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.
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).
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.
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 other worlds up close, and which are different from our own, can be very informative. We may have theories about the composition of worlds like Pluto, of how it formed, how it behaves, and what its surface is like. However, it is not until we go there that we can truly test those hypotheses. In many cases, when we are dealing with worlds vastly different from our own, what we find is surprising, mysterious, and awe-inspiring.
For instance, most pre-New Horizons models would have made Pluto out to be a fairly heavily cratered object, not unlike the Moon. However, that was not at all what was found. The first high-resolution picture released during the flyby, part of a mosaic which is still being put together, had no craters visible. None.
Given that the history of Pluto is likely to have included significant bombardment by smaller objects, this result makes us rethink our model of the processes happening on Pluto’s surface. From what we know of the frequency of impacts, these surfaces would need to have been recently (geologically speaking, so in the last ~100 Ma) formed, eroded, or otherwise modified.
It is encounters like this which help us understand and consider our models, and to recognize which properties of large rocky bodies are important under which circumstances. What is reshaping Pluto’s surface? How did the various terrains form? Do they happen elsewhere? Where does the energy for these processes come from?
Exploring other worlds keeps our thinking fresh, challenges our assumptions, and inspires us to create new models and experiments to better understand our solar system and our own Earth.
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).
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.
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, ). 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.
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.
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
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.
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.
 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.
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.
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.
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!
When purifying a mineral from whole rock, one of the most useful separations is by density. Water, being less dense than most rock, is not especially useful for this. However, lithium metatungstate (LMT, mixed with water) and sodium polytungstate (SPT, also mixed with water) can create denser—albeit more viscous—liquids, with densities approaching 2.9–3.1 g/cm3. These denser liquids are enough to separate feldspar and quartz (<2.7 g/cm3) from zircon, titanite (sphene), and barite (densities >3.5 g/cm3).
Separations are fairly straightforward. A crushed, sieved rock sample is poured into a separatory funnel filled 1/2–2/3 full with the heavy liquid. The slurry is stirred vigorously with a stirring rod, and allowed to settle (it may take a couple hours if the grain size is fine and the liquid viscous). After it settles, the dense minerals should have sunk to the bottom, while the light minerals will float. A filter funnel is then placed under the separatory funnel. When the stopcock is opened, the dense minerals and some of the heavy liquid will pour out the bottom. The stopcock is then closed when the heavy separate has passed through. A second filter funnel is then used to capture the light fraction. With good filtering, the heavy liquid can be reused. The separates can be washed with distilled water and dried.
Heavy liquid separation is often used in combination with magnetic separation to purify minerals for analysis. Depending on the difference in densities being separated, a liquid may need to be fairly precisely calibrated with larger samples of the desired minerals. Sanidine (~2.55 g/cm3) and quartz (~2.65 g/cm3) need a well-calibrated liquid to achieve good separation, while either (or both) of them from zircon can be done with any LMT solution >2.7 g/cm3.