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Northeastern Minnesota
GEOLOGIC OVERVIEW
Between 5 and 6 billion years ago the earth was a very hot ball of molten (melted) rock.
Roughly 4.6 billion years ago the earth started to cool off. A thin crust covered the earth,
cracked with lava flows and formed crust again. Oceans formed. Life began.
Some of these very old lava flows make up the rock foundation of Northeastern Minnesota.
The high cliffs of rock surrounding Lake Superior are over 600 million years old.
ROCKS
The rocks of Northeastern Minnesota are seen easily because glaciers have scraped away the
sediment. This process is called glacial scour. The rocks are part of what is called the
Canadian Shield. The Canadian Shield is the bedrock that formed from the lava flows that
occurred when the earth was first forming a crust.
Rock cycle processes in the past include the formation of igneous rock at and below the
surface of the earth and the weathering and erosion of the soils and rocks by glacial scour.
Rock cycle processes that still occur today are the weathering and erosion of the soils and
rocks due to wave action along the shore of Lake Superior and from regular rain and snow
runoff.
Granite: This igneous rock formed about 2.7 billion years ago. Granite formed from molten
material deep in the earth’s crust as it very, very slowly cooled underground. Granite is a light
colored rock; mostly gray, pink and white. It is coarse-grained allowing us to see individual
crystals of minerals that make up the rock. Granite is found easily near the border lakes of
Canada.
Greenstone: The rocks that outcrop at Ely are ancient lava flows that formed under ocean
water. These metamorphic rocks were changed by heat deep in the crust after they formed.
This rock is over 2.7 billion years old. The greenstones form many of the points and islands
of the North Shore because it is hard and does not erode easily. Greenstone often contains
major ores – gold, silver, chromium, nickel, copper and zinc – the major ore in Northeastern
Minnesota is iron.
Basalt: One of the youngest rocks in the area is the basalt lava flows. These dark gray igneous
rocks formed slightly less than 1 billion years ago. The rounded gentle basalt flows can be
found along the shore of Lake Superior.
Gabbro: This coarse-grained dark igneous rock was formed deep in the earth, allowing
crystals to cool slowly and grow large. After forming it was then pushed up and into the
basalt lava flows. This pushing is called intrusion. Great places to see the gabbro are at
Duluth and Gunflint Trail near Grand Marais.
Rhyolite: This red igneous rock is found mainly in horizontal layers as it intruded as a sill
into other igneous rock. Rhyolite is fine-grained and generally appears one color because the
crystals did not have time to grow large when it cooled. The magma that rhyolite formed
from was stickier than the basalt magma but otherwise it is very similar to basalt in texture.
Rhyolite is resistant to erosion and is seen along many of the cliffs around Lake Superior.
Diabase: A very resistant dark medium-grained igneous rock. This rock is found along the
rugged cliffs of the North Shore. This is mainly an instrusive rock, forming many igneous
formations along the North Shore.
Sandstone: This sedimentary rock formed in shallow water seas that covered Minnesota after
the igneous rocks had all formed. Fond du Lac and Grand Portage are areas where you can
find this sandstone. Sandstone is a soft rock that is easily weathered.
Anorthosite: An unusual and uncommon rock on earth forms many of the peaks and hills in
Northeastern Minnesota. Anorthosite also is a main rock of the highland areas on the moon!
It is made almost entirely of the mineral plagioclase feldspar and is very resistant to erosion.
It is coarse-grained and a light green color. The water fountain in Grand Marais is made of
anorthosite and Carlton Peak in Tofte is a rounded knob of anorthosite. Carlton Peak is the
second highest feature in Minnesota after Eagle Mountain.
Agate: This special sedimentary rock, 1.1 billion years old, has layers made of the mineral
Chalcedony. Chalcedony was deposited by hot water inside the holes of hardened lava. The
holes formed in the lava while air was escaping from the lava as it cooled. Agates are plentiful
in the North Shore beaches and in gravel pits near Cloquet and Moose Lake. Any glacial drift
sediment may have agates. This is our official state rock. A good place to hunt for agates is in
quarries when the ground is still wet from dew or recent rain. The color stands out next to
other rocks. Ask for permission first!
Fossils: If there were fossils from creatures that lived over 1 billion years ago, they are now
gone because of the heat from the lava flows or glaciers may have crushed them or moved
them as they passed over. Don’t fossil hunt in Northeast Minnesota!!
MAJOR FEATURES
Mountain Building: Before the North Shore lava flows were formed, a part of the continent
to our south pushed north toward us. 1.85 million years ago this portion of the continent
pushing north caused upheaval of the crust and fold mountains were formed in Minnesota.
These fold mountains were called the Penokian Belt. After the continent stopped pushing
northward, the entire mountain belt was eroded away by water, wind, and glaciers. Only
some rock evidence of the mountains exist today.
Vermilion Fault: Rocks break and move along faults because of great pressures in the earth.
Magma rises up and pushes land masses into or past ocean basins. The pushing causes cracks
in the earth.
The Vermilion Fault is the same type as the San Andreas Fault in California. The San
Andreas fault is still moving, causing potential danger from earthquakes. The Vermilion
Fault stopped moving millions or even billions of years ago.
Laurentian Divide: This divide separates the waters going north to the Arctic Ocean and
waters going south to the Atlantic Ocean. The Laurentian Divide runs irregularly from the
headwaters of the Minnesota and Red rivers to the headwaters of the Rainy and Pigeon rivers.
Any rain shower that falls on North America must sink into the ground, evaporate or flow
away to the north, south, east, or west. In certain places of high ground this rain shower must
split, with portions of it flowing different directions. Such a place of diversion of rainwater is
called a watershed or drainage divide. The Laurentian Divide marks the separation of waters
flowing north to the Arctic and south and east to the Atlantic.
North Shore Volcanics: Igneous rocks formed over 1 billion years ago make up the North
Shore Volcanics. Intrusive rocks formed deep in the earth and moved upward (granite,
gabbro) and extrusive rocks flowed from lava or were pushed into other rock (rhyolite, basalt,
diabase). These igneous rocks are seen all over the Northeastern Region of Minnesota.
Extrusive rocks pushing into other rocks form interesting geologic formations known as sills,
dikes, and batholiths. These features are made of hard rock and do not erode easily. The
features tell the tale of geologic time by the rock record.
Iron Formations: Iron is the fourth most common element in the earth’s crust, following
oxygen, silicon, and aluminum. Five percent of the earth’s crust is made up of iron. Iron
occurs in many kinds of rock; however its ores are concentrated only occasionally and in small
and widely scattered areas. Minnesota’s Mesabi Iron Range is one of the world’s greatest
concentrations of iron ore.
Iron formed in Minnesota in two ways. 1- The Soudan Iron Formation has pockets of iron
bearing rock folded into volcanic rock. 2 -The Mesabi and Biwabik formations were laid
down in shallow seas that dropped iron out to the bottom with the help of some algae.
Minerals and rocks that contain the iron are called iron ore. The iron ore in Minnesota are
red or bluish hematite, jaspilite, greenish white chert, limonite, and taconite.