Tag Archives: LANDSAT

Heard Island Landslide!

Landslide on Compton Glacier, Heard Island, 2017-07-21.  Image credit: processed by Bill Mitchell (CC-BY), using USGS/Landsat 8 data.
Landslide on Compton Glacier, Heard Island, 2017-07-21. Image credit: processed by Bill Mitchell (CC-BY), using USGS/Landsat 8 data.

On July 21, 2017, the Landsat 8 satellite imaged a fresh landslide on Heard Island, seen in the picture above. The slide occurred in the northeast portion of the island, on top of Compton Glacier, and I have annotated it for clarity in the image below.

Satellite image of Heard Island with annotation marking the region where the landslide is present.  Image credit: processing and annotation by Bill Mitchell (CC-BY), data from USGS/Landsat 8.
Satellite image of Heard Island with annotation marking the region where the landslide is present. Image credit: processing and annotation by Bill Mitchell (CC-BY), data from USGS/Landsat 8.

This landslide is quite easy to spot because of the relatively clear conditions over Heard Island and the very high contrast between the dark, presumably-basaltic rocks and the white snow of the glaciers. Given that it is presently austral winter and Heard Island is located south of the Antarctic Convergence, the rate of snow accumulation should be quite high. It will be interesting to see how long it takes to be covered by snow.

I am fairly convinced that this is a rock- or landslide rather than an eruption. The head of the flow is along the top of a steep ridge, and the infrared imagery shows no thermal anomaly in this part of the island.

What’s interesting to me is that this slide appears to have eroded some snow on top of the glacier which then caused a secondary avalanche from a north-facing slope. I’ve annotated this in the image below.

Region of secondary avalanche.  Image processing and annotation: Bill Mitchell (CC-BY), data from USGS/Landsat 8.
Region of secondary avalanche. Image processing and annotation: Bill Mitchell (CC-BY), data from USGS/Landsat 8.

This landslide has a run-out of about 2.5 km, an elevation drop of ~750 m, and a total affected area of ~0.8 km2. Several flow tongues are evident in the close-up image, even though the satellite imagery resolution is a modest 15 m/pixel.

Close-up of landslide on Compton Glacier, Heard Island.  Several flow paths of dark rock are evident here.  Image processing: Bill Mitchell (CC-BY), data from USGS/Landsat 8.
Close-up of landslide on Compton Glacier, Heard Island. Several flow paths of dark rock are evident here. Image processing: Bill Mitchell (CC-BY), data from USGS/Landsat 8.

From this image, it looks like the rockfall mostly happened in the portion running west-to-east, then as it turned the corner to head northeast, transitioned to a surface flow. In many ways, this reminds me of the Mt. Dixon (New Zealand) rock avalanche in 2013 (coverage by Dave Petley here and here, among others). The video below is from the Mt. Dixon (NZ) rock avalanche, but is likely similar to what occurred on Heard Island.

A fly-over after the Mt. Dixon (NZ) rock avalanche provided more video of the rock avalanche scar.

I look forward to seeing more images of this slide as they come in. Heard Island is imaged roughly every 8 days by Landsat 8, which as far as I can tell is the only publicly available high-resolution imagery for the island now that EO-1 has been decommissioned.

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Big Ben Eruption Update 2017-02-27

Mawson Peak's summit crater glows orange in this false-color infrared image (bands 7-6-5) taken February 27, 2017.  Image credit: Bill Mitchell (CC-BY) using data from USGS LANDSAT 8 (public domain).
Mawson Peak’s summit crater glows orange in this false-color infrared image (bands 7-6-5) taken February 27, 2017. Image credit: Bill Mitchell (CC-BY) using data from USGS LANDSAT 8 (public domain).

It has been three weeks since I reported on an active eruption on Heard Island seen by Landsat 8. Since then, the presence of lava at or near the surface in the summit crater of Mawson Peak has continued, and a thermal anomaly is present both in the February 27 Landsat 8 image shown above and in February 20 imagery. It is difficult to discern in the true-color imagery from February 27 whether there are any new lava/debris flows present. The two MODIS instruments (one on Aqua, one on Terra) have not picked up any thermal anomalies since early February.

Unfortunately, one of the best tools I’ve had at my disposal for keeping an eye on Mawson Peak is no longer available: NASA/USGS satellite EO-1 was decomissioned last week. EO-1 provided 10 m/pixel true-color imagery, which is significantly higher resolution than 15 m/pixel of Landsat. Archival data for both satellites remains available, but no new EO-1 data will be taken. New data from Landsat 8 typically comes in a few times each month (every 7-16 days), and I’ll be keeping an eye on it.

Mawson Peak in true color, February 27, 2017.  Image processing: Bill Mitchell (CC-BY) using USGS Landsat 8 data (public domain).
Mawson Peak in true color, February 27, 2017. Image processing: Bill Mitchell (CC-BY) using USGS Landsat 8 data (public domain).

Big Ben Eruption, 2017-02-04

Lava and debris flows radiate away from Mawson Peak on Heard Island.   February 4, 2017.  Image credit: Bill Mitchell (CC-BY) using data from Landsat 8 OLI (NASA/USGS; public domain).
Lava and debris flows radiate away from Mawson Peak on Heard Island. February 4, 2017. Image credit: Bill Mitchell (CC-BY) using data from Landsat 8 OLI (NASA/USGS; public domain).

On February 4th, Landsat 8 captured a clear view of the summit of Big Ben volcano, at Heard Island. Heard Island is a very cloudy location, so clear views are uncommon (I don’t have numbers, but would estimate <20%). However, the February 4th images are even more spectacular: they capture an ongoing volcanic eruption.

Observations
In the sharpened true-color image (above), four or five different lava/rock/debris flows are visible emanating from the summit. Using a false-color infrared image (below), two hot regions are apparent (red/orange/yellow), and are separated by about 250 meters. The longest of the flows stretches nearly 2 km, and drops from an elevation of roughly 2740 m to 1480 m (using 2002 Radarsat elevation data with 20 m contours). All three of the large flows to the west or southwest of the summit drop below 2000 m elevation at the toe.

False-color infrared imagery of Mawson Peak, Heard Island.  Two vents are visible in red/orange/yellow, separated by 250 meters. Data source: Landsat 8 OLI/TIRS bands 7-6-5.  Image credit: Bill Mitchell (CC-BY), data from NASA/USGS (public domain).
False-color infrared imagery of Mawson Peak, Heard Island. Two vents are visible in red/orange/yellow, separated by 250 meters. Data source: Landsat 8 OLI/TIRS bands 7-6-5. Image credit: Bill Mitchell (CC-BY), data from NASA/USGS (public domain).

Interpretation
In the sharpened true-color imagery, I have identified five rock and debris flows originating at the summit, as well as one potential avalanche. Annotation of these observations is found on the pictures below.

Annotation of lava/rock/debris flows from Mawson Peak, Heard Island, February 4, 2017.  Image credit: Bill Mitchell (CC-BY).
Annotation of lava/rock/debris flows from Mawson Peak, Heard Island, February 4, 2017. Image credit: Bill Mitchell (CC-BY).

The streaky, varying lightness of the flow areas, presence of snow and ice, and steep terrain lead me to believe that what is showing up here are mixed snow/rock/lava debris flows, rather than pure lava flows. A mix of rocky debris and snow would not be out of line for a supraglacial eruption on a steep mountain. The longest flow drops nearly 1300 m along its 2000 m horizontal path according to the 2002 Radarsat elevations. I’ll be the first to admit that I am distrustful of the specifics of the Radarsat contours due to the rapidly changing landscape and an intervening 15 years, but I think that it manges to get the general picture right.

Southwestern Heard Island is a high-precipitation area, so rocks exposed on the surface of the glaciers are likely quite fresh. It probably won’t be long before most of the deposits are covered in snow again.

Speaking of snow, it looks as though there is a faint outline of an avalanche scarp/deposit on the northeast side of the summit, which I annotated below in green.

Annotation of avalanche scarp and deposit, Mawson Peak, Heard Island, February 4, 2017.  Image credit: Bill Mitchell (CC-BY)
Annotation of avalanche scarp and deposit, Mawson Peak, Heard Island, February 4, 2017. Image credit: Bill Mitchell (CC-BY)

The two hot spots provide an interesting challenge for interpretation. Two scenarios come to mind quickly: there are two vents from which lava is issuing, or there is a lava tunnel from a summit crater down to a flow front or breakout. Analyzing the Landsat 8 OLI/TIRS infrared imagery from January 26th (most recent previous high-resolution image), only one hot spot is present—in the same place as the eastern hot spot in the February 4th infrared image. For spatial correlation without doing the whole image processing and GIS thing, use the forked flow to the south-southeast of the hotspot as a reference.

False-color infrared image of Mawson Peak, January 26, 2017.  Landsat 8 OLI/TIRS bands 7-6-5.  Image credit: Bill Mitchell (CC-BY) using NASA/USGS data (public domain).
False-color infrared image of Mawson Peak, January 26, 2017. Landsat 8 OLI/TIRS bands 7-6-5. Image credit: Bill Mitchell (CC-BY) using NASA/USGS data (public domain).

Due to a different time of day for imaging, there are significant shadows in the January image on the southwest side of ridges. It’s tricky to figure out what is going on for the flows (even in visible imagery), but the hot spot from January 26th is right on top of the eastern hot spot from February 4th.

Another thing which becomes apparent in the January image is the topography at the summit. The clouds form a blanket at an atmospheric boundary (and roughly-constant elevation), which is conveniently just below the elevation of the summit. A roughly circular hole in the clouds is present, and a conical mountain summit pokes through with the hot spot right in the center. That suggests that the second hot spot seen in the February 4th image is at a lower elevation—a possible flow front or breakout.

Excitement in the Mundane
Finding this eruption was a bit of a surprise to me: the low-resolution preview image for the Landsat data on EarthExplorer was so coarse that there wasn’t anything striking or out of the ordinary visible at the summit. Clouds covered most of the rest of the island. However, when I opened up the full-resolution color images (30 m/pixel), it was immediately evident that this was a special day. Sharpening the true-color bands with the high-resolution panchromatic band using QGIS made it pop all the more!

Upon seeing both the lava/debris flows and the thermal anomaly, I checked the MODIS volcanism (MODVOLC) site to see if the Terra and Aqua MODIS instruments had picked up thermal anomalies as well over the preceding week. They had, as shown below. Both satellites had recorded thermal anomalies at Heard on February 2nd and 3rd.

MODIS thermal events at Heard Island, in the week preceding February 6, 2017.  Image credit: MODVOLC.
MODIS thermal events at Heard Island, in the week preceding February 6, 2017. Image credit: MODVOLC.

Update:: Follow-up from February 27, 2017.

Cloud-Free Heard Island

Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).
Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).

Heard Island is a pretty cloudy place most of the time. However, there are occasional times when the weather clears, particularly on the southeastern (leeward) side of the island. On rare occasions, the northwest and southwest sides of the island come out from the clouds as a satellite passes over.

For the past two years, I have been watching Heard Island using true-color imagery from four satellites: Terra, Aqua, Landsat 8, and EO-1. I have posted previously about satellite imagery from these instruments. Although every image of the Island I have seen has clouds in it covering a portion of the island, I was curious whether or not I had accumulated clear imagery of the entirety of Heard Island.

In part, this question was spurred by a follower on Twitter asking about eruptive activity at Heard. I had to admit I didn’t really know whether the activity was low-level and continuous (like Kilauea) or more intermittent. Given that our knowledge of its eruptive activity is primarily from satellite observations, do the satellite “thermal anomalies” correspond to short eruptive events, or simply a cloud-free view of the volcano?

For high-resolution imagery of Heard Island, the datasets of interest are from EO-1 ALI, and Landsat 8 OLI. The two MODIS instruments (one on Aqua, one on Terra) are moderate-resolution, and while 250-m resolution is sufficient for some purposes, this one needs more. Looking through the archives, I was able to find EO-1 ALI data primarily for Mawson Peak and points southeast, and Landsat 8 OLI covered much of the island, particularly the northwest.

Not only is having cloud-free, high-resolution data important for me, but I want the data to be recent. There has been a retreat of up to 5.5 km for some of the glaciers since 1947, and the Google Maps imagery of that area (Stephenson Lagoon) is horribly outdated. Fortunately, I found most of the island covered in large swaths with images from 2014 onward, and mostly 2016. There was even good imagery from when I was on Heard Island! Our ship, the Braveheart, is visible as a few white pixels in Atlas Roads (just north of Atlas Cove), slightly closer to the Azorella Peninsula than to the Laurens Peninsula. The tents and campsite are too small and darkly colored to be visible on this image.

Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island.  Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).
Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island. Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).

A small portion of the island between Atlas Cove and Mawson Peak was the most difficult to find. With the topography of the island, the steady stream of wind, and the humid air, the 2.5 km by 2.5 km region was cloudy pretty much all the time. Eventually, using the EO-1 ALI instrument and going back to early 2010, I found a reasonably clear image of it.

Once I had the images (after combining true-color and panchromatic brightness data in QGIS), I needed to stitch them together. Thanks to the wonderful QGIS training manual, I was able to create vector (polygon) layers which corresponded to the clear region of each image (plus some surrounding ocean). At this point the troublesome mostly-cloudy spot became evident, and the search was on for imagery to fill the void.

Creating polygons for clipping the satellite imagery using QGIS.  Four polygons are shown here, including the small polygon of much cloudiness.  A fifth dataset was subsequently incorporated.
Creating polygons for clipping the satellite imagery using QGIS. Four polygons are shown here, including the small polygon of much cloudiness. A fifth dataset was subsequently incorporated.

Finally, I tried to put them together. This turned out to be more trouble than it was worth for my purposes, having only five images. Several of the images had differing resolutions (10 m/pixel for EO-1 ALI, 15 m/pixel for Landsat 8 OLI). Additionally, since I was handling these in their raw format, color balances/exposures were not consistent across images. I decided it best, then, to leave them separate, and sent them around to the Heard Island Expedition team.

Soon I had an email from the expedition leader: he was very interested in the imagery, but it wasn’t opening in Google Earth. Some searching later, I found that Google Earth works best with a certain map projection (EPSG:4326), and when exporting the GeoTIFF, I needed to select “rendered image” rather than “raw data”. I re-exported the images, zipped them up, and tested it out on another computer: success! This Google Earth friendly imagery is now available here (17 MB zip).

One continuation of this project would be to keep looking through the documentation on GeoTiffs to find out how to make the rendered images use a transparent, not white, border where there is no data. That would likely let me create a virtual raster catalog to load all of them in one go, rather than having to load them separately.

Global Warming, and Stephenson Glacier Retreat

Annual global surface temperature difference from the 20th century average.  2014 is the 38th straight year above average.  Image credit: NOAA.
Annual global surface temperature difference from the 20th century average. 2014 is the 38th straight year above average. Image credit: @NOAA.

Two things came to my attention today which are of particular interest.

First, NOAA has announced that globally, 2014 was the warmest year on record, and the 38th straight year of above-average temperatures. Continued action will be needed in 2015 to reverse this trend. Every delay makes fixing the situation more difficult.

Second, Mauri Pelto has written today about the retreat of Stephenson Glacier and the formation of a lagoon on Heard Island. In 1947-1948, when members of the Australian National Antarctic Research Expedition (ANARE) spent 15 months at Heard Island, they found Spit Point, on the southeast side of the island, was only accessible after crossing Stephenson Glacier. Imagery from LANDSAT shows substantial retreat, as do photographs from a 2004 expedition to Heard Island.

Landsat 2010 image, annotated by Mauri Pelto.  Arrows mark the toe of the glacier in 2001 (purple), 2010 (red), and 2013 (yellow).  Additional images are available on Mauri Pelto's blog.
Landsat 2010 image, annotated by Mauri Pelto. Arrows mark the toe of the glacier in 2001 (purple), 2010 (red), and 2013 (yellow). Additional images are available on Mauri Pelto’s blog.

Today, where once Stephenson Glacier met the ocean, there is now Stephenson Lagoon. The toe of the glacier has retreated inland, and to my eye appears to have moved about 4 km. With a warming at Atlas Cove of 1 °C over 1947-2001, the retreat is not surprising.