Tag Archives: Anorthosite

Split Rock Anorthosite

Looking SW from Split Rock Point (a large anorthosite block).  Note the gentle dip of the rocks toward Lake Superior.  Image credit: Bill Mitchell (CC-BY).
Looking SW from Split Rock Point (a large anorthosite block). Note the gentle dip of the rocks toward Lake Superior. Image credit: Bill Mitchell (CC-BY).

This summer, I took a field trip up to Split Rock State Park in northern Minnesota, along the north shore of Lake Superior. While I wrote a little bit about the trip, there is still a bit more to be said and shown.

Part of what makes Split Rock interesting, besides a picturesque lighthouse which I didn’t take many pictures of, is the large blocks of anorthosite. Anorthosite is a rock formed primarily of the mineral anorthite, which is a calcium-rich feldspar, and the mineral zircon—used in U/Pb dating—can be found in anorthosite as well.[1] Its appearance is generally light grey or whitish, and has relatively coarse grains (mm to cm).

Anorthosite is an intrusive igneous rock formed through the crystallization and accumulation of anorthite within a magma body. It is abundant on the Moon, and lunar anorthosites are believed to have accumulated on top of a magma ocean early in lunar history. A relatively dense magma will act as a heavy liquid, and cause the less dense anorthite to float, separating the original magma from the crystallized anorthite. These types of crystallization processes, where the magma becomes separated from crystals it produces, are called fractional crystallization, and can cause the resulting magma to be enriched in some elements or components (such as SiO2). Even with massive basalt flows, fractional crystallization can cause an occasional rhyolite flow as well, but I’ll leave discussion of the rhyolites of the North Shore for another day.

Pictured above is the view from Corundum Point, a large block of anorthosite at Split Rock State Park. Below is a close-up view of some of the anorthosite, as well as a benchmark which has been placed in the anorthosite block [Thanks to Jessica Ball (@tuff_cookie) for giving me the idea of photographing the benchmark]. Despite being far from the ocean, Minnesota is home to National Ocean Survey benchmarks.

Anorthosite with survey point, Split Rock State Park, MN.  Image credit: Bill Mitchell (CC-BY).
Anorthosite with survey point, Split Rock State Park, MN. Image credit: Bill Mitchell (CC-BY).

The name Corundum Point suggests the presence of corundum—a mineral used in abrasives—and it comes from a mining operation on the site in the early 1900s. However, the point is actually anorthosite, which was much less useful for abrasives. Between the incorrect mineral identification and a fire which burned down the crushing house, the operation was eventually shuttered.

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[1] Mark D. Schmitz, Samuel A Bowring, Trevor R Ireland, “Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: new high-precision isotope dilution thermal ionization mass spectrometry results” Geochimica et Cosmochimica Acta (2003), 67, p. 3665–3672. DOI: 10.1016/S0016-7037(03)00200-X

Update Upon further study, it appears that the naming convention of Split Rock State Park is to call this point Corundum Point. However, Google Maps displays this point as Split Rock Point, with Corundum Point a few hundred meters to the northeast. Regardless of the arbitrary common name, the benchmarks are on the point to the southwest.

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Walking on Lava (Flows)

A cascade along the Split Rock River, in Split Rock State Park (Minnesota).  Cascade is 2-3 m tall, and the lava is cold enough to touch.  Image credit: Bill Mitchell (CC-BY).
A cascade along the Split Rock River, in Split Rock State Park (Minnesota). Cascade is 2-3 m tall, and the lava is cold enough to touch. Image credit: Bill Mitchell (CC-BY).

On a conference call some weeks ago, Nigel Jolly, captain of the RV Braveheart which will be taking the Heard Island expedition to Heard Island in March and April, 2016, told the expedition members that they will be expected to be in good physical shape for this expedition. Specifically, he reminded us that not only will we need to be able to walk around on the uneven and slippery ground, but that we will need to do so while carrying heavy things (potentially fragile and expensive, and generally needed for a successful expedition). In order to prepare ourselves, we are to get out and try walking around with heavy stuff on uneven ground.

Naturally, my first thought was that he just told me I needed to go backpacking on the north shore of Lake Superior. Don’t twist my arm too hard!

I called my cousin, who I figured would also probably need some arm-twisting to go backpacking on the North Shore, and we figured out the logistics. We even managed to reserve a hike-in campsite in Split Rock State Park that was right along the shore. Before we left, I checked through Roadside Geology of Minnesota to see if there were any special features besides the anorthosite (rock almost exclusively made of the mineral anorthite, which is a feldspar) which makes up Split Rock itself, and I put a few places on the quick stop list for the drive home.

The geology along the Split Rock River did not disappoint. Here were lava flows, more than a billion years old (1 Ga). Along the river channel, columnar jointing was often evident (see the far bank of the cascade and the far canyon wall above). Most of the lava flows were massive. The opposite canyon wall in the photograph shows columns 5–10 m tall, which would have formed in a single flow. That’s a lot of lava! While hiking along, I was on the lookout for ropey pahoehoe flow-tops, but did not find any that I recognized.

Lava flows found along the North Shore are generally part of the North Shore Volcanic Group, and have an age of roughly 1.1 Ga. They were formed as part of the Mid-Continent Rift system, and now dip gently (~20°) toward the lake. Many of the flows are basalts (low silica, high iron), although there are rhyolites (high silica, low iron) in the area (such as Iona’s Beach).

Mid-Continent Rift system.  Volcanic rocks are in the striped regions, while the dotted regions indicate sediments derived from those volcanic rocks.  Not all of these rocks are at the surface; much of the area in central and southern Minnesota, Iowa, Nebraska, and Kansas are overlain by younger sediments (e.g. glacial till, Paleozoic carbonates).  Image source: Nicholson et al., via USGS.
Mid-Continent Rift system. Volcanic rocks are in the striped regions, while the dotted regions indicate sediments derived from those volcanic rocks. Not all of these rocks are at the surface; much of the area in central and southern Minnesota, Iowa, Nebraska, and Kansas are overlain by younger sediments (e.g. glacial till, Paleozoic carbonates). Image source: Nicholson et al., via USGS.

It was fun to get to see some igneous rocks up close in outcrop (I live on a lot of glacial sediments, and the bedrock is Paleozoic sediments). The backpacking definitely demonstrated that more such activities are needed, because my legs were quite sore by the end of the hiking and the next few days. However, we did have a gorgeous view from the campsite! In the photo below, you can see the gentle dip of the lava flows toward the lake. Obviously, the weather we had on the North Shore (quite comfortable!) was much, much better than is expected for Heard Island. I had a great trip, and hope to head back up some time for more hiking adventures.

A clear morning on Lake Superior.  The lava flows making up the points further down the shore can be seen dipping gently toward the lake.  Image credit: Bill Mitchell (CC-BY).
A clear morning on Lake Superior. The lava flows making up the points further down the shore can be seen dipping gently toward the lake. Image credit: Bill Mitchell (CC-BY).

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Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the midcontinent rift system of North America: Precambrian Research, 58 (1-4), p. 355-386. DOI: 10.1016/0301-9268(92)90125-8