At the Eagle Harbor Lighthouse a sequence of tilted lava flows shows us how to tell where one flow starts and the other stops. It also shows how the lava flows have hard and soft layers.
This active lava flow has a crusted upper surface which is called aa. It is broken lava crust material which gets progressively ground up by the movement of the flow, which abrades the pieces. While it breaks up, the aa flow top insulates the interior of the flow from heat loss.
Many Keweenawan lava flows have fairly smooth tops, more like pahoehoe. These smooth flow tops are bubbly, and these bubbles get filled with colored minerals which are green, white, pink, black and other colors. Here a lava flow is tilted away from us, like most of the Keweenaw lavas, but the flow top can be easily seen because of the bright bubbles.
These beach pebbles are pieces of the pahoehoe flow tops of Keweenaw lavas. The colors are the various minerals of the native copper mineralization--from hot spring water which was pumped through the porous flow tops. The filled bubbles tell the story of this hot water mineralization.
