Rocks that tell stories

by KEVIN STRAUSS

I’ve always liked geology. It is the “Sherlock Holmes” science. Unlike in biology or physics, where scientists can perform experiments to support or disprove a hypothesis (educated guess), geologists have to look at the evidence left behind on earth and determine how things got this way. Geologists can’t exactly create a massive lava flow and then watch it change over millions of years. Instead, they look at the land and rock formations as they are now and search for clues about how it got this way. Geologists follow a few simple rules like “if one layer of rock is lying on top of another layer of rock, the bottom layer of rock was deposited there first,” and “if a pebble or rock has round edges, it must have been eroded, probably by moving water and other rocks.”

With a few pieces of information, we can do the same thing in our own neighborhoods. Most of our native rocks are igneous rocks. “Igneous” comes from the same root word as “ignite,” these are rocks born of fire deep in the molten center of the earth. These rocks formed from cooling lava or magma millions of years ago, back in the days when northern Minnesota looked more like the big island of Hawaii with its warm equatorial temperatures and active volcanoes.

When magma flowed out into the open air, it cooled quickly, so quickly that crystals never formed in the rock and we say that it has a “smooth texture.” Black basalt and reddish rhyolite are examples of these quickly cooled igneous rocks. When magma flows into cracks in the earth, but then cools underground, it cools very, very slowly. When that happens, crystals form in the rock and we say it has a “bumpy” or “crystalline” texture. Black gabbro and reddish granite are two well-know slowly-cooled igneous rocks.

Rock color has a lot to do with rock ingredients, just as cake color has to do with cake ingredients. Basalt and gabbro are dark-colored rocks because they contain a lot of the dark mineral iron. Rhyolite and granite don’t contain much iron. Instead they contain a lot of silica (the same stuff that makes up quartz crystals and sand). Those light-colored minerals often have a pink or reddish color to them.

But after our lava rocks cooled, that wasn’t the end of the story. About 30,000 years ago, mile-high glaciers came bulldozing out of the north, grinding down and crushing much of our surface rock. Those glaciers stripped the northland of its soil and of the surface rock that covered up our native granite bedrock. Glaciers carried many of these broken-off pieces of granite, tumbling them in streams and depositing them in gravel and sand beds here in the northland.

So the next time you find a pebble or a rock outcrop, ask yourself some questions: Did this rock cool slowly or quickly? Does it contain a lot of iron (black color) or silica (red color)? Does this rock match the bedrock around here, or was it carried here by a glacier? Did an ancient glacial stream tumble this pebble and give it nice smooth, rounded edges?

By answering these questions, you are well on your way like geologists to reading a rock like a book. Because our rocks have been here a long time and they have lots of stories to tell.