Figure 1
Australia is the only continent without any current volcanic activity,
but it hosts one of the world's largest extinct volcanoes, the Tweed
Volcano. Rock dating methods indicate that eruptions here lasted about
three million years, ending about 20 million years ago. Twenty million
years of erosion has left this landform deeply eroded yet very
recognizable, appearing as a caldera with a central peak. The central
peak is not an old remnant landform but is instead the erosional stub of
the volcanic neck (the central pipe that carried the magma upward). It is
surrounded by ring dikes, which are circular sheets of magma that
solidified and now form erosion-resistant ridges. The central peak is
named Mount Warning.
Topography plays a central role in envisioning the volcano at its climax
and in deciphering the landscape evolution that has occurred since then.
Low-relief uplands interspersed between deeply eroded canyons form a
radial pattern that clearly defines the shape and extent of the original
volcanic dome. Erosion is most extensive on the eastern side because the
eroding streams drained directly to the ocean and therefore had the
steepest gradients. This asymmetry of erosion has been extreme enough
that the volcano has been hollowed out by the east-flowing drainage,
forming an "erosional caldera." Calderas usually form as the result of
collapse where magmas retreat within an active volcano. If collapse
occurred here, erosion may have removed the evidence, but it produced a
similar landform itself.
Three visualization methods were combined to produce this image: shading,
color coding, and synthetic stereoscopy. The shade image was derived by
computing topographic slope in the north-south direction. Northern slopes
appear bright and southern slopes appear dark. Color coding is directly
related to topographic height, with green at the lower elevations, rising
through yellow and tan, to white at the highest elevations. The
stereoscopic effect was then created by generating two differing
perspectives, one for each eye (see Figure 1). The image can be seen in
3-D by viewing the left image with the right eye and the right image with
the left eye (cross-eyed viewing) or by downloading, printing, and
splitting the image pair and viewing them with a stereoscope. When
stereoscopically merged, the result is a vertically exaggerated view
of Earth's surface in its full three dimensions. Elevations range from
sea level (shown in blue) to about 1340 meters (4400 feet) along the
northwest caldera rim.
Elevation data used in this image were acquired by the Shuttle Radar
Topography Mission aboard the Space Shuttle Endeavour, launched on Feb.
11, 2000. SRTM used the same radar instrument that comprised the
Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR)
that flew twice on the Space Shuttle Endeavour in 1994. SRTM was designed
to collect 3-D measurements of the Earth's surface. To collect the 3-D
data, engineers added a 60-meter (approximately 200-foot) mast, installed
additional C-band and X-band antennas, and improved tracking and
navigation devices. The mission is a cooperative project between NASA,
the National Geospatial-Intelligence Agency (NGA) of the U.S. Department
of Defense and the German and Italian space agencies. It is managed by
NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Earth
Science Enterprise, Washington, D.C.
Size: 102 kilometers (63 miles) by 74 kilometers (46 miles)
Location: 28.4 degrees South latitude, 153.3 degrees East longitude
Orientation: North toward the top, cylindrical projection
Image Data: Shaded and colored SRTM elevation model
Date Acquired: February 2000