Formation of the Great LakesFormation of the Great LakesPart 1Part 1

Precambrian GeologyPrecambrian Geology

History Channel VideoHistory Channel Video

Chapter 2 in GradyChapter 2 in Grady

Chapter 2 in GreenbergChapter 2 in Greenberg

Geological Time LineGeological 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 LineGeological 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

eraera eraeon

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 rocksFormation of earth ~ 4.6 bya

Origin of life ~ 3.6 bya

Early Precambrian rocks contain few fossils –so decay rates of radioactive isotopes are used to age rocks

Coal forests in

Illinois

This layer is missing in GLB

Glaciers carved out GL

basins

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

Potassium 40

Argon 40 1.25 billion yrs

Rubidium 87Strontium 87

48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207704 million years

Uranium 238 Lead 2064.47 billion years

Carbon 14Nitrogen 14

5730 years

Building the Great Lakes

Three time periods are importantPrecambrian Eon

Paleozoic Era

Pleistocene Era

Processes involvedshifting bedrocksedimentationmovements 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 collisionsometimes 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 alikeCanadian 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 rocks and granite

Formed 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 RockCanadian Shield Rock