Tag Archives: Geology

A One-Stop Heard Island Expedition Primer

North-up view of Heard Island seen from the International Space Station, Nov. 24, 2011.   Image credit: NASA (public domain)

North-up view of Heard Island seen from the International Space Station, Nov. 24, 2011. Image credit: NASA (public domain)

Heard Island, the southernmost surface exposure of the Kerguelen Plateau, is an uninhabited Australian territory situated in the southern Indian Ocean, roughly 450 km SE of the Kerguelen Islands. It is classified as a UNESCO World Heritage Site both for its geologic significance as well as its relatively undisturbed ecosystem. Heard Island is unique among sub-Antarctic islands for having no known human-introduced species.

Physically, Heard Island is roughly circular, with a diameter of 20 km. In the center is Big Ben, a volcano which reaches to 2700 m and was observed erupting within the last 10 days. To the southeast is Elephant Spit, a long sandy spit which protrudes 10 km past the circular shape of the island. In the northwest is the Laurens Peninsula, where volcanoes have added another 10 km to the windward side of the island, against the erosive power of the heavy Southern Ocean seas.

On March 10th, an expedition team of 14 scientists and a ship’s crew of five will depart Cape Town, South Africa, aboard the Braveheart and begin the roughly (and rough) 10-day voyage southeast to the island. Upon arrival at Heard Island, our team will wait for sufficiently calm surf to safely land boats on the beach. About three weeks will be spent on the island before a 10-day voyage to Fremantle, Australia.

For accommodations, two HDT Global air-beam shelters (20’x21′) will be erected at Atlas Cove, in the northwestern part of the island and in the lee of the Laurens Peninsula. A covered walkway, also from HDT Global, will allow travel between tents without full exposure to the elements. Nearby is an emergency refuge (condition unknown) from previous Australian Antarctic Division expeditions, as well as the potentially asbestos-containing ruins of the Australian research base from the 1947-1955 expedition. The ruined base is in a restricted area on account of the asbestos, and expedition members will not enter that area. Restrooms will be in the form of a portable toilet, and portable generators will provide electricity for the site.

Airbeam tent set up at HDT Global headquarters.  This is one of two which will be set up on Heard Island.  Image credit: Cordell Expeditions.
Airbeam tent set up at HDT Global headquarters. This is one of two which will be set up on Heard Island. Image credit: Cordell Expeditions.

Although featuring a large, vegetation-free sand and gravel plain, Atlas Cove is not devoid of life. Our neighbors will include elephant seals, fur seals, four species of penguin—gentoo, king, macaroni, and rockhopper—and many other types of seabirds. Leopard seals have been seen at Atlas Cove as well. To the northeast on the Azorella Peninsula, a colony of the endemic Heard Island cormorants nests atop a moss-covered lava field (access is forbidden due to the sensitive mosses and potential for lava tube collapse).

Communications is an important part of this project. Already we have done a major outreach effort in person, via the internet, and on social media. Many different levels of communications need to be covered: ship-to-civilization, ship-to-island, on-island, island-to-civilization, and amateur radio from the island. We will have several different satellite phone/data systems, marine radios for ship-to-island contacts, and amateur radios for both on-island and worldwide communications. Being able to talk with field teams, the ship, and the outside world is a important for a safe expedition.

Soon after the tents and generators are set up, the antennas used to make contacts around the world will be erected. Amateur radio operators have given generously to support this expedition, and are often curious about science. Making contacts with these amateur stations helps to bring visibility to Heard Island, its unique geology and ecology, and the science being done to better understand and protect the World Heritage Site. Large numbers of amateur radio contacts will also provide an interesting dataset, because the locations one can reach will vary depending on conditions in the upper atmosphere (ionosphere). During the expedition, near-real-time maps of contacts can be found here.

At the camp, an automated weather station will be set up. Being far from human civilization and in the middle of the ocean, a record of weather at Heard Island would be valuable for assessing climate change in an under-sampled region of the globe.

Automated weather station for temperature, pressure, relative humidity, and wind measurements.  Our station will be similar but not identical to this one.  Image credit: US Marine Corps (public domain)
Automated weather station for temperature, pressure, relative humidity, and wind measurements. Our station will be similar but not identical to this one. Image credit: US Marine Corps (public domain)

When weather permits, a small field party will venture out to collect rock samples from the Laurens Peninsula. These samples will be used to answer questions like the environmental conditions when the rock was deposited, the processes that produced the unit (glacial, marine, volcanic, etc.), the duration of deposition, and the age (via biostratigraphy or radioisotopic dating). It is unknown when volcanism began on Heard Island, and whether the volcanism has been relatively continuous or more episodic. There have been no geologic research parties on the island since 1987, so this is an opportunity to collect important samples—especially because glacial retreat has exposed areas which were previously inaccessible. Field parties will not only collect samples, but will map the extent of glaciation and vegetation using GPS.

I will be taking the lead on a different geology project: capturing high-resolution panoramic pictures. Through collaboration with Prof. Callan Bentley and the GEODE project supported by the National Science Foundation (NSF DUE 1323419), we will have a Gigapan system on Heard Island. Using a robotic camera mount and a telephoto lens, a series of images are taken from one location. Upon return to camp, the images are transferred to a computer, where they are automatically stitched together with specialized software. The resulting images, which can be several gigapixels large, can be viewed using a web browser and offer pan-and-zoom capabilities (example from Axel Heiberg Island, Nunavut, Canada). We will use these high-resolution images to provide context for geologic sampling, to document the extent of glaciers and the appearance of landforms, and potentially to estimate populations of seabirds or marine mammals. Because the images will be very large, they may not be available online until after we return to the developed world.

Another project I have in mind, which may or may not be feasible, is to do at least some basic population counts for eBird. There have been four eBird checklists submitted for Heard Island, but none in March or April. I feel fairly confident on my ability to distinguish different types of penguins (at least at close range). Other seabirds, such as albatrosses, petrels, and prions, will be more difficult for me. Perhaps another team member will be able to help out.

Rockhopper penguins in the Falkland Islands.  Image credit: David Stanley (CC-BY)
Rockhopper penguins in the Falkland Islands. Image credit: David Stanley (CC-BY)

Along the shoreline, our team will record the concentrations of anthropogenic marine debris (plastic bits, fishing gear, etc.). The amount of debris and extent to which it is interfering with seabirds and marine mammals at Heard Island is unknown, and we are particularly interested in documenting cases where skeletal remains have associated debris.

There are a few more projects, and more detailed project descriptions can be found on the expedition website project page. If the winds are calm enough a few quadcopters may even be deployed to take pictures in areas too dangerous for us to reach on foot.

Heard Island is home to virtually pristine ecosystems, and our expedition will take care to keep it that way. Rodents are a particularly high concern, so before the ship sails from Cape Town, it needs to be certified free of rodents and must follow several rat prevention protocols. All gear has to be thoroughly cleaned and sanitized before being brought onto the island. On the island, when we move between ice-free areas (Atlas Cove and Spit Bay), we have to clean everything again. Even the food we eat must be in line with ecosystem preservation: no fresh fruit or vegetables, no poultry or eggs (except egg powder kept in sealed containers opened only indoors), and no brassicas (broccoli, cabbages, turnips). This expedition isn’t just an extra-large camping trip.

After three weeks of science, radio, documentation, and outreach, we will pack everything back up onto the Braveheart and embark on a 10-day voyage to Fremantle, Western Australia. On the ship, to the extent we are functioning on what could well be very rough seas, we will probably get started on data analysis, further documentation, and the task of identifying the most compelling photographs.

Polar projection map of the route to and from Heard Island.  Image credit: Bob Schmieder/Cordell Expeditions
Polar projection map of the route to and from Heard Island. Image credit: Bob Schmieder/Cordell Expeditions

When possible, I will try to maintain my presence on Twitter (@i_rockhopper) and here on this blog during the expedition. However, I do not expect to have much time or bandwidth for such things when there is a lot of important field work to do. My hope is that I will get some posts queued up and scheduled for release during the expedition. However, failing that, the best place to find news will be the expedition website and the radio-focused website.

I’m very excited about these projects, and look forward to being on Heard Island in about six weeks!

Update Feb. 11, 2016: There has been a correction on the project collaboration for the Gigapans. The first version wrongly credited the NSF support to the MAGIC project, rather than its umbrella project, GEODE.

Eruption on Heard Island

Today there is a new video out from scientists aboard the R/V Investigator which shows a volcanic eruption occurring from Mawson Peak, Heard Island. This is an exciting video not because it is unusual for an eruption to happen on Heard Island—the Global Volcanism Program shows activity on about an annual basis for the last few years—but because it is unusual for someone to be there to see it!

In the video above, a small plume can be seen over Mawson Peak, and a few lava flows. Given the terrain near the summit and the imagery below from lava flows in 2013, I think it is safe to say that the flows are heading down the southwest flank. As someone going to this island in less than two months, the direction of lava flows is important: it is away from the campsites which we intend to use.

Lava flow on Heard Island, April 20, 2013. Image credit: NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team.
Lava flow on Heard Island, April 20, 2013. Image credit: NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team.

From the video above, this appears to be an effusive eruption, where lavas gently flow out of the volcano. That eruptive style is consistent with a hot (~1100 °C), basaltic (low-SiO2) melt—eruptions with a high SiO2 content tend to have cooler lava and are more often explosive in nature. Basalts or other lavas (trachybasalts and basanites) with low SiO2 (48–52%) are typical of the Big Ben series of lavas (Big Ben being the volcano upon which Mawson Peak is located).[1] Predicting that the lavas from this eruption would be generally low-SiO2 seems fairly safe, although our expedition is not equipped to undertake the sampling required to test that prediction.

Finally, if you’re wondering what happens when a basaltic lava flows out onto ice and snow, know that experimental volcanologists at Syracuse University have asked that question and made a video.

***

[1] Barling, J.; Goldstein, S. L.; Nicholls, I. A. (1994) Geochemistry of Heard Island (Southern Indian Ocean): Characterization of an Enriched Mantle Component and Implications for Enrichment of the Sub-Indian Ocean Mantle. Journal of Petrology 35:1017-1053, doi 10.1093/petrology/35.4.1017.

This Year in Uranium Decay

Pumice from the Bishop Tuff (~767 ka).  Zircons in this pumice are rich (relatively) in uranium, with up to 0.5% U.[1,2]  Image credit: Bill Mitchell (CC-BY).
Pumice from the Bishop Tuff (~767 ka). Zircons in this pumice are rich (relatively) in uranium, with up to 0.5% U.[1,2] Image credit: Bill Mitchell (CC-BY).

With 2016 now upon us, I felt it would be appropriate to think about what a new year means for uranium geochronology. What can we expect from the year ahead? Without getting into any of the active research going on, I felt it would be useful to address simply what is physically happening.

On Earth, there is roughly 1×1017 kg of uranium.[3] The ratio of 238U:235U is about 137.8:1, and 238U has a mass of roughly 238 g/mol (=0.238 kg/mol). Looking only at 238U, that gives us
1x1017[kg]x(137.8/138.8)/0.238[kg/mol] = 4.17x1017 mol [238U]

Radioactive decay is exponential, with the surviving proportion given by e-λt where λ is the decay constant (in units of 1/time) and t is time, or alternatively, e-ln(2)/T1/2*t, where T1/2 is the half-life and t is time.

To find the proportion that decays, we subtract the surviving proportion from 1: (1-e-λt)

Multiplying this proportion by the number of moles of 238U will give us the moles of decay, and multiplying by the molar mass will give the mass lost to decay:

(1-e-λt)*molU

Plugging in numbers, with λ238 = 1.54*10-10 y-1, t = 1 y and the moles of 238U from above, we get:

(1-e-1.54*10-10)*4.17*1017 mol [238U] = 6.4*107 mol

That yields (with proper use of metric prefixes) roughly 64 Mmol U decay, or 15 Gg of U on Earth that will decay over the next year.

Although those numbers sound very large, they are much smaller than even the increase in US CO2 emissions from 2013 to 2014 (50 Tg, or 50,000 Gg); total US CO2 emissions in 2014 were estimated at 5.4 Pg (=5.4 million Gg).[US EIA]

As for what’s in store for geochronology as a field, I think there will be a lot of discussion and consideration regarding yet another analysis of the Bishop Tuff.[4] Dating samples which are <1 Ma (refresher on geologic time and conventions) using U/Pb can be tricky, and Ickert et al. get into some of the issues when trying to get extremely high-precision dates from zircons. The paper is not open access, but the authors can be contacted for a copy (@cwmagee and @srmulcahy are active on Twitter, too!).

***
[1] J. L. Crowley, B. Schoene, S. A. Bowring. “U-Pb dating of zircon in the Bishop Tuff at the millennial scale” Geology 2007, 35, p. 1123-1126. DOI: 10.1130/G24017A.1
[2] K. J. Chamberlain, C. J. N. Wilson, J. L. Wooden, B. L. A. Charlier, T. R. Ireland. “New Perspectives on the Bishop Tuff from Zircon Textures, Ages, and Trace Elements” Journal of Petrology 2014, 55, p. 395-426. DOI: 10.1093/petrology/egt072
[3] G. Fiorentini, M. Lissia, F. Mantovani, R. Vannucci. “Geo-Neutrinos: a short review” Arxiv 2004. arXiv:hep-ph/0409152 and final DOI: 10.1016/j.nuclphysbps.2005.01.087
[4] R. B. Ickert, R. Mundil, C. W. Magee, Jr., S. R. Mulcahy. “The U-Th-Pb systematics of zircon from the Bishop Tuff: A case study in challenges to high-precision Pb/U geochronology at the millennial scale” Geochimica et Cosmochimica Acta 2015, 168, p. 88-110. DOI: 10.1016/j.gca.2015.07.018

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.

Strict Nature Reserve

Tsingy de Bemaraha Strict Nature Reserve (and UNESCO World Heritage Site).  Image credit: Oliver Lejade (CC-BY-SA).
Tsingy de Bemaraha Strict Nature Reserve (and UNESCO World Heritage Site), featuring awesome karst geology as well as lemurs. Image credit: Oliver Lejade (CC-BY-SA).

A recent email from the Australian Antarctic Division about the Heard Island Expedition permit application and plans reminded me that I haven’t spent much time discussing the protections in place for the island. As you might expect for an IUCN class 1a strict nature reserve and a UNESCO World Heritage Site, there are protections, and they are detailed.

The goal of IUCN strict nature reserves is to preserve a natural landscape or ecosystem which would “be degraded or destroyed when subjected to all but very light human impact,” and secondarily, “to preserve ecosystems, species and geodiversity features in a state as undisturbed by recent human activity as possible.” Heard Island definitely fits this category, because apart from whaling and sealing in the late 1800s, there has been very little human activity there. It is home to the Heard Island cormorant, and provides breeding habitat for millions of birds and many marine mammals. Unlike the other sub-antarctic islands, Heard Island has no known introduced species.

In order to protect Heard Island, and as is required by various classifications (IUCN trict nature reserve, UNESCO World Heritage Site) and laws, there is a comprehensive management plan which lays out policies, procedures, and best practices for preserving the integrity of the site.

Here are some illustrative excerpts which demonstrate that a trip to Heard Island is not undertaken lightly.

5.3.8: Visitors to the Reserve must minimise their use of packaging and wrapping material.

5.3.9: Only detergents which are fully biodegradable and low in phosphates may be used in the Reserve.

5.3.10: Polystyrene beads and similar particulate material must not be taken into the Reserve.

5.3.16: Washing water may be disposed of below the high water mark provided reasonable efforts have been made to remove food matter prior to disposal. Such food matter must be handled in accordance with prescriptions 5.3.13 or 5.3.17.

5.4.7b: Prior to departure for the Reserve, all items travelling in the vessel’s cargo spaces or on deck (such as equipment, stores, field accommodation, vehicles, personal gear shipped as cargo) to be taken ashore in the Territory must be hot-washed, disinfected, fumigated, or otherwise treated, and inspected for contaminants which if found must be removed and destroyed.

5.4.10: All outer clothing to be taken ashore in the Territory must be new or thoroughly cleaned and appropriately treated to kill all organisms (including reproductive material) (e.g. with a biocide or similar).

5.4.29b: Footwear to be taken or worn ashore must be thoroughly scrubbed to remove all organisms, soil, and other contaminants (which if found must be removed and destroyed) and must be treated with a biocide [e.g. bleach].

7.1.9: Intending visitors will be provided information that explains the Reserve’s values, the difficulties and dangers of visitation to the Reserve, and the need to apply for permits.

There is a whole lot more there, and it’s interesting to me at least to see what is prescribed to what level of detail. Maintaining a rodent-free ship is HUGE:

5.4.9: The Director [of the Reserve] must be promptly informed of the detection of any rodent on a vessel that is underway to the Territory. The Director will prohibit the entry of that vessel into the Territory unless the Director can be satisfied that the vessel’s entry into the Territory will not result in the escape of rodents into the Territory.

Reading through documents like this, although a little dry at times, helps set a tone for the expedition, reinforces the primary mission of the reserve: conservation. Not every strict nature reserve has such stringent requirements for entry. There’s a strict nature reserve in Madison, WI, (University of Wisconsin–Madison Lakeshore Nature Preserve) which is open to the public without any permitting process, and without thorough cleaning/disinfecting/bleaching being required of any items and clothing being taken into the park.

I’m continuing to get excited about the opportunity to visit Heard Island, which brings me to chapter 7.4 of the management plan:

Our Aims:

  • The enhancement of public awareness and appreciation of the Reserve’s values.
  • The effective use of off-site measures to present the Reserve to national and international audiences

7.4.1d-7.4.1e: A reserve website will be maintained to provide information about the Reserve. It will include maps of the Reserve [d, and e, ] images of the Reserve.

7.4.2: Where practicable, opportunities will be taken to present the Reserve in appropriate public forums.

I hope I’m already doing well on the aims and on 7.4.2, and you can bet that a lot of photography (and some mapping) on the island will be published with Creative Commons licensing.

Geoscientist’s Toolkit: Lasers

Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, open to show internals.  Image credit: Kkmurray (CC-BY).
Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, open to show internals. Image credit: Kkmurray (CC-BY).

Lasers are a fairly charismatic tool for scientists to use—using a laser is an obvious sign that science is happening in some way shape or form, especially if the laser has many hazard warnings on and around it.

Their applications, even within geoscience, are quite varied. They put the “Li” in “LiDAR.” Lasers are also used to turn very small portions of rocks into tiny dusty bits, in a process called laser ablation (the LA of LA-ICP-MS).

One tricky problem in geochemistry is that of analyzing rocks with a mass spectrometer. Mass spectrometers work only on ionized gases (or plasmas), and rocks are pretty solidly solids. In order to get them into a mass spectrometer, you need to break them down somehow, either through acid digestion or other dissolution method, or by vaporizing/blasting them with lasers.

Laser ablation works because lasers—particularly pulsed lasers—can emit a great deal of energy into a small volume very, very quickly. As I expect you know, rocks are not especially thermally conductive, so when they are heated up by all the laser energy coming in, it doesn’t have anywhere to go and the small volume of rock heats up and is broken into dust fragments and/or vaporized. By flowing a gas like helium or argon over the sample, this dust can be swept along into the plasma torch of an inductively-coupled-plasma mass spectrometer and analyzed.

Lasers used for ablation can be focused to very small spot sizes, from 2 μm to 1200 μm (=1.2 mm). These spot sizes are small enough that zones within a crystal, such as growth bands or inclusions, can be analyzed separately.

For atmospheric work, lasers can be used for spectroscopy, or at least probe the concentration of certain molecules (e.g. H2O, CO2). One of my favorite instruments (perhaps deserving its own Geoscientist’s Toolkit post) is the cavity ringdown spectrometer, where a laser illuminates a cavity with highly-reflective—but not completely reflective—mirrors containing a sample gas between them. A detector then measures the time it takes once the laser is shut off for the light to bleed out of the cavity (ms). From the ringdown time, the concentration of the gas of interest can be measured with high precision, even at very low concentrations. It’s pretty neat.

Really, there are a lot of geoscience things one can do with lasers: this is just a smattering of those uses of the tool.

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).

***
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

Exploring Capitol Rock, MT

Wide-angle view of Capitol Rock, MT.  Image credit: Bill Mitchell (CC-BY)
Wide-angle view of Capitol Rock, MT. Image credit: Bill Mitchell (CC-BY)

Several weeks ago, I took a road trip with some friends across the northern part of South Dakota as part of a ham radio adventure. When we reached northwestern South Dakota, we were having so much fun that we decided to continue into just across the border into Montana.

At the state line between South Dakota and Montana, we found that there was a relatively high point (Capitol Rock) which we could probably access with our vehicle. Capitol Rock is in a national forest, so no permission would be needed to go there. It would be a good place to do ham radio (primary goal), and it would get me close to some rocks (bonus)!

As we drew closer to the summit of the hills, I couldn’t help but think that the rocks looked a lot like the ones in my research area in northeastern Montana, in the Hell Creek region (Hell Creek and Tullock/Fort Union Formations).[1]

Sadly, I didn’t get quite as close to the outcrops as I would have liked (we were on a bit of a schedule), but I did get some pictures and made a few observations.

North half of Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
North half of Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Here we had flat-lying sedimentary strata, presumably of roughly Cretaceous-Paleogene age (somewhere around 80-50 million years ago, Ma) (introduction to geologic time). These would have been shallow marine or terrestrial sediments from along the western interior seaway, which was on its way out at the end of the Cretaceous (66 Ma, [1]). I would expect to find some fossils preserved in the sediments, and from those, a fairly accurate date on the strata could be obtained. There may even be some volcanic ash deposits which would allow for direct dating using the U-Pb system or the K-Ar system (Ar/Ar dating) .

At the top of Capitol Rock were several massive units with a slight orange color (probably from oxidized iron). Beneath those were some more finely bedded strata, with bed thicknesses probably around 3-10 cm (eyeball estimation), and displaying some rough texture (popcorn texture?). Underneath those were some fairly easily eroded strata of generally uniform grey color. The image below has these observations annotated.

Northern portion of Capitol Rock, annotated.  Image credit: Bill Mitchell (CC-BY)
Northern portion of Capitol Rock, annotated. Image credit: Bill Mitchell (CC-BY)

The ground under my feet for that previous picture was still above average terrain. Here is an additional picture, taken from the south (looking north), which shows that the light-grey sediments are underlain by more yellow-orange units.

Distant photograph of the lower portion of stratigraphy underlying Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
Distant photograph of the lower portion of stratigraphy underlying Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Upon returning home, I decided to see what description I could find online of Capitol Rock’s geology. It seems there are a number of different descriptions of it.

Capitol Rock, located in the Long Pines Unit in Montana, is a massive white limestone uplift that resembles the Nation’s capitol building.
Montana Office of Tourism

Capitol Rock, located in the Long Pines land unit in Montana, is a massive white sandstone remnant which originated as a volcanic ash deposit. This unique formation resembles the Nation’s Capitol Building in Washington, DC.
US Forest Service

The Bureau of Land Management (BLM) has an interesting discussion of the geology of this area from the perspective of firefighting, specifically in the avoidance of fibrous or asbestos-like minerals which are present in some of the formations in the area:

Brule Member, White River Formation [ed: Formations are a larger stratigraphic unit, and can include multiple Members] – may only be present at Capitol Rock (SE 1/4 sec. 17; T3S; R.62 E) in the Montana portion of the Sioux District. Located at the base of the Arikaree Formation. Massive pinkish gray, calcium containing, clayey siltstone: nodular claystone: and channel sandstone. Contains abundant vertebrate fossils. Thickness 0-30 ft. The member is composed of massive pink clay, exposed in the badlands just Southeast of Reva Gap, well-bedded, hard pale green sandstones alternation with very pale brownish gray clay.
Weathering causes a tread and riser affect much like a staircase. Both the sandstone and the clay are generally calcareous and Bentonitic. The lower portion of the vertical cliffs in Slim Buttes is generally Brule.

Chadron Member, White River Formation – only located in the southern Long Pines within Montana. Found at the base of the Arikaree formation and beneath the Brule Formation at Capitol [R]ock (SE 1/4 sec. 17 T, 3 S., R. 62 E). Basal conglomerate sandstone overlain by beds 10 to 15 ft
thick of dark gray bentonite and cream colored siltstone. Thickness 0-100 ft.
Bureau of Land Management

Well, that’s a puzzling bunch of information, isn’t it! Various sources are suggesting limestone, sandstone from volcanic ash, and a mix of sandstone and siltstone. There’s one more source to check, too: the geologic map. Specifically, we’re interested in the Ekalaka 30’x60′ quadrangle from the Montana Bureau of Mines and Geology!

In the geologic map (look along the right [eastern] edge, near the “T 19N” mark; Capitol Rock is ~1 km NE of the “Tar” label] we see the Fort Union Formation (informal Ekalaka member) at the base of the hills (i.e., under my feet), which is consistent with observations and the relatively detailed presentation from the BLM. It is also consistent with my experience that the Fort Union Formation is generally yellow-orange (in contrast to the drab, grey of the Hell Creek Formation). Then things get trickier. The rocks right at Capitol Rock are mapped as “Tar”, which is the Tertiary Arikaree Formation.

So, what is the Arikaree Formation? Well, the USGS has this to say:

Arikaree formation: gray sandstone with layers of concretions; contains volcanic ash and, locally, channels filled with conglomerate; known only in southeastern Montana.

On the other hand, the North Dakota Department of Mineral Resources breaks the Chadron, Brule, and Arikaree into distinct formations unto themselves.

I suspect this is all hitting at an important point: mapping is really hard, as is saying the rocks over here are the same as the rocks 40 km away. These difficulties are compounded when different scientists use different terminology, such as when the mapping is done by state geological surveys. The same rocks may change names when a state boundary is passed. Sometimes researchers will use the terminology from one state to apply to the rocks on both sides of the boundary, and then the literature is filled with multiple terminologies for the same rocks. It can also be very difficult to correlate rocks laterally over large distances, especially when there is poor outcrop over those distances (i.e. between buttes).

Here’s my interpretation of what’s going on at Capitol Rock: it is composed of siltstone, sandstone, and altered volcanic ash [still good for U-Pb dating!]. This volcanic ash is high in erionite, an asbestos-like mineral. Naming of the unit could include either the Arikaree Formation, or the Brule Member of the White River Formation. An age of 37–30 Ma seems reasonable.

***

[1] Renne, P. R., Deino, A. L., Hilgen, F. J., Kuiper, K. F., Mark, D. F., Mitchell, W. S., III, Morgan, L. E., Mundil, R., Smit, J. (2013) Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science 339: 684-687, doi: 10.1126/science.1230492.

Geoscientist’s Toolkit: Fluxgate Magnetometer

Fluxgate magnetometer; coil is around 1 cm in length.  Image credit: Zureks (CC-BY-SA).

The fluxgate magnetometer—not to be confused with the flux capacitor—is a nifty tool for determining the strength and direction of a magnetic field.

It works by using an alternating current to induce an alternating magnetic field in a magnetically permeable core (ferrite core), saturating the core. The magnetic field then induces a current in a secondary winding. My apologies for not having an open-use schematic, but the ones here and here are quite good, plus have a more nuanced explanation.

Absent an external field, the induced current will be equal to the driving current. However, in a magnetic field, one direction will saturate more easily and the other less easily, because the permeable core will be reacting to the external field. As a result, the secondary windings will have a current imbalance when compared to the driving winding, and the imbalance will show up both on the rise and fall of the driving waveform. The imbalance has a frequency of twice the drive frequency. Also, this design detects magnetization in one direction only. For a full 3D characterization of the direction of the magnetic field, it takes three magnetometers, each perpendicular to the others.

One of the early applications of fluxgate magnetometers was the detection of submarines (large metallic bodies). Indeed, through this type of study, the alternating magnetization of rocks along the sea floor of the Atlantic Ocean was discovered, with bands parallel to the Mid-Atlantic Ridge. These data gave strong evidence in support of plate tectonics.

Magnetic field anomalies of the world.  Image credit: J.V. Korhonen,J. Derek Fairhead, M. Hamoudi, K. Hemant, V. Lesur, M. Mandea, S. Maus, M. Purucker, D. Ravat, T. Sazonova & E. Thébault, 2007, accessed via SDSU.
Magnetic field anomalies of the world. Image credit: J.V. Korhonen,J. Derek Fairhead, M. Hamoudi, K. Hemant, V. Lesur, M. Mandea, S. Maus, M. Purucker, D. Ravat, T. Sazonova & E. Thébault, 2007, accessed via SDSU.

But the magnetometer’s usefulness doesn’t stop there! Earth’s magnetic field extends out into space, where it interacts with magnetic fields from the solar wind. By measuring the magnetic fields, scientists can study the interactions between Earth’s magnetosphere and the solar wind, interactions which can give us auroras.

Aurora in Minnesota.  Image credit:  Charlie Stinchcomb (CC-BY)
Aurora in Minnesota. Image credit: Charlie Stinchcomb (CC-BY)

Perhaps an even more exciting application is the study of magnetic fields near the Moon. NASA’s ARTEMIS mission (using repurposed THEMIS spacecraft) is flying two magnetometers around the Moon. Heidi Fuqua, a scientist at UC Berkeley, and her collaborators are using the magnetic data gathered by the ARTEMIS satellites to study the Moon’s interior. Depending on the size and conductivity of the Moon’s interior, the magnetic field will have differing responses to the induced magnetic field from the solar wind. It’s pretty neat stuff!

Geoscientist’s Toolkit: Geologic Map

Geologic map of Minnesota bedrock.  Image credit: scaled down from University of Minnesota/Minnesota Geological Survey.
Geologic map of Minnesota bedrock. Image credit: scaled down from University of Minnesota/Minnesota Geological Survey (original, 11 MB PDF).

Maps are neat. Geologic maps in particular can be quite interesting (see above, particularly the original PDF). These are the product of detailed surveys, which are undertaken both at the federal and state level, and show which rock types are found in which regions. Some of these rocks can be traced over long distances (like the sedimentary rocks of the southeastern corner of Minnesota), while others are localized.

Geologic maps give a summary of what types of rocks are in which areas. From this, you can find out search terms to get you to more information about certain rocks, or you can use the rock type to determine what used to be happening in an area. For instance, southeastern Minnesota was once covered by a warm, shallow sea, leading to sandstone, limestone, and dolostone formation. Some of the limestones are fossiliferous. Northeastern Minnesota used to be home to a volcanic rift valley (like the one presently in East Africa) and is home to volcanic rocks, such as the North Shore Volcanic Group.

In addition to the short description of the rock units, geologic maps will give the estimated age range of the rocks (if you need a refresher on geologic time, see this post). A quick glance at the time scale will show you that although you may find fossils in southeastern Minnesota, don’t expect to find any dinosaurs (they existed during the Mesozoic)!

Faults are mapped as well, either transform (offset side-to-side), thrust (compressing, one side going up), or normal (expanding, one side falling). Dikes, which are ribbon-like intrusions which cut through the local rock, are mapped as lines. Because they need to cut through the local rock, they are inherently younger than the rock which they cut through—thus a radioisotopic age for the dike will be a minimum age for the unit it intrudes.

There are also several different types of geologic maps. Bedrock maps, such as the one above, show what the primary consolidated rock is, although it may be buried beneath loosely packed, more recent sediments. Surficial maps show more recent deposits; here in Minnesota, that’s often glacial deposits of various types, but can also include features such as alluvial fans and landslide deposits.

Finding geologic maps here in the US can be a little bit tricky. The USGS has nice geologic maps (start here), but they tend to be large-area. State surveys seem to have more detailed local maps, but each state has their maps in a different location and the availability may not be consistent state to state. Montana has a nice geologic map interface on their website, while Minnesota’s geologic maps are not easily found—there are county-scale surficial geologic maps, at least for some counties, but I’ve really only been able to find them through third-party search engines. For advanced map users, the state surveys will often make the raw GIS (geographic information system) data available.

Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.
Silicic dike in the Benton Range, near Bishop, CA. Image credit: Bill Mitchell.