This
false colour image of the western Grand Canyon, Arizona, shows in red the sparse
vegetative cover of sage, mesquite, and grasses on the limestone plateau. The
sediment-laden river shown as blue-green can be seen dispersing into the clearer
waters of Lake Mead in the west of the image. The sedimentary rocks of this
area are still
generally lying in the horizontal positions in which they were deposited between
250 million and 1.25 billion years ago. Some 60-70 million years ago the Rocky
Mountains
began to form and the ancestral Colorado River was created. Then, around 17
million years ago the Colorado Plateau (in eastern Arizona and Utah) was uplifted
by about 5 km,
causing the river to cut deeper and the erosion of the rocks younger than 250
million years old. However, it was not until around 5 million years ago, when
it was probably
captured by another river, that the Colorado River ran through the western end
of the current Grand Canyon, shown here, and south to the Gulf of California.
Side canyons
can be seen crossing the main canyon in straight lines showing where erosion
along
weakened fault lines has occurred.
Ganges
Delta, India/Bangladesh (left)
At over 300 km in east-west extent and situated
at the very north of the Bay of Bengal, the 'Mouths of the Ganges' form the
world's largest delta. This low lying
region is extremely vulnerable to widespread damage and loss of life from frequent
cyclones and tidal surges. The Ganges and the Brahmaputra rivers combine in
the
centre of Bangladesh carrying the heavily sediment laden waters from the Himalayas
which have deposited to form the delta. The main outflow has progressively moved
eastward, leaving in the centre of the image, the 'abandoned delta', now only
fed by a few rivers, some of which branch off from the Ganges further up-stream.
The
current 'active' delta can be seen in the eastern half of this image. Satellite
imagery such as this provides a useful means to monitor river courses, coastlines
and mud-banks which can move hundreds of metres each year. On the western side
of the image is the mouth of the Hugli, with the elongated city of Calcutta
showing
as dark grey just to the north. The large red area, covering much of the central
delta and dissected by the India-Bangladesh border, are mangrove forests and
swamps
which line the many quasi-distributaries that are much influenced by tidal currents.
To the north of the current mangrove swamps the natural forest has been cleared
where clay soils are capable of supporting agriculture.
Rub' al Khali ('Empty Quarter'), Saudi Arabia (right)
The
Rub' al Khali or 'Empty Quarter' in the Arabian Peninsular is the world's largest
sand sea (erg) covering almost 600,000 km2. This image of part of its
south-eastern side, near the Saudi Arabia - Oman border, shows many different
types of sand dune. In the south-east are Star Dunes, which in some parts of
the world can be several hundred metres high. In the west are Complex Linear
Dunes (Seif Dunes) which run parallel to the prevailing wind. In the north-east
are Complex Cresentic Dunes (Barchans) which steadily move forward and form when
the wind direction is constant. This false colour image enhances the subtle
differences in colour between the dune sand and the inter-dune areas, which have
a higher clay composition. With much of the world's oil and gas reserves located
within such inhospitable areas, satellite imagery together with Global Positioning
Systems have provided the means to map and explore with increased safety.
Mount St Helens, Washington, USA (left and right)
Situated
to the north of the Columbia River in southern Washington State, USA, Mount
St. Helens exploded on 18th May 1980 killing 60 people. Within minutes
the volcanic blast had devastated around 400km2 of forest. The top of the
volcano slid away reducing the mountain peak by 400m to its current height
of 2550m.
Over the hours following the initial blast, volcanic ash rose some 25km into
the atmosphere. The images above show Mount St. Helens before the eruption
in
1973 and 8 years after the eruption in 1988. In the 1988 image, the southern
side of the characteristic volcanic cone can be seen 'intact', whilst apparent
to the north is the collapse which resulted in the devastating 'liquid' flow
of mud and rock. Looking north-east from over the Pacific Ocean, the Space
Shuttle
picture below shows the plume of ash rising from the volcano
Mount St Helens, Washington, USA (left and right)
