ABOUT 1.1 BILLION YEARS AGO NORTH AMERICA TRIED TO SPLIT IN TWO. The split failed but left behind a great rift or chasm of boiling igneous rock, which extruded to the surface. This Mid-Continental Rift ran 1200 miles, from what is now the North Shore of Lake Superior, where its effects are most visible and spectacular, all the way south to Kansas. The grand Lake Superior cliffs, headlands, and pebble beaches we enjoy today are a result of that great rift.
On our summer Road Scholar trips, we take an interest in such things, exploring geologic history and rocks such as those you see here. The large one is Porphyritic Basalt— in which magma slowly cooling below the earth’s surface had time to form crystals of feldspar (note the long shapes, square corners, and edges) before it erupted to the earth’s surface during the rift. There, the partially crystallized but molten magma cooled very quickly, creating the dark, smooth stone (basalt) but still containing the feldspar (porphyritic) crystals.
The two stones to the lower right are similar, but after cooling they had round gas bubbles that were later filled slowly, and over eons, with other minerals—calcite and quartz. These filled bubbles ( more rounded than the porphyry,) are called Amygdules, and the rock is thus Amygdular Basalt. Sometimes they are filled with agates!
The fourth rock is also smooth, quick-cooled basalt, but no bubbles. What it does have is a wonderful oblong shape, perfect for the palm of a human’s hand. Many of our North Shore rocks are flat or oval or elongated just like this as you will have noticed. As billions of waves wash over the beaches, round rocks are rolled back into the big lake, while flattened or elongated ones remain on the beach for us to find. And take home, and hold in our hand as a remembrance.
It is fun to learn and discover such things on a North Woods trip, as the wind moans in the pine tops and the waves wash the shore…