Formation of the Great Lakes
Part 1
Precambrian Geology
History Channel Video
Chapter 2 in Grady
Chapter 2 in Greenberg
Geological Time Line

Great Lakes are recent features of the
North America Continent, BUT

Their geological foundation was laid down
over 3 billion years ago
Geological Time Line

The Great Lakes: three important events
 Great
Lakes Basin formed
 When: 3 billion years ago
 How: Volcanism and crustal plate activity
 Individual
Lake Basins formed
 When: 10 thousand years ago
 How: Glacial activity
 Current
shorelines formed
 When: 3 thousand years ago
 How: Changes in water levels
Geological Time Line

Time hierarchy
 Eons
 Eras
 Periods
 Epochs
 Stages

The Geologic Time Line -
see handouts
era
eon
era
era
Geological Time Line

Geologic time is divided into
 Precambrian
Time
Before Cambrian (<570 mya)
 Life is mostly microscopic single celled organisms

 Phanerozoic
Time
Cambrian era and after
 Visible life (>570 mya)

Geological Time Line

Divisions of geologic time are based on
the fossil content of rocks
 Formation
 Origin
 Early
of earth ~ 4.6 bya
of life ~ 3.6 bya
Precambrian rocks contain few fossils –
so decay rates of radioactive isotopes are
used to age rocks
Glaciers carved
out GL basins
This layer
is missing
in GLB
Coal forests
in Illinois
Radioactive elements decay,
releasing particles and energy.
High energy particles may
damage living cells or DNA.
Radioactive decay occurs at a constant
exponential or geometric rate.
The rate of decay is proportional to
the number of parent atoms present
Most minerals which contain radioactive
isotopes are in igneous rocks. The
dates they give indicate the time elapsed
since the magma cooled.
Uranium and Phosphorus most common.
Half Lives for Radioactive Elements
Radioactive
Parent
Stable
Daughter
Half life
1.25 billion yrs
Potassium 40
Argon 40
Rubidium 87
Strontium
87
48.8 billion yrs
Thorium 232
Lead 208
14 billion years
Uranium 235
Lead 207
704 million years
Uranium 238
Lead 206
4.47 billion years
Carbon 14
Nitrogen 14 5730 years
Building the Great Lakes

Three time periods are important
 Precambrian
 Paleozoic
Era
 Pleistocene

Eon
Era
Processes involved
 shifting
bedrock
 sedimentation
 movements of ice, water, and wind (erosion)
Cambrian
Building the Great Lakes



Precambrian
 Bedrock of GLB formed over 3 bya
 Volcanic activity, uplift, erosion
Paleozoic
 Central North America experienced repeated
transgressions and regressions of shallow, tropical
seas
 Large areas of tropical coral reefs deposited layers
of materials that became sedimentary rocks
Pleistocene
 Series of glacial advances and retreats 10-6 kya
 Most of the topography we see around us is due to
glacial activity
Anatomy of Planet Earth
Anatomy of Planet Earth

Earth made of layers of
varying densities

Inner core makes one more
rotation than the crust every
400 years

Spins like a poorly balanced
top – wobbles on its axis
Tectonic Plates

Lithosphere and crust
broke into large irregular
chunks

Float on sluggish molten
rock of asthenosphere
and drift about freely

Collided and moved
apart many times

Process continues today
Diverging Plates

Where plates pull apart, hot molten rock (fluid
magma) emerges as lava
 New
matter is added to the plates
 New oceanic plates are formed

The place where this happens is known as a
mid-ocean ridge.
 Beneath
each of the world's great oceans there is a
mid-ocean ridge.
 Mid-ocean ridges are areas of much volcanic and
seismic activity.
The Growing Atlantic
Converging Plates

Huge plates of the earth's surface are
slowly moving together
 Edge
of one plate is gradually destroyed by
the force of collision
 sometimes the impact simply crimps the
plates' edges, thereby creating great
mountain ranges: process = orogeny.
 When one tectonic plate bends beneath the
other, it is called subduction.
Splitting Plates


A Rift or chasm is a place where the Earth's
crust and lithosphere are being pulled apart
Two rifts important in GL history
 Mid-Continent
Rift
 Saint Lawrence Rift

These rifts are responsible for the great depths
of Lakes Superior and Ontario
 Deep
valleys formed as tectonic plates pulled apart
 Valleys are now deepest regions of these two lakes
Midcontinent Rift

1.1 to 1.2 billion years ago
two previously fused tectonic
plates split apart and created
the Midcontinent Rift.

A valley was formed providing
a basin that eventually
became modern day Lake
Superior.

Rocks rich in copper and
silver in Michigan’s UP
Saint Lawrence Rift


Saint Lawrence rift, formed
around 570 million years ago
 Extends more than 1000 km
along the Saint Lawrence valley
from the Ottawa - Montreal
area
 Seismically active area
 5 magnitude >6 earthquakes
in 350 year record
 most recent earthquake in
1925
Created basins for Lakes Ontario
and Erie, along with what would
become the St. Lawrence River.
Cratons

The oldest parts of the continental crust,
known as 'shields' or 'cratons', include
some rocks that are nearly 4 billion years
old.
 Cratons
are made up of a shield-like core of
Precambrian Rock and a buried extension of
the shield.

They form the relatively stable nucleus of a
continent.
600 million years ago
Laurentian Plateau or the
North American
Precambrian Shield are
both geological terms for the
North American Craton.
USA - Precambrian Shield
Canada – Canadian Shield
Geological Provinces for the
Great Lakes Region

Provinces are geological landforms in
which all rock types are alike
 Canadian
Shield Craton made up of three
geological provinces
Superior Uplands Province
 Southern Province
 Grenville Province

 The
Central Lowlands Province contains the
lower midwest USA region of the GLB.
Geological Provinces for the
Great Lakes Region

Superior Uplands (N and NW)
 Metamorphic
 Formed
rocks and granite
4.5-2.5 bya
 Resources:
Precambrian sedimentary iron ore
deposits and copper
Geological Provinces for the
Great Lakes Region

Southern Province
 Joined
Superior Province 2.5 bya
Sedimentary rocks: limestone, shale, sandstone
 Metamorphic forms of these

Geological Provinces for the
Great Lakes Region

Grenville Province
 Joined
1.8 bya when
Canadian shield collided
with South America and
West Africa

This huge mass became
the supercontinent Rodinia
 Grenville
Orogeny –
impact formed mountain
range
Geological Provinces for the
Great Lakes Region

Grenville Orogeny
2
belts of rock types
 Central Gneiss Belt


Gneiss with granite bodies called
plutons
Central Metasedimentary Belt
Marble
 Volcanic rock
 Other metamorphosed
sedimentary rocks

Geological Provinces for the
Great Lakes Region

Grenville Orogeny
 Formed
along eastern coastline from Canada to Texas
and Mexico
 Mountains produced:
 Appalachians
 Adirondacks
 Also formed the well-studied Grenville Province of
Canada
Geological Provinces for the
Great Lakes Region

Central Lowlands (upper Midwest)
 Mostly
sedimentary rock (limestone and
dolomite) over Precambrian igneous rock
 Resources:
coal, gas, oil and oil shale, gold,
and lots of other minerals
Superior Uplands = 10
Central Lowlands = 11
From The Great Lakes:
an Environmental Atlas
and Resource Book
Canadian Shield Rock