Igneous Rocks of the North Shore
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MINNESOTA’S NORTH SHORE A Natural Laboratory for Igneous Rocks MMEW 2015 Jim Miller Magma Creation by Partial Melting Melting the mantle makes MAFIC Magma Melting the crust makes FELSIC Magma Igneous rocks crystallize over a range of temperatures Mafic Magma Formation at Mid-Ocean Ridges Mafic Magma Formation at Subduction Zones Magma to Rock Mag Mafic Magma T - 1250-1050 C Felsic Magma T - 1050-800 C Magma Differentiation by Fractional Crystallization IGNEOUS ROCK TEXTURES f (rate of cooling = crustal environment) Plutonicī Volcanic 1 mm Degree of Undercooling 1 mm Porphyritic Texture Indicative of two-stage cooling Ophitic Texture Indicative of Late Pyroxene Crystallization from a Primitive High-T Mafic Magma Ophitic Basalt Ophitic Gabbro Mafic magmas at divergent boundaries Volcano Types f (Magma type = Plate Boundary) Mixed magmas at convergent boundaries Stratovolcano Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows, are called stratovolcanoes. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions. Mt St. Helens Explosive (pyroclastic) Eruptions Costa Rica Phillipines Pillowed Basalts Submarine Eruptions Shield Volcanoes Basaltic Volcanism in Oceanic Settings Basaltic Volcanism in Continental Settings Mantle Plumes and Hot Spots LARGE IGNEOUS PROVINCES Manifestations of Plumes and Hot Spots _______________ THE MIDCONTINENT RIFT _______________ An attempt at continental rifting 1.1 billion years ago Plume Head 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 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 ROCKS Magmas trapped in the Crust Duluth Complex Coarse-grained Gabbro Layered Gabbro Beaver Bay Complex Volcanics Present-day Surface Intrusions Mt Josephine Eagle Mtn Hovland/ Brule River Reservation Devil’s Track River River Grand Marais Cascade Lutsen Mtns River Carlton Peak Temperance River Sugarloaf Finland Air Base Cove Tettegouche Split Rock Gooseberry Falls Lighthouse Silver Cliff Tunnel Two Harbors City Park Tischer Creek Twin SkylinePonds Parkway Ely’s Peak HAPPY HUNTING!
