Page 145 - Fundamentals of Geomorphology

128 STRUCTURE

Nile N Nile
0 250 km
Lake Lake
Lake Kyoga Lake
Albert Albert Kyoga

Lake Congo Lake
Edward Edward
Congo
Virunga
Drainage Mountains
from Lake
Tanganyika Lake
Kivu
Lake Rusizi Lake
Victoria River Victoria

Lukuga R.
Lake Lake
Tanganyika Tanganyika
b
a
() ()
Figure 5.10 Drainage diversion by volcanoes in central Africa. (a) The Nile drainage through the Western Rift before the
eruptions that built the Virunga Mountains. (b) The Nile drainage after the formation of the Virunga Mountains.
Source: Adapted from Francis (1993, 366)

2 km in sedimentary rocks and 4 km in crystalline features, or a combination of the two (Figure 5.12).
rocks do not have a simple bowl shape. Rather, they This is a small total compared with the number iden-
are complex structures that, in comparison with simple tified on planets retaining portions of their earliest crust.
structures, are rather shallow (Figure 5.11b). The most However, impact structures are likely to be scarce on
recent examples, such as Clearwater Lakes in Quebec, the Earth owing to the relative youthfulness and the
Canada, typically have three distinct form facets. First dynamic nature of the terrestrial geosphere. Both factors
a structurally uplifted central area, displaying shock- serve to obscure and remove the impact record by ero-
metamorphic effects in the autochthonous target rocks, sion and sedimentation. Craters would have originally
that may be exposed as a central peak or rings; second, marked sites of all impacts. Owing to erosion, older
an annular depression, partially filled by autochthonous sites are now obscure, all that remains being signs of
breccia, or an annular sheet of so-called impact melt shock metamorphism in the rocks. Thus, impacts will
rocks, or a mixture of the two; and, third, a faulted always leave a very long-lasting, though not indelible,
rim area. signature in rocks, but the landforms (craters) they pro-
Impact craters occur on all continents. As of 2 Novem- duce will gradually fade, like the face of the Cheshire
ber 2004, 172 had been identified as impact craters from cat. The current list of known impact structures is cer-
the presence of meteorite fragments, shock metamorphic tainly incomplete, for researchers discover about five