Minnesota Geology Monday - Hull Rust Mahoning Iron Mine

Just returned from a weekend trip where one of the stops included the world's largest open pit iron ore mine.  The Hull Rust Mahoning open pit mine formed through the consolidation of over 40 different smaller properties.  During the mine's peak production, during World War II, approximately one fourth of the iron mined in the United States came from this location.

This open pit mine covers 3,075 acres, nearly 3.5 miles long by 1.5 miles wide.  Iron ore has been mined from the area since the early 1890s, with ore being transported by rail to the Duluth-Superior ports on Lake Superior to be shipped to steel mills further east.

The city of Hibbing is located near the mine.  As the mine expanded during World War I, buildings in north Hibbing were placed on steel wheels and moved two miles to the south.  Today, you can still see the foundations of some of these buildings near the mine.

Delano's Water Treatment Facility

This may help you when reviewing for the Field Trip quiz on May 30/31.

Minnesota Geology Monday - Good Thunder Bay

Along Highway 61 on the north shore of Lake Superior, just a few miles before Grand Marais, is a great observation point that overlooks Good Thunder Bay and has an excellent outcrop of volcanic and sedimentary rocks.  The igneous basalt of the Mid-Continent Rift system overlies a red sandstone and siltstone.

The basalt lava flow is named the Terrace Point and is approximately 160 feet thick.  It has been mapped inland from the lake for a distance of 15 miles.  The sedimentary structure beneath the basalt is what makes the site remarkable.  Along the north shore there are numerous instances of interflow sedimentary structures, but this red sandstone or siltstone is the most easily accessible, being found alongside the major highway of the region.

The 130 foot thick red sedimentary unit represents a large amount of time between different lava flows.  This particular sequence is also one of the thickest interflow sedimentary sequences found on the north shore.  It has been interpreted as being deposited by streams, though some areas indicate deposition at the bottom of a lake.

Some locations at the top of the sedimentary sequence show that the top layers were pushed, pulled or dragged as basaltic lava came out of the rift system.

A great gigapan of the location is found at the following website:  http://www.gigapan.org/gigapans/86878.

Minnesota Geology Monday - Pahoehoe

Basaltic lava flows that have a ropy surface are called Pahoehoe.  The ropy structure forms as the fluid lava continues to flow underneath a hardening crust.  These lava flows are common on the big island of Hawaii today.  Hawaiian pahoehoe images courtesy of Wikimedia Commons.

Because of the 1,100 million year old Mid-Continent Rift system, which resulted several hundred individual lava flows on Minnesota's North Shore, there are several locations of pahoehoe lava flows easily accessible.

One location is just below the footbridge that spans the Temperance River at the Temperance River State Park (http://www.dnr.state.mn.us/state_parks/temperance_river/index.html).  The basalt here has turned red due to the oxidation of iron within the basalt.

Unfortunately I did not have a scale card or even the idea of using an object for a sense of scale.

Another location with ropy, pahoehoe lava flows is found at Sugarloaf Cove (http://www.sugarloafnorthshore.org/).  These are not as extensive or large as those found at the Temperance River and are much more lichen covered.

Green River Formation

Students in 8th Grade Earth Science at Delano Middle School were recently working with samples of the Green River Formation from Southwestern Wyoming.  The formation consists of sandstones, mudstones, siltstones, shales and other sedimentary rock types that were deposited in several freshwater lakes.  The Green River Formation is Eocene in age (48 million years old) and has several layers that are very fossiliferous.  Fossil Butte National Monument (http://www.nps.gov/fobu/index.htm) is located within the formation and is well known for the preservation of fish fossils found there.

The sediments of the Green River Formation are deposited in layers, a dark layer representing the summer months and a lighter layer representing deposition in the winter months.  A dark layer and light layer taken together represent one years worth of sediment deposition.  These pairs of layers are called varves, each varve equals one year.  The presence of varves allows students to determine how long the lake was in existence by counting varves, in a way, very similar to counting tree rings.

The varved shale was obtained from the Ulrich's Fossil Fish Gallery and Preparatory (http://www.ulrichsfossilgallery.com/) as approximately 8 inch by 10 inch sheets, about one centimeter in total thickness.

This larger piece of varved shale was then cut down into individual shale billets for student use.

Using various methods of magnification, students were then able to count the number of varves in each shale billet and determine the number of varves per millimeter.  Each block of students came up with a different class average of varves per millimeter, but our averages ranged from 7.3 - 8.1 varves per millimeter.  Stating this another way, it took between 7.3 - 8.1 years to deposit one millimeter of sediment.

Students were then able to calculate how long it would take to deposit a meter's worth of sediment (7,300 - 8,100 years) as well as the entire 260 meter thickness of the Green River Formation (1,898,000 - 2,106,000 years).  Though it may not exactly match the accepted ages for the formation, the numbers obtained by our students are close to the accepted ages.