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Earth from Space – Eruption in the Red Sea

Satellite image of a volcanic eruption in the Zubair Group

NASA Earth Observing-1 (EO-1) satellite image of a volcanic eruption in the Zubair Group of islands in the Red Sea.

AN ERUPTION OCCURRED in the Red Sea in December 2011. According to news reports, fishermen witnessed lava fountains reaching up to 30 metres tall on December 19.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua satellites observed plumes on December 20 and December 22. Meanwhile, the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite detected elevated levels of sulphur dioxide, further indicating an eruption.

The activity in the Red Sea included more than an eruption. By December 23, 2011, what looked like a new island had appeared.

The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured these high-resolution, natural-colour images on December 23, 2011 (above), and October 24, 2007 (below).

Satellite image of Zubair Group islands

A satellite image of the same region, taken in 2007, shows no sign of the new volcanic island.

The image from December 2011 shows an apparent island where there had previously been an unbroken water surface. A thick plume rises from the island, dark near the bottom and light near the top, perhaps a mixture of volcanic ash and water vapour.

The volcanic activity occurred along the Zubair Group, a collection of small islands off the west coast of Yemen. Running in a roughly northwest-southeast line, the islands poke above the sea surface, rising from a shield volcano.

This region is part of the Red Sea Rift where the African and Arabian tectonic plates pull apart and new ocean crust regularly forms.

Wider satellite image of a volcanic eruption in the Zubair Group

This wider view shows more of the islands in the Zubair Group.

Close up satellite image of a volcanic eruption in the Zubair Group

And this close up gives a better view of the new island and the huge plume of smoke and steam.

NASA Earth Observatory image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team. Text adapted from information issued by Michon Scott.

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Gravity waves on the water

Atmospheric gravity waves over the Persian Gulf

Rollercoaster airflow produces smooth and rough patches on the waters of the Persian Gulf. The effect is produced by "gravity waves".

When the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface, a phenomenon called sunglint occurs.

In the affected area of the image, smooth ocean water becomes a silvery mirror, while rougher surface waters appear dark.

Sometimes the sunglint region reveals interesting ocean or atmospheric features that the sensor does not usually record.

The image above shows a large, overlapping wave pattern in the sunglint off the Persian Gulf.

The pattern is not from large ocean waves, however. It is the “impression” of atmospheric gravity waves on the surface of the ocean.

As the name implies, atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down, a bit like a rollercoaster. On its descent into the low-point of the wave (the trough), the air touches the surface of the ocean, roughening the water.

The long, vertical dark lines show where the troughs of gravity waves have roughened the surface. The brighter regions show the crests of the atmospheric waves.

Beneath the crests, the water is calm and reflects light directly back towards the sensor.

The image was captured by the MODIS instrument aboard NASA’s Terra satellite on June 14, 2010.

Adapted from information issued by Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC.

Caspian Sea from orbit

The Caspian Sea seen from orbit

The Caspian Sea covers 371,000 square kilometres and borders five countries.

Measured by surface area, the Caspian Sea is the world’s largest inland water body. It covers roughly 371,000 square kilometres (143,200 square miles) and borders five countries. To the ancient Greeks and Persians, the lake’s immense size suggested it was an ocean, hence its name.

A large expanse of clear sky permitted an unobstructed view of the Caspian Sea in early June 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-colour image on June 4, 2010.

The colour of the Caspian Sea darkens from north to south, thanks to changes lake in depth and perhaps sediment and other runoff. The northern part of the lake is just 5 to 6 metres (16 to 20 feet) deep. The southern end, however, plunges more than 1,000 metres (3,300 feet).

See the full-size image here (3MB, new window)

Just as the lake reaches a greater depth in the south, the nearby land reaches a greater height. The mountains of northern Iran line the southern end of the giant lake, and emerald green vegetation clings to those mountain slopes.

In marked contrast to the mountains, sand seas line the southeastern and northern perimeters of the lake, and marshes occur along the lake shores in Azerbaijan to the west.

Multiple rivers empty into the Caspian Sea, the Volga being the largest. Lacking an outlet, the Caspian Sea loses water only by evaporation, leading to the accumulation of salt.

Although a lake, the Caspian is not a freshwater lake; the water delivered by the Volga River minimises the lake’s salt content at the northern end, but the Caspian grows more saline to the south. Kara-Bogaz-Gol is a saline inlet along the lake’s eastern perimeter.

Geological research indicates that the Caspian Sea was once part of a prehistoric sea known as the Paratethys. Tectonic forces uplifting the land and a drop in sea level left the Caspian landlocked more than 5 million years ago. Climate shifts have alternately raised and lowered the lake’s water levels, sometimes nearly drying it out completely.

NASA image by Jeff Schmaltz, MODIS Rapid Response Team. Text adapted from information issued by Michon Scott.