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

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 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

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 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 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 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 Eon Paleozoic Era Pleistocene 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 Solid inner core of iron. Liquid outer core of iron and nickel Mantle is semisolid: silicon, magnesium, iron Multilayered outer band asthenosphere – semiliquid, sluggish lithosphere – solid rock crust – thin and dense at bottom of oceans, thickest but less dense in mountainous regions. 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 Mountains formed when continental plates collide – Himalayas were formed when India crashed into Asia Plates can also slide under another plate. Ex. Pacific Plate is sliding under California. Will eventually cause California to break free of North American Plate.

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. Mid-ocean ridges are rarely more than about 4,920 ft.(1,500 m) high, but they may snake along the ocean bed for thousands of miles. Beneath each of the world's great oceans there is a mid-ocean ridge. An example is the Mid-Atlantic Ridge in the Atlantic Ocean, which stretches from the North Pole to the South Pole. Mid-ocean ridges are areas of much volcanic and earthquake activity.

The Growing Atlantic The Growing Atlantic Iceland sits on top of the Mid-Atlantic Ridge. This illustration shows what may be happening along the ridge as the North American and Eurasian plates gradually pull apart. The ridge - viewed here from the south - continues beyond the northern tip of Iceland.

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. Most of the time this happens because a dense oceanic plate collides with a lighter continental plate. You can see this along the Pacific coast of South-America. The oceanic plate dips beneath into the Asthenosphere. Through the heat of the Asthenosphere the subducted plate melts. At the surface an ocean trench is created, followed by an arc of islands. In this area also volcanic activities and earthquakes occur. When continental plates collide, one of the plates splits up into two layers: a lower layer of dense mantle rock and an upper layer of lighter crustal rock. As the mantle layer subducts, the upper layer is peeled off and crumples up against the other plate, thus forming mountain ranges, like the Alps. These are called crumpled mountains.

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.

Cambrian Shield is largest craton on earth Position has changed over the course of time During much of Paleozoic and Mesozoic it was near the equator – tropical seas etc.

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 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 Rock