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.

Various Interesting Articles

Thin section photomicrograph of a gabbro, (crossed polarizing filters).  Image credit: Siim Sepp (CC-BY-SA).
Thin section photomicrograph of a gabbro, (crossed polarizing filters). Image credit: Siim Sepp (CC-BY-SA).

There have been a couple of interesting articles I’ve come across recently, which are worth mentioning.

First, Emily Lakdawalla has an excellent summary of the Pluto discoveries from both the American Geophysical Union’s Fall Meeting and the [NASA] Division of Planetary Science meeting. There’s a lot of new stuff there, and it’s pretty exciting.

Second, the Joides Resolution blog (the Joides Resolution is an ocean sediment coring vessel) has a series of posts (1, 2, 3) on geologic thin sections. Not surprisingly, the thin sections pictured are from rocks such as gabbros or sheeted dikes, which are expected in oceanic crust and in ophiolites (oceanic crust exposed on land). There’s a great exposure of the Coast Range Ophiolite just west of Patterson, CA, in Del Puerto Canyon, which is described in a recent blog post by Garry Hayes.

Third, Dave Petley has a great post on The Landslide Blog about the recent landslide in Shenzhen, China. I find landslides fascinating, and always learn something when I read The Landslide Blog.

Christmas Bird Count 2015

A red-bellied woodpecker visits a backyard bird feeder.  This photo is not from Christmas Bird Count 2015, but red-bellied woodpeckers were observed on my count.  Image credit: Bill Mitchell (CC-BY).
A red-bellied woodpecker visits a backyard bird feeder. This photo is not from Christmas Bird Count 2015, but red-bellied woodpeckers were observed on my count. Image credit: Bill Mitchell (CC-BY).

Beginning in 1900, the Audubon Society began holding an annual bird population survey at Christmas (in contrast with the earlier tradition of shooting all the birds one could). Over the years, the Christmas Bird Count (CBC) has changed and grown, but still maintains its founding principles: a census of birds, taken around Christmas.

Today, counts are organized into 15-mile diameter circles, with teams of birders tallying not just how many different species they see over the course of a day, but the number of individuals of each species. In many cases, the large circle itself is subdivided, and teams of birders will count in a smaller area. Many groups meet at or before dawn for breakfast and planning. Some finish in time for lunch, others don’t stop for much of anything until it’s too dark to bird.

This weekend, I took part in the count, with a northern section of the Faribault (MN) circle. As a college student at Carleton College, I got involved in the count around Northfield, MN (within the Faribault circle), and have gone back to join the count a handful of times since graduating. One year I was in Berkeley for the CBC, which was a very different experience: no snow, and >100 species recorded.

Our group managed to see 19 species (on the higher side for that area in my experience), and around 1400 individuals. Most of those individuals, due to the relatively warm conditions here, were Canada geese (800) and mallards (450), which could be found on the open water. In years past when cold conditions have frozen over the lake and river, the waterfowl count (and total individuals count) are much lower.

Many of the other regulars were out and about: white-breasted nuthatches, red-bellied woodpeckers, goldfinches, crows, blue jays, and cardinals. We found a few surprises while counting: a belted kingfisher, a hooded merganser (among the 800 geese and 200 mallards on one small pond), a great blue heron, and five eastern bluebirds. The heron, bluebirds, and kingfisher are surprising in that they are quite uncommon in this area at this time of year (i.e. generally expected to migrate further south), but a quick search of eBird shows that they are not unheard of (note that the data include my CBC checklists).

The Christmas Bird Count is a great opportunity for new or newer birders, because you can (and probably will) be placed on a team with more experienced birders. You meet new people, see some birds—possibly adding a species or ten to your life list—and participate in citizen science. Professional scientists alone couldn’t do these detailed counts in this many areas all at once. While in many areas the annual count may have happened already, some areas might be still have a count coming up, so check to see when your local circle does its count.

Of course, if the count already happened and you don’t want to wait for next year, you can always participate in eBird. eBird is a project of the Cornell Ornithology Laboratory, and is a huge database of population counts. Participants submit their lists (including counts of individuals) with time, date, location, and some other information. The database keeps track of your life list (and many other lists), and also can show you data from all the aggregated observations. Are you wondering what birds you might see when you go on vacation? You can check that county/area in eBird, and get a graph showing the relative abundance of different birds seen in that area over the course of a year. Looking for a particular species? A map tool can show you where they have been seen, and at a detailed zoom level will show the individual observations.

To date, there have been no eBird checklists from Heard Island, or the ocean near Heard Island. However, I intend to do what I can (and I may not be alone in birding Heard Island) to get a few lists for eBird when I am there in March and April.

Communicating Science Precisely and Accurately

Newton's cradle pendulums swinging back and forth over a copy of Isaac Newton's Principia Mathematica.  Image credit: DemonDeLuxe (CC-BY-SA).
Newton’s cradle pendulums swinging back and forth over a copy of Isaac Newton’s Principia Mathematica. Image credit: DemonDeLuxe (CC-BY-SA).

Recently when I was volunteering at my local science museum, I was leading activities on resonance. I had tuning forks, tuned plastic pipes, and a series of pendula with differing lengths on an arm rotated by a much heavier pendulum. The main idea was that when the frequencies of two pendula or a tuning fork and tuned pipe match, then the energy from one could be transferred into the other, making it oscillate. When you hold a tuning fork up to a resonant cavity, and the cavity will sound. Similarly with my pendula, if the pendulum driving the rotating arm is swinging at the same frequency as the natural frequency of one of the pendulua coupled to it, that pendulum will swing too. Other pendula with faster or slower oscillations will be relatively unaffected.

In the course of talking with visitors, I was reminded of a constant challenge for science communication: being precise, accurate, and accessible. Scientific language is often used to convey precisely the conditions or idea in question. And yet, sometimes a more colloquial meaning of a word is understandable. As I was talking about how quickly this pendulum oscillated, and how slowly that one oscillated, it was difficult to maintain a clear, concise distinction between speed (distance/time) and frequency (1/time). It isn’t about the speed with which the pendulum moves, nor is it about how high (how large the amplitude of motion is) the pendulum swings. But frequency isn’t necessarily a word that visitors understand in distinction with a colloquial version of “speed”.

Next week will be a big week for science communication, and you should keep an eye on the science news. The American Geophysical Union (AGU) is having its 2015 Fall Meeting, which is a gathering of more than 20,000 scientists in San Francisco. There will be lots of new results presented, many of them esoteric or incremental, but others will be quite accessible and groundbreaking. Many science journalists will be on hand covering the proceedings, and most of them do an excellent job.

However, there are a few headlines to watch out for. “Water found on Mars!” is a fairly common one, although the announcements, if you investigate a little more deeply, are indeed new when coupled with the precise situation. This past summer, the big announcement of water on Mars was in fact new: liquid water, at the surface, presently. Another headline to watch out for is “[volcano] ready to erupt!” Yes, many volcanoes have magma chambers under them, which may or may not be larger than previously thought. However, most of the time, the magma chambers underneath the volcanoes are actually much more solid/mushy than reports make them out to be.

If you’re interested in following along, I’d recommend reading the AGU blogs, as well as Erik Klemetti’s Eruptions blog. Twitter will also be very busy using the hashtag #AGU15.

Glacial Erratics

Glacial erratics on a prairie in South Dakota.  Image credit: laikolosse (CC-BY).
Glacial erratics on a prairie in South Dakota. Image credit: laikolosse (CC-BY).

When glaciers flow down across the ground, they can break off rocks and pick them up in the ice. As the ice moves and eventually melts, those rocks are deposited. When the large rocks are exposed on the surface, they are termed glacial erratics. Much of Minnesota and the eastern Dakotas are covered under these glacial deposits, and these glacial erratics are relatively common.

Glacial deposits are also interesting because they will have grains or rocks of all sizes, from very fine silt and mud up through large boulders. This can make identifying glacial deposits in the field straightforward in some cases, because there will be many grain sizes all together. When grains settle out of the air or from water, the coarse ones deposit first, and the grains end up becoming finer as you go up the stratigraphic column.