Monitoring the earth’s volcanoes from space poses a variety of unique
problems and opportunities related to spatial and temporal scales, as well as
spectral range (e.g., the necessity for thermal IR observations). Some of these
are well posed with respect to the ASTER instrument and mission, and some
challenge ASTER’s limitations.
Volcanoes represent one of the most active features of landscape generation.
The frequency of discernable volcanic feature generation (as opposed to
indiscernible fault or landslide/glacial creep) is exceeded only by aeolian-generated
landforms and beach landforms (at times constantly changing at an observable
spatial scale). Volcanic eruptions reported during human history range in
frequency from daily (e.g., Mt. Etna, Italy; Mt. Sakurajima, Japan) to once
every several centuries (e.g., Mt. Pinatubo, Philippines; Mt. St. Helens, USA).
Currently there are of order 1000 volcanoes worldwide that are considered
active, in the sense that they could enter a restless state or erupt
more-or-less at any time (Simkin and Siebert, 1994).
The earth’s volcanic activity manifests itself at fairly well-defined spatial
scales. Lava flows typically exhibit characteristic length scales of 1-10km,
with exceptional flows reaching length scales of the order of 100km (e.g.,
ancient high temperature komatiite flows). Central vent volcanic features on the
earth have characteristic base diameters of the order of 10-100km, with basaltic
shields tending toward the high end and more compact stratovolcanoes tending
toward the lower end of the range. Thus, ASTER’s 60x60km footprint and
15-30-90m/pixel (VNIR-SWIR-TIR) resolution scale allow most central vent
volcanoes to be captured in just a few frames centered on the feature, and in
many cases with just one image. At 15m/pixel (VNIR) and 30m/pixel (SWIR), ASTER
images reveal the spatial distribution of lava flow and summit crater hotspots (Figures
1 and 2). Landsat ETM+ can provide comparable imaging and hotspot
assessments; however, its dynamic range in the SWIR region is less than ASTER.
Figure 1. A false color composite of the three ASTER VNIR channels
showing the 29 July 2001 summit eruption of Mt. Etna. Spatial resolution is
15 m/pixel. Areas appearing red are vegetated. Strong water vapor, ash, and
SO2 emissions are evident.
Figure 2. A shortwave / longwave false color composite of the same
scene as Figure 1. ASTER VNIR (Band 3), SWIR (Band 4), and TIR (Band 10) are
reconstructed here in the blue, green, and red color planes. The active lava
flows appear red and yellow, and are visible through much of the
In addition, ASTER VNIR Bands 3N (nadir-looking) and 3B (backward-looking)
provide parallax information that permits the construction of digital elevation
models (DEMs) using classic stereo photogrammetry techniques. Such products are
available on-demand from the USGS Eros Data Center in Sioux Falls, South Dakota.
Such data are useful in assessing post-eruption flow volumes, and in helping
with pre-eruption assessments of lava flow routing paths (Figure 3).
Figure 3. A three dimensional perspective view
created from an ASTER digital elevation model with a simulated natural color
ASTER image. El Misti volcano towers above the city of Arequipa, Peru, with
a population of more than one million. Geologic studies indicate that a
major eruption occurred in the 15th century. Despite the obvious hazard,
civil defense authorities see it as a remote danger, and development
continues on the volcano side of the city.
Finally, the detection and multispectral imaging of low altitude (<10km ASL)
proximal volcanic plumes with weather satellite instruments (e.g., AVHRR, GOES)
at nadir resolutions ranging from 1-4km/pixel is somewhat problematic (Hufford
et al., 2000). This is because the plumes themselves are often small with
cross-wind dimensions of order 100m and down-wind extensions of 1-10km.
Nevertheless, the detection and imaging of such plumes is important not only in
the context of basic science, but also in the arena of aviation safety.
Understanding the magnitude and distribution of ash and sulfur dioxide in such
emissions can only be accomplished utilizing TIR-based multiband techniques at
relatively high spatial resolution. In this regard, ASTER’s capabilities are
ASTER, given its 14 bands between 0.5µm and 12.6µm, can directly address a
number of volcanological issues that require a multispectral approach. Much of
what ASTER can do in this area can also be done with Landsat ETM+ or with
experimental instruments on board the EO-1 spacecraft, though at somewhat
coarser spatial resolution (e.g, identification of hydrothermal alteration and
mapping of spectrally contrasting weathered units with VNIR, mapping of thermal
radiance from active lava flows with SWIR, monitoring summit crater activity
with SWIR). ASTER, however, has the unique capability of acquiring multispectral
TIR data (5 channels) at less than 100m/pixel. This scale potentially allows the
deconvolution of eruption precursor hot spot data with relatively low thermal
contrast (theoretical limit given by NEΔT~0.3K; practical ΔT limit~5K) with
relatively modest areas (≈7x104m2), as well as geologic mapping utilizing
restrahlen signature principal-component contrasts, as was pioneered with the
TIMS airborne instrument (Kahle et al., 1987).
As an example of low temperature thermal capability, warm spots associated
with the currently erupting Chikurachki Volcano in the Kurile Islands were
detected by ASTER as early as January 2003 before the April 2003 eruption (Pieri
and Abrams, 2003). Enhanced heat flow, probably related to the subsequent
explosive eruption, was detected in about a half dozen summit crater pixels,
generating a consistent temperature contrast of between 5K and 10K above
ambient, with temperatures hovering around the melting point of water. Between
January and February 2003, the average temperature of these warm spots increased
a statistically significant 1-2K. Though this analysis was carried out
retrospectively, it shows the potential of ASTER as a very sensitive
geo-thermometer for the detection of thermal precursors of volcanic eruptions.
SO2 is a fellow traveler with ash in explosive eruptions, and is often a key
diagnostic that such an eruption has occurred. The TIR bands 10, 11, and 12 on
ASTER are sensitive to an SO2 absorption band between 8 and 9 µm (Realmuto et
al., 1994). Mt. Etna in Sicily is one of the world’s largest natural sources of
SO2, erupting between 2500-5000MT/day during non-eruptive periods and
10,000-25,000 MT/day during paroxysmal eruptions. The NASA TOMS instrument,
sensitive to UV absorption of SO2, has difficulty detecting the relatively low
altitude (~3000-4000mASL) SO2 plume from Etna because of the general absorption
of UV energy in the troposphere and TOMS band selection. ASTER, however, with
higher spatial resolution and working on the TIR SO2 absorption feature at
8.5µm, has little difficulty in picking up the Etna SO2 plume (Figure 4).
Figure 4. One of the largest recent eruptions of Mt. Etna started
on July 17, 2001 and continued until late August of that year. This ASTER
image was acquired on Sunday, July 29, 2001 and shows the sulfur dioxide
plume (in purple) originating form the summit, drifting over the city of
Catania, and continuing over the Ionian Sea. The SO2 plume is distinguished
by its absorption in ASTER TIR bands 10, 11, 12.
Even more problematic in terms of detection is the nearly constant very low
altitude (<500-1000mASL) plume that has been emanating from the Pu’u O’o vent at
Kilauea Volcano in Hawaii since 1983. While the emission rate is lower than
Etna’s, the humid tropical air and SO2 conspire to produce coastal “vog,” or
volcanic smog. Again, more weather-directed instruments like TOMS cannot see
deeply enough into the troposphere to detect such a small plume. However, ASTER
can clearly delineate the SO2 emission (Figure 5).
Figure 5. Kilauea Volcano, on the Island of Hawaii, has been in a
constant state of eruption since January 3, 1983. The Pu'u O'o vent, formed
soon after the onset of this eruption, is a persistent source of sulfur
dioxide (SO2) gas emissions, as shown in this map of the Pu'u O'o SO2 plume
derived from ASTER data. The map, produced from ASTER's thermal infrared
channels, depicts color-coded concentrations superposed onto a false-color
composite of ASTER's visible and near infrared channels. High SO2
concentrations (>4 gm/m2) are colored white, lower concentrations are red,
orange, yellow, green and blue (<0.5 gm/ m2). ASTER is the only instrument
in orbit that can detect passive venting of SO2 in plumes as small as the
Pu'u O'o plume (typically less than 1.5 km wide over land). The image was
acquired on October 30, 2001 and covers an area of 42 x 44 km. (Courtesy of
Vince Realmuto, JPL).
ASTER, with its pointable platform in low-earth orbit, occupies a kind of
temporal niche between the high spatial resolution, multispectral nadir-looking
infrequent Landsat ETM+-style observations, and the more frequent—but lower
spatial resolution—weather satellite data (e.g., AVHRR and GOES). ASTER’s VNIR
and SWIR scan platforms are capable of nominal off-nadir pointing up to 8.55º, and
occasional off-nadir pointing up to 24º. As mentioned above, this results in a
revisit interval of as short as 5 days at the equator, and much shorter revisit
intervals at higher latitudes. Repeated acquisitions of ASTER data, then, could
result in useful time-series observations of eruptions, if the eruptions have
characteristic timescales of order 10 days or more.
A graphic example of monitoring of summit craters utilizing ASTER data is
shown in Figure 6, a view of the Popocatepetl Volcano, Mexico summit crater
during 2000 and 2001. Here, the summit crater exhibits thermally active pixels
in both SWIR and TIR ASTER channels from September 2000 through the beginning of
the following year.
Figure 6. ASTER view of the summit of Popocateptl Volcano, Mexico.
Summit crater radiances in both the SWIR and TIR channels are shown on the
right. Red pixels are hot (>100ºC). Surrounding dark pixels are at
An example of a longer interval time-series was the ASTER discovery that
Chiliques Volcano in Chile was active. On January 6, 2002 an ASTER nighttime
thermal infrared image of the Chiliques volcano showed a hot spot in the summit
crater and several others along the upper flanks of the edifice, indicating new
volcanic activity (Figure 7). Examination of an earlier nighttime thermal
infrared image from May 24, 2000 showed no thermal anomaly.
Figure 7. Chiliques Volcano, Chile. ASTER nighttime thermal data
discovered a thermal anomaly in January 2002, continuing in April 2002 (two
left hand images). A visible image (two right hand images—lower one is a
blow-up) from 14 March 2002 reveals two crater lakes at summit (dark areas
against white snow), that have become hot. Chiliques has shown no historic
activity, but is re-awakening.
Chiliques volcano was previously thought to be dormant. Rising to an
elevation of 5778 m, Chiliques is a simple stratovolcano with a 500-m-diameter
circular summit crater. Officials at the Chilean Geologic Survey reported that
the summit hot spot indicated that the crater lake was heating up. During an
aircraft overflight fumaroles were observed on the volcano’s flank. Such ASTER
data point up the utility of time-series observations at high spatial resolution
and the utility of simultaneous infrared observations.
The ASTER mission represents a fundamentally new type of observational
capability with respect to studying the earth’s volcanoes. The new features of
- (a) an unprecedented number of thermal infrared channels (five);
- (b) the ability to point up to 24º off-nadir, resulting in a
five day revisit interval;
- (c) the ability to carry out simultaneous along track stereo
- (d) high spatial resolution: 15 m/pixel in the VNIR instrument,
30m/pixel in the SWIR instrument, and 90m/pixel with the TIR instrument.
Hufford, G., J.J. Simpson, L. Salinas, E. Barske, and D.C. Pieri, 2000,
Operational considerations of volcanic ash for airlines, Bulletin of the
American Meteorological. Society, 8 (4), 745-755.
Kahle A.B., 1987, Surface Emittance, Temperature, and Thermal Inertia Derived
from Thermal Infrared Multispectral Scanner (TIMS) Data For Death-Valley,
California, Geophysics 52 (7): 858-874.
Pieri D. and M. Abrams, 2003, ASTER pre-eruption thermal analysis of
Chikurachki Volcano, in preparation for Geophysical Research Letters.
Realmuto VJ, Abrams MJ, Buongiorno MF, Pieri DC, 1994, The Use of
Multispectral Thermal Infrared Image Data To Estimate the Sulfur-Dioxide Flux
from Volcanoes - A Case-Study from Mount Etna, Sicily, July 29, 1986, Journal
of Geophysical Research-Solid Earth 99 (B1): 481-488.
Simkin, T, and L Siebert, 1994, Volcanoes of the World (2nd Edition),
Geoscience Press, Inc., Tucson, and the Smithsonian Institution, Washington,