America's Volcanic Past
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
| [NOTE: This list is just a sample of various Michigan features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Michigan.] |
Michigan is unique among the states in that it is divided into two areas geographically, the Northern and Southern Peninsulas, and in that, to a considerable degree, these two geographical provinces are geologically separate and distinct. This geological dichotomy is not absolute, inasmuch as the eastern part of the Northern Peninsula is geologically similar to parts of the Southern Peninsula. The western part of the Northern Peninsula is underlain by rocks of Precambrian and Cambrian age. In contrast, underlying rocks of the eastern Northern Peninsula and the entire Southern Peninsula are younger (Ordovician to Pennsylvanian) in age. In addition to this broad split, Michigan geology also is characterized by a major horizontal break-bedrock geology vs. surficial geology. Much of the state is veneered by deposits of Pleistocene age, the results of glacial and glaciofluvial depositional processes. These sediments are unconsolidated tills, gravels, sands, silts, and clays. They effectively mask much of the bedrock geology, particularly in the Southern Peninsula. For this enormous span of time the rock record is incomplete and, indeed, within many areas, only fragmentary. Much has happened to Precambrian rocks both within Precambrian and within Phanerozoic (post-Precambrian) time.
Within Michigan, the oldest Precambrian rocks have been
subjected to at least three major periods of crustal
deformation and mountain building and to at least
three or four additional minor or local deformational episodes.
In many cases these crustal crumplings
were accompanied by the intrusion of molten masses of
granitoid igneous rocks
emplaced deep within
the crust and subsequently unveiled by uplift and erosion. Periods of volcanism (at least four)
produced lava and pyroclastic rocks, chiefly of
basaltic composition and accompanied by dikes of diabase and gabbro.
Excerpts from: Michigan Department of Environmental Quality - Geological Survey Division, 1998, Michigan Geological Survey Website, 2001 |
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The Interior Plains:2
Superior Upland Province:2 Every continent has a core of very ancient metamorphic rocks. The Superior Upland Province is the southern extension of the Laurentian Upland Province, part of the nucleus of North America called Canadian Shield . The basement rocks of the Laurentian Upland Province were metamorphosed about 2,500 million years ago in a mountain-building collision of tectonic plates called the Kenoran orogeny. The rocks of the Superior Upland are mostly Precambrian metamorphic rocks and overlying Paleozoic rocks (Cambrian) covered by a thin veneer of glacial deposits left behind when glaciers melted at the end of the Pleistocene Ice Age. If we could strip away all of the younger rocks deposited on top of buried Precambrian basement, you would see a landscape of low relief. The topography of the Precambrian rocks is very subdued, with barely 500 feet difference between the highest point and the lowest. Clearly, this region was exposed to a very long period of erosion in the very distant past which beveled the original mountainous surface to a gently undulating surface. The present surface is not much different. Hills rise just a few hundred feet above the surrounding countryside. The highest of these, such as Rib Hill, Wisconsin, are made up mostly of resistant quartzite or granite. |
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| Michigan's Precambrian |
Michigan's Precambrian:1
Within Michigan, the oldest Precambrian rocks have been subjected to at least three major periods of crustal deformation and mountain building and to at least three or four additional minor or local deformational episodes. In many cases these crustal crumplings were accompanied by the intrusion of molten masses of granitoid igneous rocks emplaced deep within the crust and subsequently unveiled by uplift and erosion. Periods of vulcanism (at least four) produced lava and pyroclastic rocks, chiefly of basaltic composition and accompanied by dikes of diabase and gabbro.
Metamorphism of varying degrees of intensity accompanied many of the disturbances and transformed sedimentary, intrusive igneous, and volcanic rocks into their metamorphic equivalents. Thus basalt became greenstone; granite became granitic gneiss; sandstone was converted to quartzite; limestone to marble; and shale became slate or mica schist. The youngest Precambrian rocks, such as those of the Keweenaw Period on the Keweenaw Peninsula, have not been metamorphosed and have remained largely unchanged as basalts, sandstones, and conglomerates. Even these nonmetamorphosed rocks, however, have been tilted, folded, and faulted.
| Mid-Continent Rift |
Mid-Continent Rift:4
The Midcontinent Rift formed when North America began to pull apart about 1 billion
years ago. As the crust pulled apart, a rift valley formed and magma flowed up from
Earth’s mantle onto the surface to form the
Portage Lake Lavas.
As rifting and eruption
of the lavas slowed, rivers flowed into the rift and deposited clastic sediments, including
the Copper Harbor Conglomerate, Nonesuch Shale and Freda Sandstone, which make
up the Oronto Group. The last sediments to form, the Jacobsville Sandstone, were
deposited from rivers that spread over a large area after the rift was completely covered
by the earlier sediments. Most of the water that filled the rift appears to have been fresh,
suggesting that the rift did not reach the ocean at the edge of the North American
continent.
The Midcontinent Rift is an arcuate zone that extends northward from Iowa through the
Lake Superior region and then turns southward through the Lower Peninsula of Michigan.
Throughout most of this area, rocks of the rift are covered by younger Paleozoic
sedimentary rocks, including those of the Michigan Basin. Rocks of the Midcontinent
Rift are exposed at the surface only around Lake Superior.
Rocks of the Midcontinent Rift have been an important source of copper and silver,
particularly in the Keweenaw Peninsula and the Porcupine Mountains. Much of the
copper and silver was in the metallic state and did not have to be smelted for use,
making it especially valuable.
.
| Michigan's Copper Deposits |
Michigan's Copper Deposits:1
The Michigan native copper deposits occur in volcanic and sedimentary rocks of the Keweenawan Series of Late Precambrian age.
The most important host rocks are certain beds of the Portage Lake Lava Series, whose total thickness in the copper country is about 15,000 feet. The series consists predominantly of basaltic lava flows, several hundred in number and ranging in thickness from a foot to as much as 1,500 feet. Conspicuous but subordinate are beds of red rhyolite conglomerate, an inch to 100 feet thick, consisting of boulders of various textural types of rhyolite and subordinate basalt in a sandy matrix. Conglomerate beds are separated by from 500 to 3,000 feet of basalt, and 22 conglomerates, numbered upward from the base of the series, have been recognized and used as the main basis for stratigraphic correlation in the district.
Most of the flow rocks are basalts or basaltic andesites. Volcanic rocks conformable with the Portage Lake Lava Series but apparently slightly younger have been recognized and mapped in the Porcupine Mountains at the southwest end of the copper belt. This "unnamed formation" also has amygdules encrusted with chlorite, calcite, epidote, quartz, chalcedony, laumontite, pumpellyite, prehnite, zoisite, and clinozoisite but lacks the native copper
| Isle Royale National Park |
Isle Royale3
Isle Royale exists as an island in many ways. It is an island of wilderness and home to wolves in a modern world. It is an island in time, a natural space in which you operate on natural time and experience the rhythms of light and dark. Days are measured by footsteps, possibly under a backpack.
Walking the island you are struck by its striated layout, its elongated forested-rock and lake patterns that parallel its backbone, the Greenstone Ridge. The island, it seems, must have been forcibly combed from northeast to southwest. The surface scene you see from the island's heights is the product of 10,000 years of natural sculpting, soil-building, and plant-pioneering and succession. Back then -- actually not long ago by nature's standards -- the island appeared beneath glacial ice, rising as the lake level dropped. The island developed soil and was colonized by plants and animals. Its many inland lakes first formed in basins gouged out by glaciers, and then began to shrink, as lakes and ponds inevitably do. Beneath the ponds, the forests, and the light soil covering, however, is a story which must be told not in increments of centuries, but by millions and billions of years. The "ridge-and-trough" pattern of the rocks is the work of millions of years, pre-dating even the formation of Lake Superior and its islands. The story begins some 1.2 billion years ago with a great rift in the earth's crust which may have extended from here southward all the way to the Gulf of Mexico.
As this series of cracks poured forth molten lava covering thousands of square kilometers, the land along the rift zone sank to form the Superior Basin, which has shaped all subsequent geological events in the region. The rock record of this cataclysmic happening -- the volcanics, sandstones, and conglomerates -- forms Isle Royale's bedrock today. Clues to the island's past abound. Smoothed, rounded, and even groove rock belies the crushing power of the last major glaciation, known as the Wisconsin. It ended here only a few thousand years ago.
| Kaweenaw Peninsula |
Kaweenaw Peninsula:1
Metamorphism of varying degrees of intensity accompanied many of the disturbances and transformed sedimentary, intrusive igneous, and volcanic rocks into their metamorphic equivalents. Thus basalt became greenstone; granite became granitic gneiss; sandstone was converted to quartzite; limestone to marble; and shale became slate or mica schist. The youngest Precambrian rocks, such as those of the Keweenaw Period on the Keweenaw Peninsula, have not been metamorphosed and have remained largely unchanged as basalts, sandstones, and conglomerates. Even these nonmetamorphosed rocks, however, have been tilted, folded, and faulted.
| Portage Lake Lavas |
Portage Lake Lavas:4
The Midcontinent Rift formed when North America began to pull apart about 1 billion
years ago. As the crust pulled apart, a rift valley formed and magma flowed up from
Earth’s mantle onto the surface to form the Portage Lake Lavas. As rifting and eruption
of the lavas slowed, rivers flowed into the rift and deposited clastic sediments, including
the Copper Harbor Conglomerate, Nonesuch Shale and Freda Sandstone, which make
up the Oronto Group. The last sediments to form, the Jacobsville Sandstone, were
deposited from rivers that spread over a large area after the rift was completely covered
by the earlier sediments. Most of the water that filled the rift appears to have been fresh,
suggesting that the rift did not reach the ocean at the edge of the North American
continent.
| Western Upper Peninsula |
Underlying Rocks:
The western part of the Northern Peninsula is underlain by rocks of Precambrian and Cambrian age. In contrast, underlying rocks of the eastern Northern Peninsula and the entire Southern Peninsula are younger (Ordovician to Pennsylvanian) in age.
Precambrian Rocks:1
Because of the enormous length of Precambrian time and the complexity and multiplicity of its major geological events, the Precambrian rocks of the Northern Peninsula of Michigan are exceedingly diverse, being represented by such groups as Igneous Rocks: Intrusive and extrusive; Sedimentary Rocks: Clastic, chemical, and biochemical, and Metamorphic Rocks: Meta-intrusive igneous rocks, meta-volcanics, and meta-sediments.
Excerpts from:
1) Michigan Department of Environmental Quality Website, 2001, 2003
2) USGS/NPS Geology of the Parks Website, August 2001
3) U.S. National Park Service, Isle Royale National Park Website, 2002
4) University of Michigan, Department of Geological Sciences Website, 2003
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