Editing Isle Royale National Park (section)

===Geology===
[[File:USGS Isle Royale geologic map.png|thumb|300px|Isle Royale [[geological map]]]]
[[File:Keweenaw_structure.jpg|thumb|Cross-section of the Lake Superior basin showing the tilted strata of volcanic rock that form Isle Royale]]
The island is composed largely of ridges, running roughly southwest-to-northeast. The main ridge, Greenstone Ridge, is over {{cvt|1000|ft|m|-2}} in many places. [[Greenstone belt]]s are exposed, with rounded stones of [[chlorastrolite]], also known as greenstone, near and in the lake.

The two main rock assemblages found on the island include the Portage Lake [[Volcanics]] and the Copper Harbor [[conglomerate (geology)|Conglomerate]], both [[Precambrian]] in age. The volcanics are mainly [[Poikilitic#Ophitic|ophitic]] [[flood basalt]]s, some 100 individual flows over an accumulated thickness of at least 10,000 feet. The conglomerate [[outcrop]]s on the southwestern portion of the island and consists of [[sedimentary rock]] derived from volcanic rocks in present-day Minnesota. [[Glacial erosion]] accentuated the ridge and valley topography from pre-glacial stream erosion. [[Glacial striation]]s indicate a generally westward movement of the glaciers as do the [[recessional moraine]]s west of Lake Desor. [[Drumlin]]s are found west of Siskiwit Lake.<ref name="Huber" />{{rp|8,14–15,21,25,42,45}}

Recent analyses by the USGS of both unmineralized basalt and copper-mineralized rock show that a small amount of naturally occurring mercury is associated with mineralization.

[[Native copper]] and [[chlorastrolite]], the official state [[gemstone|gem]] of Michigan, are [[primary minerals|secondary minerals]] filling [[pore space]]s formed by [[vesicular texture|vesicles]] and [[fracture (geology)|fractures]] within the volcanic rocks. [[Prehnite]] and [[agate]] [[amygdule]]s are also plentiful island gemstones.<ref name=Huber/>{{rp|55,58–61}}