Lake Superior Agate

Lake Superior agate, Minnesota's state gemstone, is a translucent, color-banded, reddish-brown stone formed from processes that began 1.1 billion years ago when basaltic lava repeatedly erupted in the Lake Superior region and spread across the landscape. Gas bubbles in the viscous lava were trapped as rapid cooling near the top of a flow quickly solidified the lava and created porous zones along the flow top. After the lava solidified, warm groundwater carrying varying dissolved chemicals percolated through the basalt, especially through these porous zones, and lined the gas cavities with layers of different minerals until they were filled. A common cavity-filling mineral was chalcedony, a type of quartz made of microscopic fibrous crystals, and when chalcedony formed in color bands it became agate. The bands resulted from small amounts of impurities such as iron, copper, and aluminum that were cyclically precipitated in alternating layers of the fibrous quartz, while the center of an agate was often filled with coarsely crystallized quartz. Over long spans of time, plate tectonics carried the continent slowly across tropical climate zones, where intense chemical weathering broke down the basalt but not the more resistant agate. By about two million years ago, as the Ice Age began, near-surface basalt flows were heavily weathered and rich in hard agate nodules, and advancing glaciers from the north scoured the Lake Superior basin, removed the basalt, and carried the agates away, some as far as Kansas; many agates in Flood Bay may have come from areas in Ontario where similar rocks are found.

Minnesota · United States