America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
[This list is just a sample of
various Minnesota volcanic features or events and is by no means inclusive.]
Rocks
can tell us a lot about what happened in Minnesota
long ago. They show a history of volcanoes, seas,
mountain ranges, earthquakes, and glaciers.
Volcanic action creates lava and
ash. The rocks along Minnesota's
North Shore of Lake Superior, for
example, were created from
cooling lava. Bentonite clay in
southeastern Minnesota was
formed from volcanic ash.
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Superior Upland Province:6
The Interior Plains:6 The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability. |
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Precambrian:1
Midcontinent Rift:7
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| Minnesota's Volcanic Rocks |
Minnesota's Anorthosite:2
Similar in composition to rocks on the moon,
anorthosite
is a coarse-grained intrusive igneous rock composed almost
entirely of a single mineral -- plagioclase feldspar.
In Minnesota, anorthosite was intruded into the lowermost
lava flows that formed during the opening of the Midcontinent
rift. Anorthosite is part of the Duluth Complex and the closely
related Beaver Bay Complex exposed along the North Shore
of Lake Superior. Split Rock Lighthouse sits atop an outcrop
of anorthosite.
Minnesota's Basalt:2
About 1.1 billion years ago, the continent
that had been building for billions of years began to split apart
across what is now Minnesota. The "Midcontinent rift,"
as it is called, is where the crust began to separate
to form a new ocean basin. The same process is currently
underway between Africa and Saudi Arabia.
The rifting process stopped short of producing a new
ocean basin in central North America, but the
abundant dark red-brown basaltic rocks.
now exposed along Lake Superior's north shore are a testament
to the massive outpouring of lava through fractures or
cracks along the rift.
Gooseberry Falls State Park is
an ideal place to explore these ancient lava flows.
Minnesota's Gabbro:2
Gabbro is an intrusive rock formed when molten rock is
trapped beneath the land surface and cools into a
hard, coarsely crystalline mass.
It is the intrusive equivalent to basalt. Minnesota's best examples of
gabbro are in the part of the 1.1 billion year old Midcontinent rift exposed in the large hills at Duluth,
known as the Duluth Complex. The rock is dense, dark-colored and contains varied percentages of the
minerals plagioclase, pyroxene, and olivine. The Duluth Complex contains extensive, but relatively
low-grade deposits of copper, nickel, and platinum group elements. None are currently being mined.
Minnesota's Gneiss:2
Some of the oldest rocks in the world include the
gneiss found in the Minnesota River Valley.
The Morton
Gneiss, which is 3.6 billion years old, is a coarsely
crystalline, foliated metamorphic rock. The texture and
mineral assemblage of the Morton Gneiss give
clues as to how the rock formed. The fact that it is a
crystalline rock with large visible grains
indicates that it originated as a granitic igneous rock that cooled
slowly beneath the Earth's surface.
The foliation, or alignment of the mineral grains, indicates that the
original rock was subjected to great heat
and pressure deep below the Earth's surface. Gneiss is quarried
for use as building stone and monuments.
You can find outcrops of gneiss near Morton (the famous
"Rainbow Gneiss"), Redwood Falls,
Sacred Heart and Ortonville.
Minnesota's Granite:2
Granite
is found throughout northern and central
Minnesota.
It varies in age from 2.6 billion years in the
Minnesota River Valley and northern Minnesota
to about 1.7 billion years near St. Cloud. Minnesota
granites are composed predominantly of the minerals
feldspar, quartz, mica, and hornblende. These rocks
formed deep below the surface in the roots of major
mountain ranges. These once deeply buried rocks are
now exposed at or near the surface due to uplift and
erosion. Granite is quarried for use as building stone
and monuments. You can find outcrops of granite in
Stearns, Pine, and Mille Lacs counties, and also in
places in northeastern Minnesota, including the
Boundary Waters Canoe Area.
Minnesota Granite in Des Moines, Iowa:4
The Iowa state capitol building in Des Moines,
constructed from 1872-1884 from a
variety of building stones, is a spectacular
example of late 19th-century stone construction.
The granite base was
secured from Buchanan County boulders and quarries in Minnesota.
Limestone blocks comprising the foundation
and lower levels were quarried in Iowa at
locations in Johnson and Madison counties.
The bulk of the exterior was
constructed from sandstone blocks derived
from quarries in Missouri. Additional stone,
both local and imported,
was used in the interior construction,
including a number of decorative marbles.
Minnesota's Greenstone:2
Greenstone in northern Minnesota
is somewhat younger than gneiss.
Greenstone is a weakly
metamorphosed (altered) basalt that is,
as its name suggests, greenish to gray.
This type of rock formed
about 2.7 billion years
ago when the area that is now northern
Minnesota was part of a volcanic island arc,
much like the
islands of Japan are today. Greenstone and
other associated volcanic and related rocks have
in the past been
prospected for deposits of economic metals
such as gold, copper, zinc, lead, and iron.
Iron mines at Ely
and Soudan are now closed. As yet, no other
significant metal deposits have been found.
| Minnesota's Agates |
Minnesota's Agates:2
Minnesota's state rock is the Lake Superior agate --
so named because it is found predominantly along the
coast of Lake Superior.
Agates form in cavities in
basalt.
As mineral-rich water circulates through the
cavities, silica (SiO2), or quartz,
is deposited in layers along cavity walls. Eventually, the cavities
completely fill with this banded variety of quartz.
The color variations are due to slight mineral impurities in
the water. Iron, for example, causes much
of the red and orange color seen in Lake Superior agates.
Although agates originated in the
basaltic rocks along the North Shore,
some of the best places to hunt for
agates are in gravel pits scattered
across the state. Specifically, agates are likely to be found where
operators are mining glacial sand
and gravel deposits associated with glaciers that advanced into
Minnesota from the northeast,
bringing agate-bearing gravel
into the central and southern parts of the state.
[See Shores of Lake Superior below]
| Bear Head Lake State Park |
Bear Head Lake - Volcanoes and Ancient Seas:4
Volcanoes and ancient seas produced the
bedrock of the Bear Head Lake area.
The bedrock of
Ely greenstone and Giants-Range
granite was scoured and gouged by glaciers.
Eagles Nest and
Bear Head Lakes, along with the
park's rolling hills, were formed
from the glacial action. Along
the lakes and road cuts, visitors
can see many different shapes and
sizes of rock left by the
glaciers.
| Cascade River State Park |
Cascade River State Park:4
Aptly named, the Cascade River
flows down one ledge after another
for a total drop of 900 feet
in the last three miles of its
journey to Lake Superior.
The park setting is a boreal
hardwood-conifer forest of aspen,
birch, fir, spruce and cedar.
Visitors can stand on the
footbridge that spans the river,
or at any of the viewing spots
above the river, and feel the
vibration of the rushing torrent of
water as it cascades down a volcanic canyon.
The geology of the North Shore
and the Cascade River is complex.
It started billions of years
ago when the ancient bedrock was
covered with molten lava from volcanoes.
Later, shallow seas covered the area and
deposited sediment over the lava beds.
The sedimentary rock formed from the seas
was bent by great mountain building forces.
Other complex geological
processes took place before
the glaciers scoured the area
several times leaving the present
Lake Superior basin.
| Central Minnesota |
Central Minnesota Geology:2
There are four basic subdivisions to the
geology of the central Minnesota area.
The youngest material, which
makes up nearly 100% of the present
land surface, consists of unlithified
glacial till and glacial outwash.
These Pleistocene glacial deposits
cover older bedrock units, and are
typically in the range of 100 to 200-feet
thick but may be as much as 400 feet thick.
Cretaceous sedimentary rocks, the next oldest unit,
consist primarily of poorly lithified shale,
sandstone, and limestone.
The Cretaceous rocks in central
Minnesota occur mostly as scattered outliers
or erosional remnants on top of older Precambrian rocks,
but become substantially thicker to the
south and west.
Paleozoic sedimentary rocks,
mainly restricted to southeastern Minnesota,
consist of interlayered sequences of sandstone,
shale, and limestone that decrease in thickness
to the northwest of the Twin Cities. Paleozoic
rocks are not present in most of central Minnesota,
but are very important aquifers in the Twin Cities
and southeastern Minnesota. The counties fringing
the northwestern metropolitan area are sites
of recharge for these aquifers of the Twin Cities.
Precambrian rocks are the first bedrock encountered
in most of central Minnesota. The
Precambrian is subdivided by age into three major
groups - the middle Proterozoic (Keweenawan) (youngest),
the Early Proterozoic, and the
Archean (oldest). Rocks of the Keweenawan system run
along the axis of Lake Superior and continue south
along the Minnesota/Wisconsin
border.
These rocks occur along what is known as the
midcontinent rift, and in central Minnesota
consist primarily of basalt flows overlain by
clean sandstones, which are very similar to
overlying Paleozoic sandstones.
The next oldest
subdivision of the Precambrian, the Early
Proterozoic, consists of a wide variety of
rocks ranging from slate, schist,
and gneiss to granite and gabbro.
The northern extent of these rocks is
roughly outlined by the Mesabi and Cuyuna iron
ranges to the north, and they continue south
and southeast beneath Cretaceous, Paleozoic, and
Keweenawan rocks. The oldest Precambrian rocks,
Archean, are made up primarily of bands of
various types of granites situated between
linear greenstone belts, and in west-central
and southern Minnesota, gneiss.
Northeast-trending, steeply-dipping
diabase dikes are common
in central Minnesota. The dikes follow preexisting brittle
fractures in granitic rocks.
Granite
is found throughout northern and central
Minnesota.
It varies in age from 2.6 billion years in the
Minnesota River Valley and northern Minnesota
to about 1.7 billion years near St. Cloud. Minnesota
granites are composed predominantly of the minerals
feldspar, quartz, mica, and hornblende. These rocks
formed deep below the surface in the roots of major
mountain ranges. These once deeply buried rocks are
now exposed at or near the surface due to uplift and
erosion. Granite is quarried for use as building stone
and monuments. You can find outcrops of granite in
Stearns, Pine, and Mille Lacs counties, and also in
places in northeastern Minnesota, including the
Boundary Waters Canoe Area.
Saprolite is composed of clay minerals
that result from in-situ weathering of
bedrock by acidic groundwater. Weathering is
concentrated along vertical and horizontal
fracture planes that may intersect to
leave rounded, relatively unweathered
corestones (a relict mass of
unaltered bedrock, surrounded by saprolite,
Corestones are very
numerous in the thick saprolites
typically found in Minnesota, and may
account for the substantial number of
"pre-rounded" glacial erratic
boulders found in the state).
The texture of the original
granite is visible in the saprolite
as patches of green and white clay.
Saprolite in Minnesota is locally
overlain by thin deposits of upper
Cretaceous sedimentary rocks and may
have formed either before, or during
Cretaceous time. Kaolin-rich saprolite
is mined in the southwestern part of
the state as a filler in the manufacture
of cement.
Large, smooth, granitic rocks are
found in the valley of the Sauk River.
This type of surface is thought to have
formed when softer and more easily eroded
saprolite was removed by
fast-moving water in what is now an
abandoned glacial outwash channel.
In effect these rounded surface boulders
were at one time corestones,
and reflect the irregular distribution of
weathering patterns in the saprolite.
| George Crosby Manitou State Park |
Country Wilderness Park:4
George Crosby Manitou State Park is a country wilderness park where
waterfalls cascade through a volcanic
canyon surrounded by majestic forest.
Volcanoes spewed fiery lava which
cooled and built up in thick layers.
Later,
when glaciers moved down from the north,
they scraped and dislodged the rock. As the
melting glacier retreated,
they left piles of rock and soil
again changing the shape of the park.
The last glacier left a large
meltwater lake behind the shoreline
ridge which drained into Lake
Superior. This drainage route
is now the Manitou River.
| Gooseberry Falls State Park |
Gooseberry Lava Flows:4
Geologists have determined that about one billion years ago,
the earth began to split apart
along the area now known as the North Shore.
Lava flowed out onto the earth and cooled to
form volcanic bedrock.
Several lava flows can be seen at the Upper, Middle, and Lower Falls and
south of the Gooseberry River along Lake Superior.
These basalt lava flows are also the
birthplaces of Lake Superior agates.
About two million years ago, glaciers (up to a mile high)
advanced into the region.
As they ground across the area, they changed the landscape
dramatically. About 10,000 years ago
the last glacier melted back, filling the infant Lake
Superior and beginning the erosional
process that creates waterfalls. Today, water, wind, and
weather continue to shape the North Shore.
| Grand Portage |
Grand Portage:4
The geology of high falls is a story of fire and ice.
According to geologists, it begins in the
middle of the Precambrian era about
1.9 billion years ago. An ancient sea
covered what is now
Minnesota and deposited mud that
hardened over time into a layer of shales
and slates known
as the Rove Formation. These are the
layered rocks you can see along the inside of the high
falls gorge.
About 1.2 billion year ago,
the continent began to spread along a rift that
extended from what is now eastern
Lake Superior to Kansas. Basaltic lava flowed over this
zone, while here in Grand Portage,
diabase intrusions penetrated the older Rove Shales
turning the contact area shales
into slate.
The cliff the falls run
over is a diabase dike and is
very resistant to erosion.
Huge glaciers bulldozed away the softer rock
(shale and slate) leaving
the harder and more resistant
rock (basalt) standing.
| Interstate State Park |
Interstate State Park:3
In this park you
can see the effects of glaciers 11,000 years ago.
Potholes were created by rivers draining the glacial
lake to the north.
The basalt rock formation from 1.1
billion years ago confined the river to a gorge, and the
river cut through overlying siltstones and sandstones
and some of the basalt to create potholes.
Geologic Background:4
About 1.1 billion years ago,
volcanoes erupted from Taylors Falls to Lake Superior.
At least
10 different lava flows were the result.
The hardened basalt rock from these lava flows was
partly responsible for the formation
of the Dalles of the St. Croix and the bottom of the river.
From 530 million years ago up until
70 million years ago, the state was washed by advancing
and retreating seas. Evidence of
these ancient seas is revealed in the sedimentary rocks and
formations found in the park.
These rocks contain fossil remains of ancient animals, evidence of
various creatures, and ripple
marks left in stone by the now vanished seas. Actually, there have
been many different St. Croix Valleys
down through the ages. The original one was formed
about 70 million years ago.
The last one, which exists today, was formed about 10,000 years
ago. This last glacier formed
both Glacial Lake Duluth, now Lake Superior, and Glacial Lake
Grantsburg, now extinct. Glacial Lake Duluth
was much larger than Lake Superior is now. As
this giant body of ice thawed,
its meltwaters roared south to carve out the broad valley of the
St. Croix. Only very resistant basalts
were able to partially withstand the torrent, resulted in
the dalles, potholes, and cliffs.
| Jay Cooke State Park |
Volcanic Dikes:4
The geological makeup of
Jay Cooke State Park is one of slate,
graywacke and red clay.
Streams have exposed the
bedrock in many places.
The Precambrian Thomson Formation is
located within Jay Cooke State Park.
The formation is a
layer of mud that was compressed
and hardened into shale, then compressed further into
slate, and finally folded.
This formation is about two billion years old.
Later,
masses of molten rock were forced
through fractures in the beds
and when they cooled, these
intrusions formed dikes which can
be seen in the river bed today.
| McCarthy Beach State Park |
McCarthy Beach:4
The glaciers that moved through the area gouged
and tore at the bedrock, the cooled lava of
ancient volcanoes.
During that period, glaciers flowed south, retreated north, and flowed south
again. The first glacier stopped where McCarthy Beach is now located. It left low rolling hills
with steep sides called moraines. Between the hills, the glaciers gouged valleys and in the
larger valleys, lakes were formed. Trails from the beach lead visitors along the ridge tops of the
park's moraines. From these vistas, visitors can imagine the titanic forces that moved and
shaped the region.
| Moose Lake State Park |
Moose Lake State Park:2,3
Learn
more about Minnesota's state gemstone, the Lake
Superior agate, while visiting this park. The Lake
Superior agate is the oldest of the world's agates, 1.1
billion years old. The agate is composed of quartz
with distinctive red and white banding.
Agates form in cavities in basalt.
As
mineral-rich water circulates through the cavities,
silica (SiO2), or quartz, is deposited in
layers along cavity walls. Eventually, the cavities
completely fill with this banded variety of
quartz. The color variations are due to slight mineral
impurities in the water. Iron, for
example, causes much of the red and orange color seen in
Lake Superior agates.
[See Shores of Lake Superior below]
| Palisade Head |
Palisade Head:4
The north shore of Lake Superior
is a combination of rock cliffs, pebble beaches, and bold
headlands. The landscape owes its
character to the erosion of bedrock by running water and
glaciers. Glacial action resulted
in thin soils, scoured lake basins, and numerous rock
outcroppings. The North Shore
drainage pattern is one of short, steep rivers with many
waterfalls and deeply eroded gorges.
Some present day features, however, are the result of
much older geologic processes.
About 1.1 billion years ago, North America began to spread
apart along a rift that extended
from what is now Lake Superior all the way to Kansas. The
deep basaltic lava that poured
out of this rift formed virtually
all of the bedrock of the North
Shore.
Removal of lava from
beneath what is now Lake Superior
caused the flows to tilt to the
southeast. Spectacular examples
of these lava flows are Palisade Head and Shovel Point.
| Red Lake Indian Reservation |
Red Lake Indian Reservation:5
The Red Lake Indian Reservation contains 564,447 acres in parts of eight
northern Minnesota counties.
Bedrock underlying reservation land is Archean granite-greenstone terrane
that comprises the southern margin of the Canadian Shield.
Although glacial
cover obscures most bedrock,
the main rock types present are mafic
volcanic rocks,
felsic volcanic rocks,
graywacke-type sedimentary rocks,
and granitic intrusive rocks.
These define southern exposures of the
Wawa and Wabigoon greenstone belts,
which extend northward into Canada.
Major structural features include suture zones,
shear zones, and several generations of
folding in which early recumbent
nappes were refolded into structures with steeply dipping limbs.
| Sawtooth Mountains |
Brule River Rhyolite Flow
and the Sawtooth Mountains:4
The bedrock exposed along Lake Superior's
North Shore has a geologic history that goes back
some 1.2 billion years. During the
mountain-building, volcanic activity of
that time,
molten lava poured through great fissures that
developed in the earth's crust.
One particular flow complex,
the Brule River rhyolite flow is
thought to be as much as 3,500 feet thick.
As these flows
accumulated, the land along the
rift zone sank to form a great basin,
presently occupied by
Lake Superior. Long periods of
erosion followed. The local
Sawtooth Mountains of the Grand
Marais area in Minnesota are the
remnants of this ancient mountain range.
More recently,
glaciers also took their toll on
the area as massive ice sheets gouged out basins and scoured
the surface of the bedrock.
| Shores of Lake Superior |
Shores of Lake Superior3
Rocks
can tell us a lot about what happened in Minnesota
long ago.
They show a history of volcanoes, seas,
mountain ranges, earthquakes, and glaciers.
Volcanic action creates lava and
ash. The rocks along Minnesota's
North Shore of Lake Superior, for
example, were created from
cooling lava.
More than a billion years ago, the North American
continent began to split apart into two separate
continents. This catastrophic event, spurred by molten
rock moving deep within the earth, poured out
massive, iron-rich lava flows. These flows now are
exposed along the north and south shores of Lake
Superior.
The tectonic forces that attempted to pull the continent
apart, and which left behind the lava flows, also
created the Superior trough. The trough eventually
became the basin of Lake Superior and the lava flows
became the birthplace of Lake Superior agates.
Minnesota Agates:3
More than a billion years ago, the North American
continent began to split apart into two separate
continents. This catastrophic event, spurred by molten
rock moving deep within the earth, poured out
massive, iron-rich lava flows. These flows now are
exposed along the north and south shores of Lake
Superior.
The tectonic forces that attempted to pull the continent
apart, and which left behind the lava flows, also
created the Superior trough. The trough eventually
became the basin of Lake Superior and the lava flows
became the birthplace of Lake Superior agates.
Water vapor and carbon dioxide became trapped within the solidified flows in the form of millions of bubbles, called gas pockets or vesicles. Later, groundwater carrying ferric iron, quartz, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions crystallized into concentric bands of fine-grained quartz called chalcedony. Over the next billion years, some of these quartz-filled, banded vesicles -- agates -- were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.
About 2 million years ago, the world's climate grew
colder signaling the beginning of the Great Ice Age. A
lobe of glacial ice, the Superior lobe, moved into
Minnesota 10,000 to 15,000 years ago. It followed the
agate-filled Superior trough. The glacier picked up
surface agates and carried them south. Its crushing
action and cycle of freezing and thawing at its base
also freed many agates from within the lava flows and
transported them, too. The advancing glacier acted like
an enormous rock tumbler, abrading, fracturing, and
rough-polishing the agates.
| Shovel Point |
Shovel Point:4
The north shore of Lake Superior
is a combination of rock cliffs, pebble beaches, and bold
headlands. The landscape owes its
character to the erosion of bedrock by running water and
glaciers. Glacial action resulted
in thin soils, scoured lake basins, and numerous rock
outcroppings. The North Shore
drainage pattern is one of short, steep rivers with many
waterfalls and deeply eroded gorges.
Some present day features, however, are the result of
much older geologic processes.
About 1.1 billion years ago, North America began to spread
apart along a rift that extended
from what is now Lake Superior all the way to Kansas. The
deep basaltic lava that poured
out of this rift formed virtually
all of the bedrock of the North
Shore.
Removal of lava from
beneath what is now Lake Superior
caused the flows to tilt to the
southeast. Spectacular examples
of these lava flows are Palisade Head and Shovel Point.
| Split Rock Lighthouse |
Anorthosite:2
Similar in composition to rocks on the moon,
anorthosite
is a coarse-grained intrusive igneous rock composed almost
entirely of a single mineral -- plagioclase feldspar.
In Minnesota, anorthosite was intruded into the lowermost
lava flows that formed during the opening of the Midcontinent
rift. Anorthosite is part of the Duluth Complex and the closely
related Beaver Bay Complex exposed along the North Shore
of Lake Superior. Split Rock Lighthouse sits atop an outcrop
of anorthosite.
Magnificent Bluffs:4
The magnificent bluffs upon which the lighthouse
is built are formations of anorthosite.
The bedrock is a result of ancient lava flows.
Then over an immense time span, many complex
geological processes occurred. Water
covered the area depositing sediment over the bedrock.
Later this rock was bent and compressed by
mountain-building forces. Eventually a series of
glaciers scraped the landscape, scouring
out the Lake Superior Basin and shearing off the
shoreline cliffs. Then the meltwaters of
the glaciers flowed over the upland rock cliffs, carved
away the less resistant rock and filled
the basin we know as Lake Superior.
| Soudan Underground Mine State Park |
Soudan Underground Mine State Park:3
Go almost 1/2 mile below the surface to view the
world of underground mining. Opened in 1883, the
Soudan Mine is Minnesota's oldest and deepest iron
ore mine.
Ely Greenstone, volcanic rocks and
sediments formed in oceans over 2.7 billion years
old, may be seen here.
| Southeastern Minnesota |
Volcanic Ash:3
Bentonite clay in
southeastern Minnesota was
formed from volcanic ash.
| St. Cloud |
St. Cloud:3
The first granite
quarry was opened in the St. Cloud area in 1868.
| St. Croix State Park |
Ancient Lava Bedrock:4
During the last glacial period, about 10,000 years ago,
the St. Croix River valley served as a
major drainage channel for glacial meltwater
from Lake Superior.
As these waters carved the
way for the river seen today, the waters left
behind a variety of soils and sediment that cover
the ancient lava bedrock far below,
including a glacial outwash plain of sand left by Glacial Lake
Grantsburg. Many springs along the river banks
occur where the river valley has eroded
through the glacial gravel to release water
trapped between the sediments. The final wave of
glacial meltwater cleaned debris from the
Kettle River valley, leaving much of the basalt and
sandstone bedrock exposed. A stop at the Kettle River
Highbanks is a step back over millions
of years.
| Temperance River State Park |
Potholes:4
One of the most interesting geologic
features in the park is the narrow Temperance River
gorge with its many waterfalls.
The rapidly falling river cut through
the soft lava of the river
bed.
Swirling water carried sand
and gravel which wore away the soft lava and created
potholes.
Over thousands of years,
these potholes were dug deeper and wider, eventually
connecting and creating the deep,
narrow gorge. Nearby, more
potholes were left high and dry
as the river found its new and present channel.
| Tettegouche State Park |
Lava Flows:4
The north shore of Lake Superior
is a combination of rock cliffs, pebble beaches, and bold
headlands. The landscape owes its
character to the erosion of bedrock by running water and
glaciers. Glacial action resulted
in thin soils, scoured lake basins, and numerous rock
outcroppings. The North Shore
drainage pattern is one of short, steep rivers with many
waterfalls and deeply eroded gorges.
Some present day features, however, are the result of
much older geologic processes.
About 1.1 billion years ago, North America began to spread
apart along a rift that extended
from what is now Lake Superior all the way to Kansas. The
deep basaltic lava that poured
out of this rift formed virtually
all of the bedrock of the North
Shore.
Removal of lava from
beneath what is now Lake Superior
caused the flows to tilt to the
southeast. Spectacular examples
of these lava flows are Palisade Head and Shovel Point.
Excerpts from:
1) University of Minnesota "Minnesota Geology";
University of Minnesota "Minnesota at a Glance", 2001
2) Minnesota Geological Survey Website, 2001
3) Minnesota's Department of Natural Resources Website, 2002
4) Iowa Department of Natural Resources Website, 2001
5) Bureau of Indian Affairs, Division of Energy and Minerals Website,
2001
6) USGS/NPS Geology in the Parks Website, 2001
7) Morey, G.B., 2001, Compositions of Rift-Related Volcanic Rocks of the
Keweenawan Supergroup Atop the ST. Croix Horst, Southeastern Minnesota:
Minnesota Geological Survey Information Circular 47, 27p.
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