Minnesota Geology Monday - Folded Banded Iron Formation

Minnesota's first iron ore mine opened in 1882 and is called the Soudan Mine, initially there were several open pit mines in the area.  In 1892, the work moved underground and continued until 1962.  During the time that the mine was in operation, over 14 million metric tons of iron ore were removed.  The state bought the mine and approximately 1,200 acres surrounding the mine for $1, on the condition that the area be converted to a park.  Today visitors to Soudan Underground Mine State Park can ride a metal skip to the 27th level (2,341 feet below the surface) for a tour of the mine or visit a physics laboratory that is also operated at this depth.

Near the mine is also a classic pavement outcrop of folded banded iron formation.  Some have called it the most photographed outcrop in Minnesota and it was recently added as one of the 101 American Geo-Sites You've Gotta See.  The day that I visited the site, it was unfortunately late in the day, with a slight drizzle and thunderstorm moving in, but here are some of the pictures taken that day.

The site consists of layers of metallic hematite, red jasper and white chert.  These layers were folded multiple times, though questions arise about whether the folding occurred while the sediments were still soft or after they had lithified.

The site is a short walk away from the mines headframe, the stands over the mine shaft supporting the cables and skips that brought people and ore from the lower levels to the surface.  Because of several small roads in the vicinity, you do not need to enter through the park's main entrance to get to the outcrop, Stunz Bay Road climbs a small hill and leads practically right past the outcrop.

In many places, the layers of red jasper and white chert have small fractures that are filled with milky quartz.  This suggests that the jasper and chert were brittle during the folding event.

Stromatolites in the Gunflint Iron Formation & the Sudbury Impact Layer

A few weeks ago, I took my two boys (Kieran, age 6 and Luke, 3) camping on the North Shore of Lake Superior and at the end of the Gunflint trail.  Day 4 of our camping trip found us almost sixty miles away from Lake Superior at the Trail's End campground at the end of the Gunflint Trail.  The plan for the morning was to accompany a group of Earth Science teachers and several geology professors on a couple of hikes to sites of geologic importance in the area.  The group of teachers were part of TIMES XIV, the TIMES Project is an intensive two-week long class teaching inquiry-based teaching methods and focusing them on Minnesota Earth Science.  As a past participant of TIMES (summer of 2009) and a member of the Minnesota Earth Science Teachers Association's Board of Directors, we were invited to go along on the days hikes.  My job would be to talk Earth Science with the teachers and get to know what they do in their classrooms, while giving them examples of what I do differently as a result of the TIMES Project.  Kieran and Luke's job for the day was to follow along and just be themselves, they actually do a great job with long hikes in uneven terrain for kids their age.

The day’s hikes were led by Jim Miller, geology professor of the University of Minnesota, Duluth and the Director of the Precambrian Research Center.  We met the group in the morning at the Magnetic Rock trailhead, the same place that we had hiked the day before, though we would not be repeating the entire hike this day.  Magnetic Rock is a large slab of the Gunflint Iron Formation that was tipped vertically by the late-Wisconsin glaciation.  The focus of the day were features found around the 1,878 million year old Gunflint Iron Formation.  An iron mine, the Paulson mine, opened near here in the late 1800s, but failed rather quickly because the Mesabi Iron Range (located to the south) was already in operation and a financial panic scared investors in 1893.  Before the discovery of iron, early explorers and/or voyageurs to the area, acquired black chert from the iron formation to be used as flint in their black powder muskets.

The group hiked approximately half way to Magnetic Rock to view stromatolites that are found along the trail.  Stromatolites are nearly circular algal mounds that can be several feet in diameter.  While alive, cyanobacteria (the living organism of the stromatolite) grow upward from the seafloor by trapping sediment on a sticky mat.  These organisms obtain energy by using the process of photosynthesis, essentially using carbon dioxide and releasing oxygen as a waste product.  Stromatolites of this time period are remarkable because of their role in the Great Oxidation Event (GOE).  They are the reason that we have the iron deposits around the world, before this period in Earth's history, the level of atmospheric oxygen was very low which allowed soluble iron to build in the world's oceans.  As the stromatolites began to release oxygen as a waste product, the iron in the world's oceans basically rusted (or precipitated) out of solution and was deposited on the ocean floor.  Stromatolites of this age are found around the world near most of the world's major iron formations.

Stromatolites are not unique to just this time period in Earth's history, they are found in many sedimentary rock sequences.  The much younger Ordovincian rocks of Southeastern Minnesota also have great examples of stromatolites.  They can even be found today in hypersaline waters, where predators are scarce, the best example being Shark Bay in Australia.

The next stop for the day was a short drive and short hike away that a catastrophic few hours in Earth's past are recorded in the rock record.  Approximately 1,850 million years ago, a meteorite struck near Sudbury, Ontario, almost 450 miles away from Minnesota.  It is the second largest impact structure found and very few are older than this impact.  The Minnesota outcrops showing evidence of the impact were found because a forest fire raged through the area in 2007 and removed most of the overlying vegetation, making it much easier to see the underlying rocks, especially the breccias of the Sudbury Impact Layer.

Essentially, the greater than 10 magnitude earthquakes caused by the impact fractured the iron formation and resulting tsunami re-worked or re-arranged the angular pieces of iron formation.  Near the base of the impact layer, these angular fragments are meters in length and cemented together in a green matrix.  Near the top of the formation, the size of the iron formation fragments are much smaller.  Curiously, Jim Miller explained that iron ceased to be deposited worldwide after the impact.  For a much more detailed read on the formation of the impact layer and a history of the impact layer itself, read the document published by the Minnesota Geological Survey.

The picture below shows Kieran sitting on the burnt remains of a tree, killed during the forest fire in 2007 that granted accessibility to the Sudbury Impact Layer.  This was just about one of the last outcrops that we would visit with the TIMES group, though they would continue on for at least one more stop before returning to the Twin Cities that evening.  It is located just a short hike down National Forest Service Road 1347 from the previous stop.

At this particular outcrop, you are able to stand near the uppermost portion of the Sudbury Impact Layer.  Deposited here are accretionary lapilli, essentially ash pellets that were thrown away from the impact site.  Using very high powered microscopes, shocked quartz (which is only found at impact sites) can be found in these centimeter-sized pellets, commonly called ejecta, and the surrounding matix.  Look for the small, concentric circle structures in the pictures below.

Many of the teachers in the group we were accompanying were amazed that two boys, like Kieran and Luke, could go through some of these hikes at their age.  A few were even surprised that they could camp in locations like these, this isn't a state park with a lot of people, it's fairly remote and there is no cell phone reception or any technology that we're used to using.  Both boys tend to do a great job doing their own thing when camping in locations like these and dad got a few hours of adult interaction (a very rare thing for a week-long father/son camping trip).  

Minnesota Geology Monday - Granites of the St. Cloud Area

St. Cloud has the nickname "The Granite City" for good reason.  There were almost one hundred granite quarries in the area over time, but currently there are four in operation in towns like Cold Spring, Rockville and St. Cloud.  Numerous granite plutons are found in the region, together they are known as the East-Central Batholith.  The individual granite plutons have also been informally named, including the Richmond, Rockville and St. Cloud Red, among others.  Radiometric ages of the granites range from 1,800 to 1,750 million years.

The granite is quarried in the area by several companies for use as either dimension stone or aggregate.  Cold Spring Granite maintains several quarries and a processing plant for decorative uses on buildings, countertops, headstones, etc.  The quarry pictured here are from the Rockville Quarry #2, which is sold by the tradename Rockville White. 

Located just above the granite in the area, and seen well in many quarries, are rounded boulders of granite.  These are corestones.  The granites in the area are jointed, weathering of the granite affect the corners of the joints first, eventually smoothing the corners and producing rounded boulders.  The weathering event most likely took place during the Cretaceous, the same period which formed the kaolinite clays found in the upper Minnesota River Valley.

The region is also swarmed by many diabase intrusions.  These intrusions are easily seen within the Martin Marietta aggregate quarry.

Below is a picture of me standing in front of a diabase dike intruding on a body of St. Cloud Red granite.

Because quarries are privately owned and a potential safety hazard, exploring them without guidance or permission is not allowed.  Quarry Park and Nature Reserve is a public park that allows easy access to many of the features found within the granite throughout the region.  Like the name suggests, the area was once home to almost 30 granite quarries.

The granite present in the park is known as the St. Cloud Red granite.  Most of the former quarries are now filled with water, one is used as a swimming pond that is 112 feet deep.  There are also large piles of quarried rock found throughout the park.

Walking the many hiking trails within the park, it's easy to examine the relationship between the granite and diabase intrusions.

Upon closer inspection, you are also able to find places where pieces of granite were broken off by the intruding magma.  These inclusions of granite are now frozen within the diabase intrusion.

Minnesota Geology Monday - Palisade Head Rhyolite

The 1,100 million year old Midcontinent Rift System stretches from northwestern Ohio and Michigan to the Lake Superior Basin, through Minnesota and Iowa all the way to Kansas.  Throughout the midcontinent rift, there are 60,000 feet of lava flows and 480,000 cubic miles of volcanic rock.  The spreading associated with the midcontinental rift is similar to the events that undid earlier supercontinents and could have placed Minnesota on the western edge of an ocean if it had continued.

The majority of the rock associated with the Midcontinental Rift System (especially on Lake Superior's north shore) are mafic gabbros and basalts.  Though there are some examples of felsic granites and rhyolites in the Lake Superior region.  The Palisade Head Rhyolite is a reddish, fine-grained, volcanic rock that outcrops near Tettegouche State Park.  It has a radiometric age of 1,096.6 million years.  Upon closer inspection, there are numerous orthoclase feldspar and quartz crystals.

This particular extrusive lava flow is 2 miles wide and 3 miles long.  It is as thick as 310 feet in some places.  As the lava cooled, large columns formed.  These columns are visible in topographic features like Shovel Point (picture below) and Palisade Head.  Joints between the columns are weak places between the rhyolite, weathering and erosion removes these columns.  The best ways to view the columns are by kayak or rock climbing the high cliffs of Shovel Point and Palisade Head.

Palisade Head (in the distance with the tower on top), rises 350 feet above Lake Superior.  Because felsic rhyolite consists of higher amounts of silica and less iron than the mafic basalts that are also found in the area, it is much more resistant to weathering via the process of oxidation.  This is the reason that Palisade Head is a topographical high in the region.

The picture below shows Palisage Head in the foreground and Shovel Point in the distance, both are easily reached by hiking paths within the state park.

The Palisade Head Rhyolite also extends away from the lake and is partly responsible for one of the parks most visited features.  The High Falls of the Baptism River is a 70 foot waterfall, the highest waterfall completely within the boundaries of Minnesota.  The Baptism River actually has created several waterfalls over the Palisade Head Rhyolite on it's way to Lake Superior.