Minnesota's Rocky Roots — September - October 1995

young naturalists digging out fossils at Lilydale Regional Park in St. Paul

By Lisa Westberg Peters

The earth is made of rock, inside and out. Even the soil on the earth's surface comes from rock.

What is a rock? It is a cluster of one or more minerals, such as quartz, feldspar, and clay.

Rocks come in every color—pink, red, even green.

Rocks Start Out As Hot Magma

Magma is molten rock deep in the earth. It may harden underground, or ooze up as lava.

When lava hardens, it forms igneous rock. Shovel Point on the North Shore of Lake Superior formed from lava flowing over the surface of the earth 1.1 billion years ago.

Metamorphic rocks, such as the tilted layers of slate in Jay Cooke State Park, form when igneous or sedimentary rocks are heated up or squeezed inside the earth until new minerals form.

Sedimentary rocks form when mountains erode and rivers carry the sand and mud to sea. Layers of sand and mud pile up on the sea bed and harden. This sandstone cliff near the Mississippi River formed about 450 million years ago in a sea that has since disappeared.

Building a Continent

A few billion years ago, the only life on earth was single-celled bacteria. Continents were much smaller than they are today. They moved around slowly and grew whenever they collided with islands or each other.

Scientists believe northern Minnesota was a chain of volcanic islands near the edge of a small continent. When the volcanoes erupted, red-hot lava flowed into the sea. The lava cooled and hardened into pillow-shaped black rock called basalt. Later, warm seawater seeped through the basalt and formed greenstone.

The islands collided with the small continent and crumpled into mountains at the seam. Much later another collision created a second mountain range. Gneiss (pronounced "nice") and granite rocks formed deep beneath the mountains. The mountains eventually wore down, exposing the rocks.

At the same time, Minnesota's iron ore deposits formed from volcanic sediments at the bottom of the sea.

The earth's continents are still moving. Maybe someday, mountains will rise in Minnesota again!

The Continent Cracks

Long after it formed, the North American continent nearly split in two. About 1.1 billion years ago, huge cracks in the earth's crust ran from northeastern Minnesota all the way to Kansas.

For millions of years, lava from volcanoes and cracks in the crust spilled onto the land in Minnesota. Then, for reasons no one knows, the great North American split-up stopped.

Today, dirt and younger rocks cover most of the old lava flows. But you can still see thick lava flows in the cliffs of Lake Superior and Saint Croix Dalles.

Explosion of Life

Life on earth quietly "exploded" about 600 million years ago. It evolved from single-celled bacteria to complex marine animals, in a fairly short time.

It was hot in North America. The continent, nearly flat, lay near the equator. Many times the tropical sea flooded the land and then drew back. Sand and mud settled on the sea bottom. The sediments, like a quiet graveyard, held the fossils of plants and animals that lived and died in the rivers and shallow areas of the sea.

The sea invaded Minnesota one last time about 100 million years ago. Those ocean muds formed the youngest rocks in Minnesota.


A few million years ago, the earth cooled off and the most recent ice age began. Snow fell on the continents and piled up into thick sheets of ice. So much water froze that sea levels fell. When the sheets of ice grew thick enough, they began to move.

The glaciers moved south from Canada into Minnesota. When they finally melted, they left behind boulders and huge piles of sand and gravel known as moraines, kames, and eskers. Our state is a showcase of glacial features.

Some of our lakes formed in the holes that glaciers scoured in solid bedrock. Others formed because the glaciers dumped their loads of sand and gravel unevenly. The low spots filled with water. Sometimes sediments buried big blocks of ice. When the ice melted, the sediments collapsed, making depressions.

She Digs Her Work

"I like to dig in the dirt," says Barb Lusardi, who is a geologist for the Minnesota Geological Survey in St. Paul. Barb collected rocks as a kid and became interested in geology in high school. Now she studies what glaciers left behind in Minnesota.

Barb digs in and studies rocks and dirt moved by glaciers. Her work helps people find underground water for wells and good places to build roads and houses.

Will the glaciers ever come back to Minnesota? "I would think so," says Barb, because glaciers seem to come in cycles. But they won't come soon enough for any of us to see them.

Six Minnesota Rocks

  1. Morton Gneiss. Metamorphic rock, 3.6 billion years old, is found near Morton. It is used for buildings, such as the downtown St. Paul telephone building.
  2. Decorah Shale. Sedimentary rock, 450 million years old, is the hardened mud from the bottom of an old ocean. Found in St. Paul and southeastern Minnesota, it is filled with sea fossils.
  3. Agate. This special sedimentary rock, 1.1 billion years old, has layers made of a mineral called chalcedony, deposited by hot water inside the holes of hardened lava. Agates are plentiful on North Shore beaches and gravel pits near Cloquet and Moose Lake. This is our official state rock.
  4. St. Peter Sandstone. Sedimentary rock, 450 million years old, looks like crumbly white sand. It is found in St. Paul and southeastern Minnesota.
  5. Granite. Speckled pink or gray igneous rock, 1.8 billion years old, is found in many places. Minnesota granite was used in parts of the Statue of Liberty and in the Minnesota History Center in St. Paul.
  6. Basalt. Black or dark gray igneous rock, 1.1 billion years old, is found along the North Shore and the St. Croix River at Taylors Falls.

Hard facts

Two good books for rockhounds are Minnesota's Geology by Richard W. Ojakangas and Charles L. Matsch (University of Minnesota Press, 1982), and Minnesota Underfoot by Constance Jefferson Sansome (Voyageur Press, 1983).

Minnesota Mining Mix-up

The scrambled words are rocks and minerals mined in Minnesota that are used to make things you use every day. Read the following hints, then unscramble the words.

  1. Speckled igneous rock used for building stone: eitgnar
  2. Coarse rock dug from glacial deposits and used for landscaping and construction: vlgare
  3. Reddish rock dug from open pits in northern Minnesota and used to make cars and refrigerators: nrio reo
  4. A sedimentary rock used for building stone: eontsndas

Want to find out more about rocks? Order Digging into Minnesota Minerals, a free children's publication from the Department of Natural Resources. Call 651-259-5959.

Lisa Westberg Peters is a children's book author and geology enthusiast, who lives in St. Paul with her rock-collecting family.

A complete copy of the article can be found in the September - October 1995 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.