INTRODUCTION TO GEOLOGICAL PROCESSES
 
 

I. Structure of the Earth

        Inner Core: Solid Iron-Nickel

        Outer Core: Molten Iron-Nickel. Magnetic field of Earth created by currents set up
            in this molten layer

        Mantle: Patially melted at top, solid below, basic Fe-Mg rich silicates. Convection
            cells within the upper section of the mantle leads to movement of crustal plates

        Crust: Solid, light, acidic Al rich silicates. Fragmented into approxiamtely 12 major plates.
 

II. Mechanisms of plate tectonics

         Convection cells develop in upper mantle

         Light crustal rocks are pulled along on top of these cells

         Types of plate boundaries:

        Divergent -- where plates move apart from upwelling mantle rock. These areas are usually
            deep in the ocean, and support chemosynthetic food webs based upon sulfur bacteria.

        Convergent -- where plates crash together, forming mountain belts

         Parallel -- where plates slip past one another
 

III. The rock cycle and rock types

        Igneous -- once molten rock. varies from acidic to basic, depending upon origin of melted
            materials. Acidic igneous rocks come from melted crust, while basic igneous rocks come
            from the upper mantle.

         Extrusive: molten rock cools after being erupted on surface. As it cools quickly, mineral
            crystals in it will be microscopic. Acidic = Rhyolite; Basic = Basalt

 Intrusive: molten rock cools inside of crust. As cools slowly, mineral crystals become large. Acidic = Granite; Basic = Gabbro

Sedimentary -- rock created by the erosion of other rocks, by wind or water action, or by biological processes, which eventually become hardened.

Sandstone: water or wind deposition of sand eroded from igneous or metamorphic rock

Conglomerate: water deposition of pebble and larger sized rock fragments

Shale: water deposition of clay and silt

Limestone: Calcium carbonate deposition by various life forms (cyanobacteria, algae, corals, mollusks)

Coal: accumulation of dead plant materials in anerobic situations

Metamorphic -- alteration of igneous or sedimentary rocks through exposure to heat, pressure, and chemically active fluids. Typically occurs at convergent crustal boundaries.

 
IV. Geological History of the Upper Midwest  Earth's history broken into four major eras:

Precambrian (4.5 billion-570 million years ago)

Paleozoic (570-230 million years ago)

Mesozoic (230-65 million years ago)

Cenozoic (65 million years ago to today)

 

Precambrian: early on, Earth's atmosphere is anerobic (lacking oxygen). Early photosynthesizers produce O as a waste product, which builds up in atmosphere. Eventually Atmospheric O is high enough to cause Fe to precipitate out of oceans, forming iron deposits of N. Minnesota, Wisconsin, and Michigan. Much mountain-building in the region, with granites, gabbros, and metamorphic rocks being formed. in late Precambrian (600 mya), a rift system begins to form in Lake Superior region, splitting North America in two. This rift fails, North America remains a single continent, but massive quantities of basalt are erupted during this time.

Paleozoic: from Cambrian (570 mya)-Pennsylvanian (310 mya) most of the region is inundated by shallow seas 6 times; in total 1/2-1/3 of this period is spent with the region under water. During this time a wide assortment of sedimentary rocks (sandstone, shale, limestone/dolomite) are deposited. At this time the North American plate was at or south of the equator; thus, the marine life found in these oceans was tropical and much like modern coral reefs. At end of Paleozoic, the collision of the African, Eurasian, and North American plates uplifts the eastern side of the continent, causing region to raise above sea level.

Mesozoic: except for a brief, shallow inundation during the late-Cretaceous (ca. 90 mya), region is above water and part of a tropical desert ecosystem. While dinosaurs certainly lived here, almost none of their remains have been preserved, as no sediments from that time were deposited. Break-up the supercontinent, Pangea, begins in late-Triassic to early-Jurassic (175-150 mya), creating proto-Atlantic Ocean, with the first mountain-building in the west starting in the late Cretaceous.

Cenozoic: region remains above sea level and is warm-tropical until ca. 10 million years ago, when drift of the North American plate moves region too far north. Starting 2.4 mya, climatic fluctuations begin which cause a series of at least 12 ice ages, re-occurring at approximately 300,000 year intervals. The last of these, which ended around 15,000 years ago covered the northern and eastern parts of Wisconsin, and carved out the basins of lakes MIchigan and Superior. Soil substrates of region are largely made from rock ground up and carried south by these glaciers (glacial till) or from deposits of wind-blown silt (loess) which accumulated south of the glacial margin. Rapid climatic change at end of this ice age (called the Wisconsinan) leads to extinction of many species, including large mammals like the mastodon and mammoth.
 

Home