Outline of "Minnesota's Geology" Pages 3-13 Introductory Background Material Introduction
Perhaps the most obvious feature on a satellite image of the state of Minnesota is the Minnesota River Valley. The valley forms a deep trench across the southern third of the state, from the border with South Dakota, southeast to Mankato and northeast to its junction with the Mississippi River at Fort Snelling, St. Paul. A trip along this river valley is a trip through geologic time, from the oldest crustal rocks in North America, upward through the Phanerozoic cover of sedimentary rocks, to the glacial deposits of the last great advance of Pleistocene ice.
- nearly 200 feet deep and 3 miles wide
- carved by catastrophic floods draining a huge glacial lake in the Red River Valley to the north less than 12,000 years ago
- the modern Minnesota River is far too small to have carved this deep gash in the surface and is merely the modern successor to the great River Warren
Oldest rocks in North America
- dark red gneisses on the floor of the valley near Montevideo and in quarries at Morton
- these regionally metamorphosed rocks have a long and tortured history including melting, crystallization and deformation at depths of 10 or more miles beneath the surface of the crust
- modern exposures are the result of ancient tectonic uplift together with the erosional stripping of huge amounts of younger rocks
Cretaceous rocks
- near Redwood Falls, exposures of sandstones and shales reflect weathering and erosion of the ancient gneisses and transport and deposition of the resulting sediment in a variety of terrestrial and marine environments
- fossils in these rocks indicate Cretaceous age and climates were probably subtropical
- downstream beyond Mankato, Paleozoic sandstone, shale and limestone are exposed in bluffs along the river
- these rocks record repeated invasions and retreats of an ancient sea across this part of Minnesota around 450 million years ago
- beyond the valley on the uplands are rocky unconsolidated sediments that record advances of glacial ice across the state as little as 12,000 years ago
And elsewhere in Minnesota, the rock record includes evidence of not only these events, but events of a more complex sort including continental rifting and volcanic activity. All of these events which have shaped the area we now call Minnesota can be understood by studying the record of history in the rocks.
Minerals and Rocks
The authors of the text go on to describe minerals and rocks and relate several geologic features of Minnesota to different types of earth materials. We have talked about minerals and rocks in our introductory lecture, and provided web references on the topic. You can also find more on minerals and rocks here. In this outline, we will simply make note of localities in Minnesota where different types of rocks are exposed.
Igneous rocks - compare rocks along the north shore of Lake Superior near Duluth with rocks in the quarries near St. Cloud
- both are igneous rocks, cooled and solidified from magma, but the rocks near Duluth are dark colored, mafic, finely crystalline volcanic rocks, while the rocks near St. Cloud are light-colored, sialic, coarsely crystalline intrusive rocks.
- the rock near Duluth is called basalt and represents the rapid solidification of lava flows like those that form modern day oceanic crust. Basalt, which is finely crystalline, is also found as far south as the Dalles of the St. Croix River at Taylor's Falls. The rock near St. Cloud is called granite. In the St. Cloud area, granite is quarried for building stone. Granite is coarse-grained and represents the slow solidification of less dense, more silica-rich magma, which makes up continental crust
Sedimentary rocks
- Ojakangas and Matsch give a very nice brief yet thorough description of the formation of sedimentary rocks, like the sandstones, shales, limestones and dolomites exposed in the bluffs that guard the Mississippi Valley from the Twin Cities south to the Iowa border. The following is a quote from their book.
- "Sedimentary rock...is the reconstituted debris that results from the destruction of earth materials in the surface environment. Bedrock exposed to weathering and erosion either breaks up to form sediments or decomposes chemically. Eventually the rock and mineral fragments, along with dissolved material, are swept away by the moving fluids that bathe the entire surface of the earth, especially running water. From the continents, most of the debris is washed into the ocean basins. There, waves winnow out finer material, leaving behind sandy beaches, and finer muds settle in the quiet depths. Under the proper circumstances the dissolved material is precipitated in the form of chemical sediments. Compaction and cementation turn the sediment into rock." (Ojakangas and Matsch, 1982, p. 4).
Metamorphic rocks
- metamorphic rocks, like the Morton Gneiss described earlier, are among the oldest rocks in Minnesota and are exposed in the western part of the Minnesota River Valley, as well as in the northern half of the state. They represent changes in preexisting or parent rocks during regional metamorphism in the cores of great and ancient mountain ranges, long since worn down by earth's surface processes.
Soils
Ojakangas and Matsch discuss the formation of soils which are important to Minnesota's agricultural economy. You should review this section carefully, as we have not discussed soils in lecture.
- soil is the ultimate product of the weathering of bedrock at the earth's surface.
- a more general term than soil that can be applied to the loose aggregate of rock and mineral fragments that blankets the surface is regolith.
- soil is layered, consisting of a dark gray to black organic-rich zone at the surface, followed by a finer-grained to clayey reddish brown to brown iron-rich horizon, followed by weathered but recognizable bedrock
- the uppermost horizon (A horizon) reflects plant and animal activity and the removal of material like iron and clay and calcite by downward-percolating water.
- in the relatively temperate climate of Minnesota, the middle horizon (B horizon) reflects deposition of the clay and iron, while calcite is washed away or leached from this zone. (If the climate were arid, calcite would be deposited in this horizon to produce a hard whitish calcareous material called caliche.)
- many variations may occur in soil profiles, caused by variations in bedrock, climate, topography, vegetation, and time of exposure.
Fossils
Much of what we know about earth history comes from a study of fossils, which are the remains of organisms that were buried and preserved in the rock record. Because the most common sorts of environments in which organisms lived and died are sedimentary environments, and because sedimentary processes are often amenable to the preservation of organic remains, fossils are most often found in sedimentary rocks.
- fossils include hard parts of organisms, impressions of organisms called molds, casts of organisms formed by the filling of molds with sedimentary material, and also traces such as trackways and burrows, made by the organisms as they acted out their lives in the sedimentary environments. Mounds of sticky algae, living in shallow marine water, can trap carbnate sediment and create mound-like layers in the resulting limestones and dolomites. Such structures are called stromatolites.
- study of the fossil record from older rocks to younger rocks reveals patterns of change reflecting greater complexity and diversity of species through time.
- this pattern of change, called evolution, results in extinction of some species and appearance of new species. Such changes enable geologists to correlate sedimentary rocks according to the time they were formed by comparing key index fossils in localities that may be located far apart from one another.
- fossils also enable geologists to reconstruct ancient environments in which the sediments containing the fossils were deposited. This is done by comparing the environments in which modern organisms live, with those of related organisms from the fossil record. Such studies, which utilize observations of modern earth processes and products to interpret the ancient, make use of the principle of uniformitarianism. This principle is sometimes simplified to simply say that the present is the key to the past.
Geologic time
This topic has been studied in detail in lecture notes 5. Only new material in Ojakangas and Matsch will be outlined here.
Absolute age determinations by radiometric dating
- the Morton gneiss in the Minnesota River valley has been dated by radiometric methods.
- a minor silicate mineral in the gneiss, zircon, has small amounts of the element uranium trapped in its crystal structure.
- Isotopes of uranium are radioactive and decay to stable isotopes of lead
- decay rates for uranium isotopes are known. For example, U-238 decays to Pb-206 at a rate such that one-half of the total parent atoms disappear in 4.5 billion years. This is the half-life of U-238, which, coincidentally, is close to the age of the Earth which is 4.6 billion years.
- measurements of the ratio of the parent U-238 to the daughter Pb-206 in the zircon of the Morton gneiss indicate that the decay process has been going on for about 3.6 billion years. This is the age of the rock itself, a measure of the time when the metamorphism occurred. Of course, the parent rock which was itself metamorphosed, must have been still older than this!
- other radiometric clocks can be used for dating. Isotopic pairs of the following parents and daughters are most useful in different rocks of different ages. rubidium-strontium, potassium-argon, and carbon-14-nitrogen. Carbon-14 is useful for dating organic remains younger than about 50,000 years.
Age of the Earth and the divisions of geologic time
We have already discussed this topic in class (lecture notes 5). We should here emphasize that the Earth is of great antiquity, around 4.6 billion years old, and that throughout its history, geologic processes which seem to operate slowly compared to our human frame of reference, can achieve great results over long spans of time. Thus, sea-floor spreading, which operates at rates of a few cemtimeters per year, can result in the opening of the Atlantic Ocean basin in less than 200 million years!
When we consider the divisions of geologic time in our calendar, called the geologic time scale, we should note that early divisions of the time scale were based on the record of past life, the fossil record. Thus, the Paleozoic was so named because of the ancient forms of life preserved as fossils, the Mesozoic was so named because it reflected a middle stage in the evolution of organisms, and the Cenozoic was so named because it reflected life that is most-recent. We sometimes think of the Paleozoic as being dominated by marine invertebrate organisms, the Mesozoic as the "age of reptiles", and the Cenozoic as the "age of mammals".
Earth structures
Deformation of the crust, generally resulting from plate tectonic processes, has resulted in the folding and faulting of rocks. These structures, folds and faults, can be used to interpret the history of deformation in the rock record.
- mountain-building events are often referred to by the term orogeny
- orogeny results in folding of rocks
- up-folds are called anticlines
- down-folds are called synclines
- folds may be so strongly deformed that they are turned over on their sides (overturned folds). The asymmetry of such folds indicates the direction from which the major deforming force came.
- if the folding of rocks exceeds their strength, they may break along faults
- events that uplift the crust but do not deform it significantly are often referred to by the term epeirogeny.
Plate tectonics
We have discussed this concept in some detail. Ojakangas and Matsch emphasize the place of Minnesota with respect to modern plate boundaries. Minnesota lies in the interior of the North American plate, which stretches from the San Andreas fault of California and the trench off Oregon and Washington, eastward to the mid-Atlantic ridge.
While internal processes are apparent at the plate edges and are manifest by earthquakes, volcanic activity and mountain building at the leading edge (convergent margin), the plate interior is largely devoid of the evidence of modern active internal processes. Rather, since the end of the Precambrian, Minnesota has only experience broad up and downwarping of the crust.
However, during Precambrian time, Minnesota's place relative to plate boundaries and the location of mountain building was different, and this part of the Earth's crust went through several episodes of strong deformation. We will explore this record in greater detail.
