Lava Flow Anatomy: Triangular Motif, Amygdaloidal Minerals

The Keweenaw features an assymetrical profile, with noticeable slopes.  If you once notice it, you see it everywhere.

On Isle Royale the rocks are the same, and you find the same triangular motif---but the layers dip the other way. This dipping of layers has a big influence on every day life, landscapes, roads, water supplies, shorelines, harbors and many other things.

Questions to explore: Once you know about the tilting of layers that is fundamental to the Keweenaw and Isle Royale--what does it lead to? Why does this structure concern us?  How can we use this information?

How does the dipping rock affect life in Houghton and Hancock, which are built right on them?  How does it affect the Peninsula?

In Houghton 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  in Guatemala 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.

Amygdaloidal Minerals of Michigan’s Keweenaw Peninsula Lavas

The most obvious properties of the amygdule minerals can be used to identify them. The sport of mineral collecting is done by many visitors who use this kind of mineral Identification to find local treasures, including agates and datolites.


What’s next?  After mastering the mineral identifications in the boulders, students can also look at amygdular minerals to study the order that minerals deposited in those vesicles, what mineralogists call paragenesis.