Minnesota Geology Monday - Tombolo

Grand Marais is one of the best natural harbors on Minnesota's North Shore of Lake Superior.  The area was a natural attraction to European traders because of its use as a harbor.  The area has likely changed since those days, Grand Marais is French for 'great marsh', indicating the harbor was likely a marsh.

The harbor owes it's creation to the erosion and weathering of relatively soft lava flows that is found between more resistant flows.  One of the more resistant lava flows is/was an island that is now connected to the mainland by a gravel bar.  Wave action and currents in the lake built the gravel bar up to the point that it connected the land to the island providing a natural harbor.  The name for a connecting bar like this is a tombolo.

Minnesota Geology Monday - Conglomerate

On the south side of US highway 14 near New Ulm, justwest of the intersection of County Road 37 is a small outcrop of Proterozoic conglomerate.  The location is across the Minnesota River from New Ulm, though within the river valley.  The outcrop itself is small, fairly covered by grasses, mosses and other plants.

The conglomerate is lies above Archean gneisses and is the base of the early Proterozoic Sioux Quartzite.  The 1,700 million year old Sioux Quartzite is found throughout SW Minnesota, NE Iowa and South Dakota and correlates well with other quartzites in other states like the Baraboo Quartzite in Wisconsin.

Pieces of the Archean gneisses are incorporated into the conglomerate.  Over one billion years of weathering of these gneisses occurred before being cemeted into the conglomerate.  This basal conglomerate is interpreted to be braided stream deposits close to the source rock.

Plate Boundaries

When preparing for the test coming next week, the fourth outcome on our 'Vocabulary Page' says:
Be able to predict the types of earth surface features that might be present when given the type of plate boundary.  Example - what would you expect to find at the boundary between: a subduction zone, divergent seafloor plates, convergent continental plates and/or transform plate boundary.

The pictures identifies examples of different types of plate boundaries, you should be able to predict what occurs at these boundaries based on the maps you've made in class.
For next weeks test, also plan on being able to identify different plate boundaries (convergent, divergent, transform), i.e. use the picture to help in preparation.

Discovering Plate Boundaries

For the past few class periods, Delano 8th Grade Earth Science students have been working on the activity called 'Discovering Plate Boundaries' developed by Dale Sawyer at Rice University.  This great activity allows students to discover what occurs at tectonic plate boundaries by making observations of real data.  A paper was published in the January 2005 Journal of Geoscience Education describing the activity.

The activity focuses on four different data maps:
1.  Volcanology - this maps shows the locations of currently or historically active volcanoes or volcanic features..

2.  Geography - the map demonstrates the elevation above or depth below sea level in meters.

3.  Geochronolgy - the data on this map indicates teh age of the ocean floor in millions of years.

4.  Seismology - this map shows earthquakes with magnitudes greater than 4 that occurred from 1990 - 1996.  The map also indicates at which depth the earthquakes occurred.

During the activity, students make observations and classify different plate boundaries based on the data provided.  Students first focus on one of the four specialty types described on four data maps, actively placing plate boundaries into different groups due to observations made from the data.  Students are then placed in new plate groups, where each group has a specialist in Volcanology, Geography, Geochronology and Seismology.  This group then classifies the plate boundaries around one specific plate, i.e. the North American plate.

After a short presentation from each group on the different types of plate boundaries surrounding their plate, students are introduced to the three different types of plate boundaries (divergent, convergent, and transform).

    1.  Divergent - areas where plates are moving away from each other.
    2.  Convergent - areas where plates are moving towards each other.
    3.  Transform - areas where plates are sliding past each other.

Different plate boundaries have different characteristics or features that students are now able to identify easily on their maps because they have a knowledge of what is occuring at these plate boundaries.

Minnesota Geology Monday - Mill City Conglomerate

Within Interstate State Park near Taylor's Falls, Minnesota are several unique geologic formations.  One of these is the informally named, Mill City Conglomerate, a basalt boulder conglomerate of Cambrian age.

About a half mile south of Taylor's Falls, near the 'Welcome to Minnesota' sign is a conglomerate outcrop.  Walking an old railroad bed from the road sign towards Taylor's Falls, you come to a notch in the cliff, this is the site of the conglomerate.

The conglomerate was deposited 504 million years ago along the shoreline of a Cambrian sea, quite possibly near basaltic islands.  Fossils of brachiopods found in the conglomerate provide evidence for the ancient sea.  The majority of the boulders of basalt have rounded edges, indicating a high energy environment.  The basalt boulders are cemented together by a tan or reddish matrix of sand and silt.

Near a hiking trail, not far from the park's campground, one can locate the contact between the conglomerate and the underlying basalt.  This contact represents nearly 600 million years of time from the underlying basalt lava flow to the deposition of the conglomerate.  The Taylor's Falls area has at least ten different basalt lava flows that are the result of the Midcontinent Rift System that formed about 1,100 million years old.

Minnesota Geology Monday - Stromatolites

As part of the 2011 Minnesota Minerals Education Workshop (http://www.d.umn.edu/prc/MMEW/index.html) a field trip to the inactive LTV iron mine, that formerly mined the Biwabik Iron Formation, was offered.  This was the second taconite mine put in operation on the Mesabi Iron Range and most of the operation facitities have since been bought by the PolyMet Mining Company to be used to process Cu-Ni-PGE ore from a nearby deposit.

The Biwabik Iron Formation was deposited 1,900 to 1,885 million years ago when the area was located at the edge of the Animikie Sea.  Due to the erosion of the continent to the north, surface waters were eroding and depositing fine materials like silt and clay into the shore areas of the Animikie Sea.  Prior to the time of iron deposition, the atmosphere and oceans would not have had free oxygen molecules, causing reduced iron to be easily dissolved and concentrated in the sea.   The iron deposition occurred at a unique period in Earth's history as photosynthetic organisms appear worldwide in the rock record, including the Mesabi Iron Range.  These organisms, called stromatolites, formed algal reefs along the shorelines of the world and as a waste product of photosynthesis, they created oxygen.  This oxygen caused the iron to precipitate out of the water and be deposited as iron minerals.  This process is called the Great Oxidation Event (GOE) and formed most of the iron formations found throughout the world.

As the deposited sediments were lithified to form rock, the iron materials turned into magnetite while silica became chert, a form of quartz.  Other sediments were deposited in deeper waters forming clay and eventually slate.  In the Biwabik Iron Formation, the iron is found between deeper water 'slatey' layers and the shallower water 'cherty' layers.  Our first stop, in Pit 5E, demonstrated an entire stratigraphic section of the Lower Cherty and Lower Slatey members of the Biwabik Iron Formation.

Near the bottom of the cherty layer, stromatolites can be found.  These can be recognized by the columns of bright red jasper.

In Pit 2E, at the top of the Upper Cherty member of the Biwabik Iron Formation is an area that contains a large number of algal stromatolite mounds.  Each dome shaped structure in the attached pictures would have been a stromatolite mound.  You need to imagine the scene 1,900 million years ago as these algal mounds would have been found near the shore of the Animikie Sea precipitating iron material to the ocean floor.

Locations like these are rare in todays world, as predators quickly consume the defenseless algae, though Shark Bay in western Australia is one location you would be able to see a similar sight of algal mounds.

Each stromatolite mound is made of individual, column structures that are build upwards.

This particular field of stromatolite domes will be preserved, there are no plans to mine the area in the future.  Though during the active mining period, blasting began to separate a section of the field, resulting in a chasm 2-3 feet wide and 10-15 feet deep.