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The Eruption
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This map shows in more detail the approximate area covered
by the SAR images on this page. In the Southeast corner is Mount
Svianukar, at 1720 meters (5659 feet) one of the highest points in the
glaciated area, while stretching away to the Northwest lies the GrĖmsvtn
caldera |
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This SAR image was acquired from the European Space
Agency's ERS2 satellite on September 1, 1996, a month before the event.
It shows the normal scene on the glacier. The valley or crevasse that lies
between Mounts Svianukar and GrĖmsvtn is clearly visible in the Southeast
corner, while the ridge of Mount GrĖmsvtn is just visible at the surface
of the ice to the Northwest. Apart from these features and two other ice
features in the west of the image there is no sign of anything but the
smooth, wet surface of the snow-covered ice cap. |
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Late on the evening of September 30, 1996 an eruption
started beneath the glacier. Over the previous 24 hours a sequence of
earthquakes had been recorded around the Bardarbunga caldera. Similar
earthquakes have occurred beneath the volcano many times during the last
22 years, but none of the previous large earthquakes had significant
aftershocks, or were followed by magmatic activity such as this last
earthquake.
Numerous earthquakes, including 5 with magnitude over 3,
were recorded in two hours. Shortly after 1300 hours Science Institute
seismologists informed the Civil Defence authorities as well as the
scientific community about this unusual seismic activity and the
possibility of impending eruptive activity. |
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The eruption site was discovered early Tuesday morning
(Oct. 1) from an aircraft. By that time two elongated, 1-2 km wide
subsidence cauldrons had formed on the ice surface of Bardarbunga, on the
northern flank of the neighbouring GrĖmsvtn volcano. The cauldron
formation indicated that the glacier was being melted by an eruption on a
4 km long fissure beneath the glacier, which is 400-600 m thick here. The
meltwater drained into the GrĖmsvtn caldera under the ice shelf of the
lake. In less than 24 hours a third of a cubic km of water had been added
to the lake. |
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By October 2 one of the active craters had melted its way
through the glacier and a massive steam column rose from the cauldron up
to an elevation of 10,000 meters. |
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This image, acquired by Kiruna station just 4 days after
the eruption began, shows how the heat has broken through the surface of
the ice. An irregular white line represents the steep slopes of a canyon
formed by ice melting. At the top of this line the black streak towards
the north shows meltwater on the top of the ice. |
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By October 9 the eruption was taking place on a 9 km long
fissure and volcanic products piled up above the fissure forming a
mountain ridge which in places approached 200 m high. About half of the
area of Vatnajokull was covered by a thin layer of ash. |
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Interferometry results
The ERS radar satellites have also another capability: through a special
technique, called interferometry, it is possible to detect land movement and
build 3-dimensional images using a couple of images. Thanks to the availability
of 2 satellites, ERS-1 and ERS-2, with only one day difference (the
interferometric results shown here have been obtained with the tandem pair
acquired the days 21th and 22nd of October), the event can be shown clearly
and measured with great precision. For interferometry the data used have also
the phases values, that in the standard product (PRI) is omitted.
Since the data was acquired only one day apart, a reasonably good coherence
over the whole scene is present, with the exception of a pear-shaded area around
the fissure. This area was still subject to strong vertical and horizontal
movements between the acquisitions, which make the data acquired in successive
days uncorrelated for the retrieval of any reliable phase information.
The images below show an area of approximately 36 km per 44 km around the
eruption site.
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Intensity
It is obtained by summing the intensity values of the two images. Due
to the very short timeframe between the two acquisitions, the result is
quite similar to the intensity of one acquisition only. |
Coherence
It is obtained comparing the two images in intensity and in phase. In
this way it is possible to detect very small changes. The dark areas
indicate where the changes occurred (low coherence). |
Phases
This image shows the phase difference between the two images. Each gray
cycle (from black to white, with values form 0 to 360 degrees) represents
a height difference. Due to this phase repetitivity, several cycles are
visible. |
Unwrapped phases
This image is obtained from the previous one by adding 360 degrees
several times, in order to unwrap the phases. Dark blue indicates the
lower part of the image, while red the higher ones. The phases values are
converted in altitude values. |
The area around the fissure can be easily detected as the low coherence
pear-shaded region, at the bottom-center part of the coherence image. This lack
of coherence is translated in a completely noisy area in the interferometric
phase image. The quality of the applied unwrapping is appreciated on the
unwrapped phase map, since errors due to lack of coherence are not propagated
outside the low coherence region.
The image below is obtained from the unwrapped one on a larger area with
respect to the above images.
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