This area was covered by hundreds of large lava flows approximately 1.1 billion years ago. When lava erupts, gasses which had been trapped in the lava bubble to the surface and create a frothy layer, similar to a can of soda when opened. The lava cooled and formed bedrock with a solid bottom part and a porous top part (similar to a sponge in appearance). Each subsequent lava flow buried the previous one, creating a layered effect: solid basalt (the grayish color rock in the area), porous basalt, solid basalt, the porous basalt and so on. The eruptions occurred for an approximately 2 million year long time span. The basalt lava flows covered an area from the Keweenaw Peninsula to Isle Royale.
As the basalt lava continued erupting, after it ceased, parts of the area were covered by reddish colored conglomerates (rhyolite). These conglomerates were then covered by the reddish sandstone common to this area later to be know as Jacobsville Sandstone. These deposits occurred roughly 1070 mya (million years ago).
The stack of lava flows, or basalt, conglomerates, and sandstones (layers of bedrock) was then compressed from the sides due to tremendous pressures caused by the moving of earth's crust (plate tectonics). The bedrock layers began to bend from the pressure and changed from basically flat lying to bowl shaped. Picture taking a thick book and pressing on it from the ends. Originally the pages are flat and straight, but push hard enough and the book will begin to bend into a "U" shape. One end of the "U" became the Keweenaw Peninsula, the other end, Isle Royale, and the space in the middle (the inside of the "U") is Lake Superior.
Glaciers have eroded away the bedrock unevenly, so the layers at the surface are not quite as obvious as they were when formed. Glaciers have also carved out most of the land features including Adventure Mountain, but in most places you still find sandstones and conglomerates along the lake shore and basalt further away from the lake.
Where did the copper come from?
Many people believe the copper here was basically melted metal contained within the basalt lava flows; saying that when the lava cooled, the copper solidified within the rock. This is NOT TRUE! The copper here was deposited AFTER the lava eruptions were finished and after the formations of the conglomerates and sandstones. Basically, the copper precipitated out of warm ground water (hydrothermal) solutions. Many people that have well water get iron or rust stains in their sinks from dripping water. This iron precipitates from their well water and deposits itself onto their sink. The copper in the bedrock formed the same way.
Given enough time, large quantities of copper were deposited by filling voids in the bedrock where the water was flowing through. Now, to put this all together, if water is flowing through bedrock, there must be cracks and porous sections of rock that allow this to occur; the copper was deposited only in the top porous sections of the basalt lava flows. This is how we came to have parallel lodes of copper in the Adventure Mine. The copper occurs only in the lava flow top sections, so only those areas were ever mined. The solid part of the basalt lava has no copper.
As discussed before, the bedrock layers were bent from pressure and this is why the lodes at the Adventure Mine are inclined at 45 degrees. If you walk in the mine and see some of the larger stopes (areas of copper lodes that have been mined away and are now open), they are all inclined upward at approximately a 45 degree incline. The conglomerates had areas of rich copper deposits where copper had filled in areas between the stones and cobbles. There were also some shales that had enough open space to allow copper deposits mostly in the cracks and fissures in the rocks. Most of the very large masses of native copper mined were formed as the result of a copper filled fissure. A single mass of copper was removed from the Minnesota Mine that weighed in excess of 500 tons.
This is the reason for our mining heritage. All the copper mined here occurs in metallic, natural (native) form. It is not chemically combined with any other minerals. This is VERY rare. The ONLY other place in the world where native copper occurs in quantities significant enough to mine is in Coro Coro, Bolivia in South America. The Keweenaw is the only place in the world where copper occurs in natural pieces weighing more than a few pounds. The largest piece mined from the Adventure weighed approximately 24 tons. Most of all the copper mined elsewhere today occurs in oxidized copper deposits containing little or no metallic copper. These oxidized deposits must be treated by various chemical processes to yield metallic copper.
Large piece of copper (under the two nails) on the prospectors tour.
The large piece of copper on the Prospector's Tour (in the Evergreen Lode). This section of the piece is approximately the size of a basket ball.