Page 143 - Fundamentals of Geomorphology

126 STRUCTURE

Caldera
Volcano (Wizard Island)

Figure 5.8 Crater Lake caldera, Oregon, USA.
Source: Adapted from MacDonald (1972, 301)

calderas between 22 million and 30 million years old. down established valleys. Erosion then reduces the adja-
2
Ignimbrites from these calderas cover 25,000 km . cent hillside leaving the more resistant volcanic rock as a
ridge between two valleys. Such inverted relief is remark-
ably common (Pain and Ollier 1995). On Eigg, a small
Indirect effects of volcanoes Hebridean island in Scotland, a Tertiary rhyolite lava
Volcanoes have several indirect impacts on landforms. flow originally filled a river valley eroded into older basalt
Two important effects are drainage modification and lavas. The rhyolite is now preserved on the Scuir of Eigg,
relief inversion. an imposing 400-m-high and 5-km-long ridge standing
Radial drainage patterns often develop on volca- well above the existing valleys.
noes, and the pattern may last well after the volcano
has been eroded. In addition, volcanoes bury pre-
existing landscapes under lava and, in doing so, may IMPACT CRATERS
radically alter the drainage patterns. A good example
is the diversion of the drainage in the central African The remains of craters formed by the impact of asteroids,
rift valley (Figure 5.10). Five million years ago, volca- meteoroids, and comets scar the Earth’s surface. Over
noes associated with the construction of the Virunga 170 craters and geological structures discovered so far
Mountains impounded Lake Kivu. Formerly, drainage showstrongsignsofanimpactorigin (seeHuggett2006).
was northward to join the Nile by way of Lake Albert Admittedly, impact craters are relatively rare landforms,
(Figure 5.10a). When stopped from flowing north- but they are of interest.
wards by the Virunga Mountains, the waters eventually In terms of morphology, terrestrial impact structures
overflowed Lake Kivu and spilled southwards at the are either simple or complex (Figure 5.11). Simple
southern end of the rift through the Ruzizi River into structures, such as Brent crater in Ontario, Canada, are
LakeTanganyika (Figure 5.10b). From LakeTanganyika, bowl-shaped (Figure 5.11a).The rim area is uplifted and,
the waters reached the River Congo through the River in the most recent cases, is surmounted by an overturned
Lukuga, and so were diverted from the Mediterranean flap of near-surface target rocks with inverted stratigra-
via the Nile to the Atlantic via the Congo (King 1942, phy. Fallout ejecta commonly lie on the overturned flap.
153–4). Autochthonous target rock that is fractured and brec-
Occasionally,lavaflowssetintrainasequenceofevents ciated marks the base of a simple crater. A lens of shocked
that ultimately inverts the relief – valleys become hills and unshocked allochthonous target rock partially fills
and hills become valleys (cf. p. 156). Lava tends to flow the true crater. Craters with diameters larger than about