Tag Archives: Rocks

Heard Island’s Most Spectacular Outcrop

Head-on view of a block of Drygalski Formation (mixed volcanics and glacial sediments, here glacial sediments with volcanic clasts).  Image credit: Bill Mitchell (CC-BY).
Head-on view of a block of Drygalski Formation (mixed volcanics and glacial sediments, here glacial sediments with volcanic clasts). 53° 01.927′ S, 73° 23.704′ E. Image credit: Bill Mitchell (CC-BY).

Heard Island is home to a spectacular outcrop. It’s the coolest outcrop I’ve ever seen, besting the Bishop Tuff tablelands, the potholes along the St. Croix River at Taylor’s Falls, Zion Canyon, The Badlands, and various outcrops in Yellowstone and Grand Teton. Admittedly this outcrop doesn’t intrinsically have the scale of many of the others just mentioned—it’s a roughly car-sized block—but the power that went into creating it and the effect it created is truly amazing.

On its face (see above), it looks quite pedestrian: a block of lithified glacial till with clasts of vesicular basalt reaching up to grapefruit size. However, it’s important to consider it from a different perspective.

Side view of a block of Drygalski Formation.  From this view, it is much easier to see this is a ventifact (carved by the wind).  There is a pile of sand on leeward (left) side. Image credit: Bill Mitchell (CC-BY).
Side view of a block of Drygalski Formation. From this view, it is much easier to see this is a ventifact (carved by the wind). There is a pile of sand on leeward (left) side.
Image credit: Bill Mitchell (CC-BY).

When viewed from the side, a pile of sand in on the leeward (left, east) side of the block is evident. Additionally, the basaltic clasts of the rock face seem to be protecting the softer, tan-colored glacial matrix from the sand-blasting.

Here’s a close-up from an oblique angle:

Close-up, oblique view of the outcrop face.  Here the differential weathering (resistant grey clasts, weak tan matrix) is very apparent.  Spires of matrix are left to the leeward of the clasts, and are roughly horizontal. Image credit: Bill Mitchell (CC-BY).
Close-up, oblique view of the outcrop face. Here the differential weathering (resistant grey clasts, weak tan matrix) is very apparent. Spires of matrix are left to the leeward of the clasts, and are roughly horizontal.
Image credit: Bill Mitchell (CC-BY).

In the oblique view, the volcanic clasts making up the face of the outcrop are seen sheltering the matrix directly to the leeward from mechanical erosion. To tie all of these views together, I took a short video (embedded below).

This outcrop is located on the edge of a volcanic sand plain roughly 1.5×1.5 km. Strong westerly winds are present most of the time (9 m/s is average, measured at a site nearby).[1] In fact, the audio which accompanies the video is mostly wind noise, though there’s a little unintelligible chatter with my field partner, Carlos. Winds when the recording was made were “moderate” (for Heard Island) and from the west, exactly the kind of winds that shaped this outcrop. At the time of the recording, the winds weren’t strong enough to kick up much sand, nor were ice pellets falling from the sky, but on a gustier, stormier day, the face of this outcrop will take a beating.

Looking toward the ventifact outcrop from Windy City, Heard Island.  Although the outcrop itself is hidden behind the small reddish rise at center, this image illustrates the expanse of vegetation-free sand plain. Image credit: Bill Mitchell (CC-BY).
Looking toward the ventifact outcrop from Windy City, Heard Island. Although the outcrop itself is hidden behind the small reddish rise at center, this image illustrates the expanse of vegetation-free sand plain.
Image credit: Bill Mitchell (CC-BY).

In my travels and geo-adventures, I’ve seen differential weathering and ventifacts (outcrops shaped by wind), but never so strikingly combined as at this outcrop on Heard Island. That’s why I can confidently say it’s the coolest outcrop I’ve seen on Heard Island or anywhere else in the world.

[1] Thost, D., Allison, I. “The climate of Heard Island” in Heard Island: Southern Ocean Sentinel, ed by K. Green and E. Woehler. Surrey Beatty & Sons, Chipping Norton 2005, p. 52-68.

The Making of the Windy City Gigapan

Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors. Image credit: Carlos Nascimento
Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors.
Image credit: Carlos Nascimento

In my previous post, I discussed the gigapan of Windy City. However, the making of that gigapan was quite the adventure in field work.

After the Azorella Peninsula gigapan, the unit was packed up and taken back aboard the Braveheart for a trip to the southeast portion of the island. Rough north winds were expected, and with no protection afforded against those winds and swells from Atlas Cove, the ship had to move. Our expedition leader and two scientists not involved in the radio operations left camp and went to ride out the storm south of Stephenson Lagoon. At that time, it had become clear that I personally would not be able to go to Stephenson Lagoon—an area which was an extremely high priority for a gigapan image. I put fresh batteries into the gigapan mount, and sent it on its way. Sadly, in the almost four hours the team had on the shores of Stephenson Lagoon, they did not have an opportunity to take a gigapan. I’ll have to go back for that one!

Upon their return to camp, I knew since they had not attempted any gigapanning that there were fresh batteries in the unit. As the end of the expedition drew near, it was time to get the gigapan done at Windy City. Mid-morning, Carlos joined me for a trip to the outcrop (about 1.4 km each way). Although we didn’t have a bright sunny day, it was dry with a temperature around 5 °C. When we reached the outcrop and everything was set up, I turned on the gigapan mount. Nothing happened. With new batteries and a limited task, I hadn’t brought the whole kit with me. We headed back to camp, arriving in time for lunch.

Several of the rechargeable batteries I had for the gigapan had been sitting on the charger and were ready to go. I tossed those into the battery holder, put it under my arm to keep warm, and headed out with Carlos once again. At the outcrop I set up the rig again. When everything was set to go, I removed the batteries from inside my jacket, and put them into their slot. I powered it on. The LCD display brightened, but displayed an error message: Button-pusher disconnected or plugged in backwards. Cycling the power on and off didn’t fix it. Everything was as it had been before when it worked. Once again, this was a problem I was unable to deal with at the outcrop.

Back in camp, Carlos looked online for a solution while I tried to see if anything was likely to have come disconnected, although our team had been very gentle with the unit. Nothing stood out. Eventually we found online that the error is commonly caused not by a disconnected or backwards button-pusher, but by a low voltage. That made a bit more sense. Out came the volt-meter, and two sets of six AA alkaline batteries were verified to be fresh. One set went into the battery holder, the other went into a storage case. Now that it was late in the afternoon, Carlos had to report for radio duty, but Adam was willing to come with me—I needed this gigapan before the light died, as there was no guarantee that I would have the weather conditions or time to get it later.

Adam and I hurried over to the outcrop, the light already beginning to fade. I set up quickly, got the batteries out from my jacket, and set up the gigapan.

Please, light, stay with us long enough to complete this shot. Please, batteries, keep up your voltage!

It was clear from the beginning that the shot would not be truly completed. Somewhere in the middle either the light would die or the batteries would. Eventually, both did at about the same time. We quickly put everything back into the packs and headed back for camp. It was getting dark, but we arrived just in time for dinner and the start of my shift at the radio desk.

Although it was too late to be of use, I asked on Twitter what some of the other cold-weather folks had done about their gigapans. By the end of my four-hour radio shift, I had responses from @rschott and @callanbentley. Evidently this is a common problem, which is fought by insulating the gigapan unit as well as possible, and using hand/toe-warmers to add a little heat.

I think it’s time to ask Gigapan to make some design adjustments to improve the cold-weather operation of the units. It wasn’t all that cold where I was gigapanning, yet I still couldn’t get 15 minutes of operation on fresh batteries at 3–5 °C.

Windy City Gigapan

Processing the Windy City gigapan.  Image credit: Bill Mitchell (CC-BY).
Processing the Windy City gigapan. Image credit: Bill Mitchell (CC-BY).

This is the third in a series of three posts about the gigapan images taken on Heard Island (1: Big Ben, 2: Azorella Peninsula), with more information about the Windy City gigapan.

Windy City is located about 200 meters south of Atlas Cove, in the northwest portion of Heard Island. It comes from a fin of Drygalski Formation rocks, which are a mix of glacial sediments and volcanics, and is mostly surrounded by sand and gravel plains.

Looking closely at the outcrop, there are a number of interesting things to observe. First, there are the striking roughly-horizontal marks, which are particularly evident toward the base of the outcrop. Second, the outcrop is made of massive, fine-grained jointed rocks with few vesicles. Third, there are quite a few fractures within the rock, with discolorations along many of the cracks.

All of these observations combine into a remarkable tale of how Windy City has been formed. The massive, fine-grained, and jointed appearance leads to the conclusion that we are looking at a volcanic outcrop, rather than glacial sediments. Fracturing and discoloration have been brought on by weathering from the very wet, near-freezing environment. Finally, the wind has been a huge factor! Sand, gravel, snow, and graupel (ice pellets) have all been blasted against the side of this outcrop, primarily from the west (at right). On Heard Island, a 9 m/s wind is typical, with maximum recorded gusts exceeding 50 m/s on three days during the 1948-1954 period.[1] The high winds sandblast the outcrop, leading to the horizontal striations.

Here are a few wider-angle shots for context, and with better light than I ended up with for the gigapan.

Windy City outcrop, viewed from the north.  The gigapan image covers from my right arm to roughly the center of this image.  Image credit: Carlos Nascimento
Windy City outcrop, viewed from the north. The gigapan image covers from my right arm to roughly the center of this image. Image credit: Carlos Nascimento

Looking eastward at Windy City, with a person for scale.  The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion.  The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors. Image credit: Carlos Nascimento
Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors.
Image credit: Carlos Nascimento

I also managed a close-up shot of one of the pieces of float.

Float rock at Windy City.  The 1:1000 metric scale at right is effectively a mm scale.  Some olive/green crystals are visible, mostly 1-5 mm in their longest dimension, which are likely olivine (possibly clinopyroxene). Image credit: Bill Mitchell (CC-BY).
Float rock at Windy City. The 1:1000 metric scale at right is effectively a mm scale. Some olive/green crystals are visible, mostly 1-5 mm in their longest dimension, which are likely olivine (possibly clinopyroxene).
Image credit: Bill Mitchell (CC-BY).

[1] Thost, D., Allison, I. “The climate of Heard Island” in Heard Island: Southern Ocean Sentinel, ed by K. Green and E. Woehler. Surrey Beatty & Sons, Chipping Norton 2005, p. 52-68.

Azorella Peninsula Gigapan

Processing the Azorella Peninsula gigapan.  Image credit: Bill Mitchell (CC-BY).
Processing the Azorella Peninsula gigapan. Image credit: Bill Mitchell (CC-BY).

This is the second in a series of three posts about the gigapan images taken on Heard Island (1: Big Ben, 3: Windy City), with more information about the Azorella Peninsula gigapan.

The Azorella Peninsula is on the northern edge of the main part of Heard Island, east of the Laurens Peninsula. It forms the eastern boundary of Atlas Cove (Laurens Peninsula forms the western boundary; see map below). At the west end of the Azorella Peninsula’s southern margin is the heritage zone around the ANARE campsite, two water-tank shelters, a green “apple” shelter, and the area where our expedition made camp. That many of the camps are all in the same area is no accident: Atlas Cove is probably the best harbor on the island (though still not sheltered from a northerly swell), there is a convenient beach for boat landings, and a small step up of elevation from the lava flows of the Azorella Peninsula provides higher ground than the sometimes-inundated Nullarbor.

Heard Island Map, 1985. Image credit: excerpt from the Division of National Mapping.
Heard Island Map, 1985. Image credit: excerpt from the Division of National Mapping.

Getting a gigapan here was not as straightforward as I had hoped. Although there were plenty of pahoehoe flow tops, cracks where a flow had deflated and collapsed in on itself, and other lava flow features, few of them were of a scale and in a location which enabled them to be nicely gigapanned with the tripod I had. With another 3–5 m of elevation, the gigapan would be spectacular.

As it was, there were some additional features besides the lava flows which I wanted to include. For one, the landscape has significant erosional processes happening, and there are sandy areas which get washed when it rains. Even more than the rain, though, the wind creates eolian features. Many of the small rocks have a little dune in their lee, and often the Kerguelen cabbage and Azorella moss grow on the leeward side of rock barriers as well. Some of this organization is visible in the gigapan.

At the top right of the Gigapan image, and lost to the fog and overexposure of the image, is a strongly layered prominence: Corinth Head. Although I would have liked to go see this outcrop up close, our permit did not allow that—the area is a major nesting site for burrowing seabirds, and in places there are lava tubes with thin ceilings which may give way underfoot.

Corinth Head, Heard Island, viewed from the south (further east than the Gigapan was taken).  Layering is clearly visible, and is likely of igneous origin.  Image credit: Bill Mitchell (CC-BY).
Corinth Head, Heard Island, viewed from the south (further east than the Gigapan was taken). Layering is clearly visible, and is likely of igneous origin. Image credit: Bill Mitchell (CC-BY).

Where the Azorella Peninsula lava flow field meets the Nullarbor, there was a little flow which caught my eye. There, one flow clearly traveled through an older channel or tube. Weathering has removed some of the older flow, giving a cross-sectional view of the dark vesicular rock.

Lava flows of the Azorella Peninsula meet the Nullarbor.  An older, grey unit is visible with a redder unit in the middle.  Notebook is 19 cm wide.  Image credit: Bill Mitchell (CC-BY).
Lava flows of the Azorella Peninsula meet the Nullarbor. An older, grey unit is visible with a redder unit in the middle. Notebook is 19 cm wide. Image credit: Bill Mitchell (CC-BY).

Some lava tubes showed obvious signs of deflation or lava tube collapse. The one seen below had eolian features nearby, and the Kerguelen cabbage and Azorella moss can be seen growing on the leeward side of the rocks. An elephant seal is also present.

Deflated lava flow beside the Nullarbor on the Azorella Peninsula, Heard Island.  Some eolian features are present.  Note the Azorella moss and Kerguelen cabbage at right, in the lee of the solid rocks.  The tan mass at right is an elephant seal.  Notebook is 12x19 cm.  Image credit: Bill Mitchell (CC-BY).
Deflated lava flow beside the Nullarbor on the Azorella Peninsula, Heard Island. Some eolian features are present. Note the Azorella moss and Kerguelen cabbage at right, in the lee of the solid rocks. The tan mass at right is an elephant seal. Notebook is 12×19 cm. Image credit: Bill Mitchell (CC-BY).

Finally, here is another example of a collapsed lava tube, which shows off a cross-section of the top of the lava tube as well as some pahoehoe flow tops.

A collapsed lava tube on the Azorella Peninsula, Heard Island, gives a cross-sectional view of the roof of the lava tube.  Kerguelen cabbage plants in foreground are roughly 25 cm across.  Several pahoehoe flow tops are visible: small-scale in the foreground, and large-scale in the center toward the top of the image.  Image credit: Bill Mitchell (CC-BY).
A collapsed lava tube on the Azorella Peninsula, Heard Island, gives a cross-sectional view of the roof of the lava tube. Kerguelen cabbage plants in foreground are roughly 25 cm across. Several pahoehoe flow tops are visible: small-scale in the foreground, and large-scale in the center toward the top of the image. Image credit: Bill Mitchell (CC-BY).

Big Ben Gigapan

Processing the Big Ben gigapan.  Screenshot by Bill Mitchell.
Processing the Big Ben gigapan. Screenshot by Bill Mitchell.

This post is the first in a series of three on the gigapans I took on Heard Island. (Part 2, Part 3)

My first gigapan on Heard Island, this one of Big Ben, came unexpectedly. As I was out hiking one afternoon, my hiking partner, Arliss, noticed that we had a clear view of the summit of Big Ben. Clearings like this can be relatively short and infrequent, so we took a few pictures immediately. We headed back to base camp just east of Atlas Cove, arriving under an hour before sunset. The mountain was still visible, so I moved quickly to set everything up and get the gigapan taken before the light faded.

From camp, Big Ben is situated to the southeast, rising up beyond the flat sandy plain of the nullarbor. In this view, the moraines and glaciers begin about 2 km from the camera. To the right of the image is the eastern slope of Mt. Drygalski. The edge of the Azorella Peninsula lava flow is in the bottom left corner.

Glacial features dominate the landscape, including a prominent moraine now covered in vegetation (lower right). Coming toward the camera are the Schmidt and Baudissin glaciers. I think this view covers from the Allison and Vahsel glaciers (at right) to the Ealey glacier (at left).

On the Nullarbor, there are a few king penguins and elephant seals, primarily to the left of center.

Big Ben itself has a range of rock types, including basanites, alkali basalts, and trachybasalts, overlying limestones and volcanic/glacial deposits.[1-4]

[1] Quilty, P. G.; Wheller, G. (2000) Heard Island and The McDonald Islands: a Window into the Kerguelen Plateau. Papers and Proceedings of the Royal Society of Tasmania. 133 (2), 1–12.

[2] Barling, J.; Goldstein, S. L. (1990) Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature 348:59–62, doi 10.1038/348059a0.

[3] 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.

[4] Stephenson, J.; Barling, J.; Wheller, G.; Clarke, I. “The geology and volcanic geomorphology of Heard Island”, in Heard Island: Southern Ocean Sentinel (Eds K. Green and E. Woehler) Surrey Beatey & Sons, 2006, p. 10–27.

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.

Geoscientist’s Toolkit: Paleomagnetic Coring

Recording rock core orientation for paleomagnetic analysis.  Image credit: Bill Mitchell.
Recording rock core orientation for paleomagnetic analysis. Image credit: Bill Mitchell.

I’ve touched on paleomagnetism a little bit before, both as a technique for tying rocks in to the geologic timescale, and as something which can be found by using a fluxgate magnetometer. It’s a pretty interesting set of techniques and uses some cool science tools, so I thought I’d explain a little bit more.

Magnetism from the Earth’s magnetic field can be retained by individual layers of rocks, at least under some circumstances. If you have a bunch of layers stacked on top of each other like pancakes, the different layers (beds) can have different magnetic directions.

Stack of banana-walnut pancakes.  Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely.  Image credit: Jack and Jason's Pancakes (CC-BY-SA).
Stack of banana-walnut pancakes. Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely. Image credit: Jack and Jason’s Pancakes (CC-BY-SA).

As you might expect, the equipment needed to make sensitive measurements of the magnetic field are not particularly portable (and may be a topic for another post). Samples need to be collected in the field and brought back to the lab, and the sample orientation must be marked and recorded in such a way that the measured magnetic field can be related back to the magnetic field in the rock itself.*

To do that, paleomagnetists (or paleomagicians) will drill a small (1″ diameter by a few inches long) annular hole into the rock, leaving a plug of rock in the center. That will become the sample. Before it can be removed from the hole, a mark is made on the top of the plug with a brass rod. The direction of the hole is determined with a compass (or a sun compass when conditions allow), as is the angle away from vertical of the core (the hade).

When the plug is freed from the rock, the down-hole direction is marked with arrows along the mark using a permanent marker. The samples (several from each bed) are then placed into sample bags, labelled appropriately, and carefully transported back to the lab.

Are you irresistibly attracted to such a magnetic field of study? This is probably the best place to go for more information, and is freely accessible online.[1]

***
[1] Tauxe, L., Banerjee, S.K., Butler, R.F. and van der Voo R, Essentials of Paleomagnetism, 3rd Web Edition, 2014. [accessed Aug. 27, 2015]

* The field magnetic field?