Thermohaline Circulation

In class this week, we've been looking at the factors that affect ocean currents.  Thermohaline circulation is the name given to large-scale ocean circulation patterns.  It refers to how differences in the densities between warm/cold water and fresh/salt water and how these different densities circulation patterns.

Two types of ocean currents are:
1.  Surface currents - ocean currents that travel at or near the oceans surface.  These tend to be warm water currents.  The Gulf Stream in the North Atlantic is a great example of a surface current.
2.  Deep currents - ocean currents that travel far below the surface of the ocean.  These tend to be cold water currents.

Differences in temperature really drive the ocean circulation.  Warm water is less dense, causing it to rise to the surface.  Cold water is more dense, meaning cold water will sink.  Warm water flows from a low latitude (near the equator, where it's warmed) to a high latitude (the poles, where the water is cooled) as a surface current.  The cold water flows back towards the equator as a deep current.  The picture belows shows warm water sitting on top of cold water because of differences in density.

Salinity, the total measure of dissolved salts or solids in a liquid, also affects density.  Because of the differences in density, less dense fresh water will always sit on top of the more dense saltwater.  The picture belows shows blue fresh water sitting on top of the clear salt water.

Large influxes of freshwater to the oceans can slow or temporarily stop the thermohaline circulation because fresh water will not sink below the more dense saltwater.

In the picture below, knowing that one container holds saltwater and the other fresh water, can you determine which jar holds which type of water?

Minnesota Geology Monday - Anorthosite

Anorthosite is an intrusive igneous rock made up of more than 90% of the mineral plagioclase.  The minerals with Anorthosite crystallize at depth within Earth's crust.  Because Anorthosite contains such a small percentage of minerals that tend to oxidize, this rock is extremely resistant to forms of chemical weather.

The Anorthosite has been brought to the surface in several locations along the North Shore of Lake Superior by associated magmas and lavas of the Mid-Continent Rift 1,100 years ago as large blocks that formed at great depths in the crust.  As the magmas of the rift rose, they carried the blocks of Anorthosite upwards.  The contact of the Anorthosite and mafic lavas can be very distinct.

Near Carlton Peak, within the Temperance River State Park (http://www.dnr.state.mn.us/state_parks/temperance_river/index.html) is an inactive Anorthosite quarry.  Rock quarried here were used for breakwaters at locations near the quarry.  The quarry has several distinct levels that were active in the past.

From the quarry or nearby Carlton Peak, views out over Lake Superior are fantastic on clear days.  Carlton Peak has an elevation of 1526 feet above sea level, this is 924 feet above the surface of Lake Superior.  To reach the quarry or parking area to Carlton Peak, you must drive a winding 1.5 mile un-improved gravel road inland from MN highway 61.

At all levels of the inactive quarry are large waste rock piles and the impressive vertical faces where rock material was removed.

Another location with a large block of Anorthosite is at Split Rock Lighthouse.  The lighthouse sits on one block of Anorthosite during the rifting event, more will come later on the geology of the rift and the Split Rock Lighthouse area.

Minnesota Geology Monday - Minnehaha Falls

Minnehaha Park (http://www.minneapolisparks.org/default.asp?PageID=4&parkid=252) is home to Minnehaha Falls.  The first Google Earth image shows the waterfall's location in comparision to the Mississippi and Minnesota river (which enters the picture from the bottom left).  The second image gives a sense of the changes in elevation in the immediate area around the falls.

Minnehaha Creek flows approximately 22 miles from it's source, Lake Minnetonka, to the Mississippi River.  Near the confluence of the creek and Mississippi River is Minnehaha Falls, which is a 53-foot high waterfall.

Geology of the Twin Cities area provides an excellent record of the transgression and regression of Paleozoic oceans.  The three Ordovincian rock formations (from top to bottom) found at the Minnehaha Falls are the Platteville Limestone, the Glenwood shale and the St. Peter Limestone.  The Platteville Limestone is fossiliferous, including numerous brachiopods, bryozoans, corals, etc.  The Glenwood Shale is found in a thin layer throughout the region.  The St. Peter Sandstone is world famous as a well sorted, nearly pure quartz sandstone.

As water of Minnehaha Creek flows over the resistant Platteville Limestone, it erodes the easily erodable St. Peter Sandstone.  In the valley below the falls, large blocks of Platteview Limestone can be found that have fallen as the sandstone was eroded away, effectively undercutting the limestone.   This results in the gradual movement of the waterfall upstream.

Minnehaha Falls formed when Glacial River Warren (an outlet of Glacial Lake Agassiz) suddenly discharged carving an enormous valley through southern Minnesota.  In the Twin Cities area, Glacial River Warren exposed the resistant Platteville Limestone six miles south of present Fort Snelling forming a waterfall nearly one mile wide.  As this waterfall moved six miles upstream to Fort Snelling, the much smaller post-glacial Mississippi River flowed into the larger channel, continually eroding the sedimentary rocks, now forming St. Anthony Falls.

St. Anthony Falls moved upstream at about 2.5 feet per year, and when it reached the confluence of the Minnehaha Creek, the waterfall began to move up both the river and the creek.  Minnehaha Falls continues to move upstream, while further movement of St. Anthony Falls on the Mississippi River was prevented with the construction of a concrete spillway in 1956.

The pictures above show the Minnehaha Falls during June of 2009 when the outlet on Gray's Bay (Lake Minnetonka) was closed due to low water of the lake.

Minnesota Geology Monday - Morton Gneiss

The Google Earth image shows the town of Morton, which is found near the intersection of US highway 71 and MN highway 19, alongside the Minnesota River.  Morton is the type locality of the Archean-aged Morton Gneiss.  The gneiss also outcrops at various location throughout the upper Minnesota River valley.

Several roadcuts and/or quarries expose the gneiss near Morton.  Northwest of the old Morton High School are several abandoned gneiss quarries,  while Cold Spring Granite (http://www.coldspringgranite.com/ ) actively quarries gneiss out of the 'Rainbow Quarry' south of MN 19.

A few miles upstream the Minnesota River from Morton, a sign claims World's Oldest Rock.  At one point the Morton Gneiss was thought to be the oldest rock in the world (now thought to be the Acasta Gneiss in Canada).  Recent studies have dated the Morton Gneiss at 3,524 million years.

Due to the large crystalline grains, the gneiss is thought to be originally a granite.  After subjected to great temperatures and pressures beneath the surface of the Earth, the original granite was metamorphosed into the foliated pink and black banded gneiss.

The Morton Gneiss can also be considered as a migmatite, a mixed metamorphic rock that consists of two components, a schist or gneissose component and a grantite component.  In some areas, the gneiss is very coarse grained.

The Morton Gneiss has be actively quarried for use as a dimension stone, usually under the trade name of 'Rainbow Gneiss' or 'Rainbow Granite'.  It can be found adorning many buildings throughout the United States, including:  the West Publishing building in St. Paul, the Banker's Life Insurance building in Des Moines, and the AIG building in New York, among others.