page_title: Isle Royale
section_title: Geology
hierarchical_section_title: Isle Royale / Geology
caption_attribution_description: Figure 38, Keweenaw Structure
context_page_description: Isle Royale is an island of the Great Lakes, located in the northwest of Lake Superior, in Laurentia. It is part of the U.S. state of Michigan. The island and the 450 surrounding smaller islands and waters make up Isle Royale National Park.
  Isle Royale is 45 miles long and 9 miles wide, with an area of 206.73 square miles, making it the fourth-largest lake island in the world. In addition, it is the largest natural island in Lake Superior, the second-largest island in the Great Lakes, the third-largest in the contiguous United States, and the 33rd-largest island in the United States.
  Isle Royale is defined by the United States Census Bureau as Census Tract 9603 of Keweenaw County, Michigan. As of the 2000 census there was no permanent population. After the island was made a national park, some existing residents were allowed to stay, and a few leases are still in effect.
context_section_description: Isle Royale boasts the purest copper to be found anywhere in the world.  A large volcanic event in the distant past twisted the copper-bearing bedrock above the water line, which permitted all sulfur impurities to burn away in the open air.   
   The island is composed largely of ridges, running roughly southwest-to-northeast.  The main ridge, Greenstone Ridge, is over 1,000 feet (300 m) in many places.  Greenstone belts are exposed, with rounded stones of chlorastrolite, also known as greenstone, near and in the lake.
  According to the National Park Service, the north sides of the ridges tend to be steeper than the south sides.  Coastal areas were once submerged beneath prehistoric lake waters, and contain many tumbled boulders and other large rocks.
caption: Cross-section of the Lake Superior basin showing the tilted strata of volcanic rock that form Isle Royale
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