Accretionary Wedge #42 - Countertop Geology & Decorative Stone

The latest Accretionary Wedge, the monthly gathering of the geoblogosphere, is being hosted by Ian Saginor at his 'volcanoclast.com' blog and is asking for 'any countertop or decorative stone that has been separated by humans from it's source.'  While thinking about this topic, I immediately thought of the dimension stone seen while walking the streets of St. Paul, Minnesota to locations or events like the Xcel Energy Center, the St. Paul Winter Carnival, Park Square Theatre or even the Minnesota State Capitol building.

The Google Earth image below attempts to give a sense of location or place for some of the dimension stones shown seen in this post.  Downtown St. Paul is located alongside the Mississippi River.

The old West Publishing Building is faced with 3,600 million-year-old Morton Gneiss at street level.  Type locality and quarry location is found alongside the Minnesota River in Morton, Minnesota.  This particular gneiss is sold commercially as dimension stone under the name "Rainbow Gneiss."

The James J. Hill library is constructed of pink marble from the state of Tennessee.  During his lifetime (1838-1916), James J. Hill was the chief executive officer of the Great Northern Railway while the railroad completed a transcontinental line to Seattle, Washington.  The library is also on the National List of Historic Places.

A recent addition to the city is the Hamm Plaza,which was created in 1992 in the center of St. Paul's entertainment district.  The walkway of the plaza is constructed of at least four different Minnesota igneous rocks - granite, gabbro and anorthosite.  The intrusive red and gray granites (radiometric ages between 1,780 and 1,730 million years ago), are quarried near St. Cloud, Minnesota.  The gabbro and anorthosite are common intrusive components of the Duluth Complex of Mid-Continent Rift System (1,100 million years) along Minnesota's North Shore.

The Church of Assumption, the oldest existing church in St. Paul (1874) is also found on the National Register of Historic Places.  The church and rectory are constructed of locally quarried limestone bedrock.  Underlying most of St. Paul is the fossiliferous Ordovician Plateville Limestone and upon close inspection of these buildings, one can see numerous invertebrate fossils.

Like most states in the United States, Minnesota has numerous companies quarrying rock for use as dimension stone.  Some quarry Morton Gneiss in the Minnesota River Valley, granitefrom the St. Cloud area or the intrusive igneous rocks of the Mid-Continent Rift System, other companies quarry limestone or sandstone from other areas of the state.

Minnesota Geology Monday - Saganaga Tonalite

Driving north out of Grand Marais (Cook County, extreme Northeast Minnesota) on the Gunflint Trail (County Road 12) leads us to numerous noteworthy locations, but todays post focuses on the Saganaga Tonalite.  In the late 1800s, as mines were being opened in the Gunflint Iron Formation near Gunflint Lake on the Canadian border, the trail was initially developed to lead from Lake Superior to the area.  The Gunflint Trail is now a paved 57 mile road from Grand Marais (on Lake Superior) to Saganaga Lake which is in the Bounary Waters Canoe Area (BWCA).  The Google Earth image below shows the relation or distance between Grand Marais and the Trail's End campground, part of the Superior National Forest.

Next June, my boys and I will be camping at the Trail's End campgound, shown in the lower left of this Google Earth image.  In the upper right of the image, outcrops of the light-colored, granitic Saganaga Tonalite can be seen.

Driving north on the Gunflint Trail, the first exposure of the Saganaga Tonalite that can be seen is at the Laurentian Divide scenic overlook pull out.  Water south of this point flows to Lake Superior and ultimately the Atlantic Ocean, water north of this point flows to the Hudson Bay.  Looking north over Gunflint Lake from the divide are numerous light-colored outcrops of Saganaga Tonalite.  These outcrops are on the Canadian side of the border in Ontario.

The Saganaga Tonalite is considered a quartz diorite due to it's mineral composition of quartz, plagioclase and horneblende.  Because it is lacking potassium feldspar it is not considered a true granite.  The Saganaga Tonalite is considered unique due to the large gray crystals of quartz.  The tonalite has a radiometric age of 2,689 million years.

Because a wind storm (with wind speeds in excess of 90 mph) in the summer of 1999, resulted in a large area of trees being flattened, numerous exposures of the Saganaga Tonalite have become accessible in the Trail's End campground and surrounding BWCA.  Fires, both controlled and wild, have also removed some of the vegetation allowing easy access to many outcrops.

Minnesota's Glacial Story (briefly)

Here's a brief (five minutes or so) narration and/or video diagraming major events in Minnesota's most recent glaciation that students may (or may not) use in preparation for their coming test.

Minnesota Geology Monday - Pillowed Greenstone

With 2012 just starting, my new goal is to publish a weekly post on selected topics or locations of Minnesota's unique geology.  This first post will look at a classic location, the pillowed greenstone located in Gilbert, Minnesota.  The greenstone is Archean in age, 2,700 million years old.  This pillowed greenstone is indicative of lava erupting underwater into an ocean or sea.  Lava that erupts into water forms rounded structures that are called pillows.

Looking at the attached Google Earth image of the area, the exposure is located behind the Eveleth-Gilbert Junior High School at the north end of Wisconsin Avenue near the athletic fields.

From the outcrop and looking towards the south, you see how the location of the school and ball fields are in close proximity to the pillowed greenstone exposure.

Just north of the greenstone outcrop is a fairly wooded area, grasses, lichens and other types of vegetation exist in depressions or cracks in the greenstone.

The lava that orginally erupted into the ocean or sea present at this location would have been basaltic in nature and would have consisted of the usual minerals found in basalt, primarily plagioclase and pyroxene.  Through weak metamorphism that involved higher temperatures and pressures, these minerals were changed to minerals like chlorite and epidote.  Since minerals like chlorite and epidote are green in color, they have given the rock an overall green color so the rock can be called greenstone.

Boundaries between individual pillows are distinguished by areas of much finer or smaller crystals than the rest of the greenstone.  Since the lava erupted into substantially colder water, the edges of this lava cooled quickly resulting in much smaller crystals.  The quicker that lava cools, the smaller the crystals are as a result.  Lava that was farther away from the seawater interface, cooled relatively slower and the crystals that were formed were bigger.

Also found on the exposure is a small portion of red hematitic chert of the Biwabik Iron Formation that would have covered the greenstone earlier in time.  The differences in time between these two units, the greenstone and the Biwabik Iron Formation, represents a difference of 800 million years.

Late Wisonsin glaciation is also represented well in this location.  The greenstone exposure has numerous examples of glacial striations or scratches that trend northeast to southwest from the Rainy Provenance.  These striations and glacial deposits that match the Rainy Provenance as a source location, indicate that the Rainy lobe moved through the area.

Glacial History of the Rush River County Park

Back on October 28^th, 167 8^th grade students from Delano Middle School visited two sites in the Minnesota River Valley and the Rush River County Park with a long-term goal of discovering the basic geologic history of the area.  At this point of our year, we are most interested in the recent glacial history of the Rush River area.

The Rush River is located entirely within Sibley County, though its watershed includes a portion of two surrounding counties.  The Rush River flows for a distance of 20 miles with an overall change in elevation close to 259 feet.  The calculated gradient of the Rush River is then 12.95 feet per mile.
Within the Rush River valley there are numerous examples of large, rocky point bars that are comprised of nonnative rocks including, basalt, rhyolite, granite, shale, limestone and a few Lake Superior agates.  In many or most cases, these rocks have been deposited hundreds of miles of their original location.  Students on this day spent a large amount of time identifying these different types of rocks and discovering the source areas for these rocks within the region.

The source of the rocks that make up the rocky point bars are found within the river valley.  Glacial till is being continually being eroded from the valley walls.  The picture below shows a prime location of this erosion.  This particular location is comprised of at least three distinct till layers, each with a different source location.  The top two layers contain tills from the late Wisconsin glaciation. 

The uppermost layer has its source in what is called Riding Mountain provenance and is commonly called the Des Moines lobe.  Till or sediment deposits from the Des Moines lobe are at or near the surface for a large portion of the state of Minnesota.  The color of the till is commonly buff or a yellowish brown.  A distinctive characteristic of the till is the presence of a large amount of Cretaceous Shale, the gray Pierre Shale.  Carbonate rocks, like limestone, are also found commonly within this till layer.

The middle till layer seen in the picture above is derived from sediments from the Superior provenance and is commonly called the Superior Lobe.  Till from the Superior lobe is much redder in color and tends to contain more clay material.  Rock types present within the till are indicative of the source area, a large grouping of crystalline rocks including basalt, rhyolite, granite and gabbro and some sedimentary rocks including red sandstone and limestone.  Also found within this layer of till and occasionally on the point bars at the Rush River are Lake Superior Agates.

The lowest layer of till on the picture above (very near the surface of the river) was deposited before the late Wisconsin glaciation and is often referred to as the old, gray till.  This till layer was not used in class and/or referred to often.

Students in our 8^th grade Earth Science classroom have recently been completing lab work on identifying general characteristics (texture and lithological) of four known Minnesota glacial tills (Superior, Wadena, Rainy and Des Moines lobe) from the late Wisconsin glaciation.  When students have identified characteristics of these four known glacial tills, they use this information to identify the source of two unknown glacial tills from the Rush River County Park in Sibley County, Minnesota.  The two unknown tills represent the upper and middle till layers described above or the Des Moines and Superior lobes.

That the Superior lobe advanced on what is now the Rush River County Park first and was followed by the Des Moines lobe is just part of the geologic history of the area.  To complete the story, the relatively high gradient of the river, at least for rivers in the area, needs to be explained thoroughly during a future post on Minnesota’s glacial history.  For a quick (and non-illustrated) version, near the end of the late Wisconsin glaciation, an immense lake called Glacial Lake Agassiz formed from meltwa

ter.  This lake catastrophically discharged forming what is called Glacial River Warren that carved a valley (now occupied by the Minnesota River) across Minnesota several kilometers wide and at least 100 meters deep.  This large valley created ‘knick points’ which resulted in large changes in river/stream channel slopes.  Since the incision of the valley by Glacial River Warren, rivers and streams have been eroding to the base level of the new valley floor in an attempt to level this steep slope.  Since Glacial River Warren carved a valley with steep sides, rivers (including the Rush Rivers) flowing into this valley have higher gradients that also increases their erosional energy.

Some Examples of Weathering

Weathering can be defined as the gradual breakdown of rock materials.  It primarily results from the physical breakdown of rock material (mechanical weathering) or via the chemical breakdown of rock through chemical reactions (chemical weathering).

A nice example of mechanical weathering (especially pertinent for places like Minnesota) is through an action called ice wedging.  As liquid water flows into the cracks of rock materials and freezes during periods of low temperatures, the frozen water expands, widening the crack.  This action can reduce very large boulders to much smaller remants as shown in the pictures below.

These large granite boulders are found just outside Pipestone National Monument near Pipestone, Minnesota.  Granite is not native to the area and would have been deposited there after transport by glaciers.  The boulders are called the 'three maidens', at one point in time there would have been just three boulders of granite, but the repeated freezing and thawing of water have split the boulders into many pieces.  Largely because of how out of the ordinary granite is to the area, a Native American legend grew out of these large pieces of granite.  Native Americans believed that the granite boulders held the spirits of three maidens who required offerings before the quarrying nearby of catlinite (or pipestone) in what is now the National Monument.

Another form of mechanical weathering is abrasion, which is the grinding and wearing away of material through the action of wind or water.  The photographs below show great examples of abrasion at Iona's Beach, a Scientific and Natural Area maintained by the Minnesota Department of Resources along the Lake Superior shore.  On the north end of the beach a large rhyolite flow is found.  Waves break this rhyolite flow down and largely through wave action, these smaller pieces of rhyolite are rounded and smoothed before eventually being deposited on the beach.

Chemical weathering is the breakdown of rock material through a chemical reaction.  This occurs largely through weak acids that are found naturally in our rain or snow and through the oxidation of other materials.  The picture below (taken in Summit Cemetery, Waukesha County, Wisconsin) is a nice example of chemical weathering, over the last 150 years the rock has been exposed to a large portion of natural acids through precipitation.  A closer look at the headstone proves that the original carving into the stone has become much more difficult to read.

Another form of weathering is called differential weathering, which refers to how different rock materials weather (or breakdown chemically or mechanically) at different rates.  The two photographs below show a nice example of differential weathering, the pink feldspar crystals weather more slowly, and as such, seem to stand out from the rest of the granite.

Another very nice example of differential weathering is Devil's Tower National Monument in Wyoming.  The land area around Devil's Tower is comprised of sedimentary rocks, which weather (and then are eroded or transported away) at a much faster rate than the igneous rocks that comprise the monument.  The igneous rocks are much more resistant to weathering than the sedimentary rocks.  Devil's Tower formed as an intrusion of igneous material that, after the surrounding sedimentary rocks weathered and eroded away, was left standing over 1,200 feet above the immediate area.

In our classroom, students recently examined some examples of both mechanical and chemical weathering.  We used different rock types (limestone, rhyolite, basalt, sandstone, marble, gabbro) in our weathering lab.  Students placed these different rock types in weak solutions of carbonic acid to determine the effects of weathering.  The next class period, mechanical weathering through the process of abrasion was explored before comparing both activities.

Working with Stream Tables

These last few weeks our 8th grade students have been working with a stream table designed to simulate and teach basic river principles, including: how river channels form and change over time and how sediment is transported and deposited within river systems.

The stream table was built with an old wood household door that was no longer being used as the base.  It has the dimensions of 1.91 meters long by .85 meters wide.  There are numerous coats of silicon to prevent the leaking of water, these coats are especially thick near joints (after three years of use, there haven't been any leaks yet).  At any given time there is also 25-30 gallons of water being circulated throughout the system by a submersible pond pump.

The modeling media inside the stream table is manufactered thermoset plastic from Composition Materials Company (http://www.compomat.com/) in Milford, CT.  It is sold by them as 'Stream Table Mix' and consists of various sizes and densities that do an exceptional job of modeling on sediment is transported and deposited in natural river systems.  We use anywhere between 50-80 pounds of plastic within the stream table for student use.

The idea of using a large stream table came from seeing an example created by the folks at Little River Research & Design (http://www.emriver.com/).