July, 2007
October, 2007 (1) 
Figure 1: Okmok Caldera on Umnak Island. Click to see the larger image (335kB).
Satellite: ALOS
Sensor: AVNIR-2
Observation date: March 27, 2007
Color: 0.61-0.69ƒÊm as Red, 0.52-0.60ƒÊm as Green, 0.42-0.50ƒÊm as Blue. Near-infrared 0.76-0.89ƒÊm band is also used as brightness to display the color of sea area.
The left picture (Figure 1) shows Okmok Volcano on the northern part of Umnak Island, Alaska (larger image). The picture was taken by the Advanced Visible and Near Infrared Radiometer-2 (AVNIR-2) on the Advanced Land Observing Satellite (ALOS) "Daichi" on March 27, 2007. The whole island is covered with white snow. White puffs of clouds are above the dark green sea area.
Figure 2: Map of Umnak Island.
Umnak Island (Figure 2) is one of the Aleutian Islands stretching for 2000km between southwest Alaska and Kamchatskaya. The area of the Umnak, which is the third largest island in the Aleutians, is about 1777km2. There are many active volcanoes in the Aleutians because the North American plate is subducting northward along the south side of the islands.
Okmok Volcano was formed by two large-scale eruptions 12000 and 2050 years ago. The volcano is classified as a caldera: a volcanic feature like "cauldron", formed by the collapse of land following a volcanic eruption. Okmok Volcano is very active even now. The latest eruption occured in 1997 inside the caldera. Also, snow melting by geothermal heat was observed around the crater in 2005.
The volcano has unique geological features, therefore many researchers are investigating it. First, the crustal movement is detected in the center of the caldera although its crater is located at the edge of the caldera. Second, the volcano is a shield volcano while most of the Aleutian volcanoes are composite volcanoes. In addition, the gentle slope and sparse vegetation of the mountain are favorable characteristics for SAR (Synthetic Aperture Radar). Observations by satellite SARs have been carried out many times.
Figure 3: A birds-eye view of Umnak Island.
Figure 3 is a birds-eye view of Umnak Island, which is created from the image and digital elevation data (GTOPO30). The elevation is amplified by doubling actual numbers.
The northern part of the island is occupied by the 1000-meter-high caldera. The southern part is composed by two mountains and flats lower than 100m. The population of the island is 20-30, and is concentrated on a settlement in the southern part.
Reference:
Miyagi, Y. et al., Surface deformation caused by shallow magmatic activity at Okmok volcano, Alaska, detected by GPS campaigns 2000-2002, Earth Planets Space, 56, e29-e32, 2004.
http://www.terrapub.co.jp/journals/
EPS/pdf/2004e/5610e029.pdf
