Formation of the Great Lakes Part 1 Precambrian Geology

1. Formation of the Great LakesPart 1Precambrian Geology History Channel Video Chapter 2 in Grady Chapter 2 in Greenberg

2. 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

3. 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

4. Geological Time Line • Time hierarchy • Eons • Eras • Periods • Epochs • Stages • The Geologic Time Line - see handouts

5. eon era era era

8. 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)

9. 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

10. Glaciers carved out GL basins This layer is missing in GLB Coal forests in Illinois

11. Radioactive elements decay, releasing particles and energy. High energy particles may damage living cells or DNA.

13. Radioactive decay occurs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present

14. 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.

15. Half Lives for Radioactive Elements

17. 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)

18. Cambrian

19. 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

20. Anatomy of Planet Earth

21. 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

23. 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

24. 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.

25. The Growing Atlantic

26. 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.

28. 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

29. 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

30. 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.

32. 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.

34. 600 million years ago Laurentian Plateau or the North AmericanPrecambrian Shieldare both geological terms for the North American Craton. USA - Precambrian Shield Canada – Canadian Shield

35. 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.

37. 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

39. Geological Provinces for the Great Lakes Region • Southern Province • Joined Superior Province 2.5 bya • Sedimentary rocks: limestone, shale, sandstone • Metamorphic forms of these

41. 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

43. 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

44. 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

46. 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

47. Superior Uplands = 10 Central Lowlands = 11

48. From The Great Lakes: an Environmental Atlas and Resource Book

49. Canadian Shield Rock