Every Rock Has A Story - Amygdaloidal Basalt

This basalt specimen is a volcanic rock from the Keweenaw Peninsula’s Poratge Lake volcanic series. It formed from lava flows associated with continental rifting in the mid-continent region of North America over 1 billion years ago. Gas bubbles in the rock were secondarily filled with minerals brought in by hydrothermal solutions. These mineral filled voids are called amygdules and the rock is commonly referred to as amygdoloidal basalt. A variety of minerals are concentrated in this fashion including agates, pure copper and Michigan’s state gem, greenstone or chlorastrolite.