a central uplift. The age of the impact event is estimated to be about 450 million years (Ordovician). Another source estimates the age at 800-500 million years (late Proterozoic to early Paleozoic). The islands are not made of slate; the rock is mainly of volcanic and sedimentary origin. Allogenic breccia is present, notably on the east and north sides of the islands.
Also located in the islands are good examples of shatter cones, rare geological features formed in bedrock by the high velocity shock waves created by meteorite impacts. They have a distinctively conical shape with thin grooves (striae) that radiate from the top (apex) of the cone. The Slate Islands are home to a shatter cone measuring 9 m (30 ft), one of the largest examples in the world (pictured here).