Lake Superior Born of Fire and Ice Jim Miller University of Minnesota Duluth Plate Tectonic Theory The Earth’s outer shell (lithosphere) is composed of rigid plates that are moving relative to one another. Convergent Boundaries Divergent Boundaries Convegent Boundaries •Explosive volcanism •Earthquakes •Mountain-building Divergent Boundaries Continental Rifting Making Ocean Crust Mantle Plumes and Hot Spots 200 Ma The Break-up of Pangea Laurasia Gondwanaland 125 Ma 75 Ma Radiometric Age Dating UNIFORMITARIANISM - The Present is the Key to the Past NORTH SHORE ICELAND HAWAII North America (Laurentia) in the Late Precambrian (1,100,000,000 years ago) _______________ THE MIDCONTINENT RIFT _______________ An attempt at continental rifting 1.1 billion years ago A. Rift Magmatism Basalt Flows Gabbro Crust Mantle The Evolution of Lake Superior Part I - FIRE 1,109-1,086 Ma Mantle Plume B. Sediment Infilling Sandstone 1,090-900 Ma C. Compression 1,000-900 Ma Fissure Eruption Massive Basalt Amygdaloidal Basalt Amygdules – Gas bubble fillings Chlorite & Zeolite Epidote & Feldspar Zeolite Thompsonite LAKE SUPERIOR AGATES Gemstone of Minnesota Formation of Lake Superior Agates BASALT FLOW SURFACES AA AA Pahoehoe HAWAII Pahoehoe NORTH SHORE High Falls on the Baptism River Shovel Point RHYOLITE The other volcanic rock type Palisade Head INTRUSIVE IGNEOUS ROCKS Gabbro Magmas trapped in the Crust Duluth Complex Layered Gabbro Beaver Bay Complex Granite Origin of MCR Igneous Rocks SEDIMENTARY ROCKS of the Midcontinent Rift Cutface Creek Sandstone Freda Sandstone LATE-STAGE FAULTING The Douglas Fault at Amnicon Falls State Park Basalt Sandstone Why isn’t Duluth a Sea-side Resort? Blame it on the Grenville Geologic Events Recorded in Minnesota’s Rocks The Ice Age: 1,800,000 yrs – Present? Last Glacial Maximum Minnesota’s Landscape: The Legacy of the Glaciers Glaciers: Nature’s Bulldozer Advance Erosion Retreat Deposition Powerful Agents of Erosion Deposits from Ice Till Deposits from Meltwater Outwash The Land of 10,000 Lakes The Last Glacial Retreat out of Lake Superior The Excavation of the Midcontinent Rift A. Rift Magmatism Basalt Flows Gabbro Crust Mantle The Evolution of Lake Superior Part I - FIRE 1,109-1,086 Ma Mantle Plume B. Sediment Infilling Sandstone 1,090-900 Ma C. Compression 1,000-900 Ma The Evolution of Lake Superior Part II - ICE D. Surface Weathering 900 – 2 Ma E. Glacial Erosion 2 Ma – 12,000y F. Lake Superior NE Minnesota Today Isle Royale Keweenaw Peninsula For More Information John Green Ron Morton Scott Wolter Coming in 2009! Geology Underfoot in Minnesota by Jim Meyers & Jim Miller