Slate Islands
Key facts
- Country
- Canada
- Diameter
- 30 km
- Age
- about 450 million years (poorly known)
- Location
- 48.67° N, 87.00° W
A cluster of wooded islands in Lake Superior about 10 km off the Ontario shore. Shoreline outcrops show superb shatter cones and veins of black, glassy breccia.
The Slate Islands are a group of islands in northern Lake Superior, off the town of Terrace Bay in Ontario, Canada. They are the eroded central uplift of an impact crater that was once about 30 km across, most of which now lies under the lake.
What it hit. The impactor struck ancient crystalline rock of the Canadian Shield: volcanic rocks and granites more than 2.5 billion years old. The rebound of the crater floor raised these rocks from depth to form a central peak. Erosion, including scouring by ice sheets, has since removed the crater rim and much of the fill, leaving the islands as the hard core.
Age. The age is not well fixed. Most estimates put it at about 450 million years, in the Ordovician, but the evidence is indirect and the true age could be several tens of millions of years different.
Proof of impact. The islands are famous for their shatter cones, cone-shaped fracture surfaces with fine, branching grooves that form only when a shock wave passes through rock. Some of the cones here are many metres long, among the largest known. The rocks also contain pseudotachylite, dark veins of rock melted by friction as blocks slid against each other, and quartz grains with shock features. The impact origin was established in the 1970s.
Wildlife. The islands are a provincial park and were long home to a small population of woodland caribou, which reached them across winter ice. Visitors arrive by boat.
Visiting. Lake Superior is cold and its weather changes fast, so trips need careful planning. The park is protected: look at the shatter cones but do not hammer or collect them.
Why it matters. The Slate Islands are one of the best places in the world to see shatter cones, and they show what remains of a crater's centre after deep erosion. The impactor is unknown.
Did you know?
Some shatter cones on the Slate Islands are several metres long, among the largest anywhere.
The islands are the central peak of a crater whose rim lies under Lake Superior.
Woodland caribou lived on the islands for many years.