Geoökologisches Kolloquium WS 2003/04

Dr. Pavel Havlicek
Czech Geological Survey, Prague
Thursday, 18.12.2003 16:15, H6

Quaternary, Palaeopedology and Natural Hazards of Nicaragua and El Salvador

The non volcanic Quaternary in Central America is represented above all by fluvial, deluviofluvial and slope (colluvial) sediments. The fluvial deposits form three low terraces with surfaces at 4, 1.5 and 0.75 m above the valley floor. They are composed mostly of re-deposited pyroclastic materials. The 4 m terrace is double, being made of two gravel bodies separated by a brown soil which evidences a break in deposition. The deluviofluvial sediments have usually a form of small flat out-wash fans developed either in areas with higher relief or at the mouths of tributaries when entering flat floor of a larger valley.
The existence of two generations of slope (colluvial) sediments was documented. The older, compacted one is composed of re-deposited strongly weathered and fragmented solid rocks (mostly andesites), slags and pyroclastics. In the younger generation loose redeposited pyroclastics prevail.
The paleosols are mostly of braunlehm or rotlehm type. They evidence relatively long breaks and stillstands in volcanic activities which duration can be assessed in the rate of milleniums. Formation of initial paleosols needed considerably shorter periods of time, probably decades only.
Natural hazards are both of endogenic and exogenic origin. The endogenic hazards include above all volcanic activity and tectonic unrest resulting usually in earthquakes and volcanic eruptions. On the other hand strong weathering, erosion, mass wasting, gravity movements and flash floods with debris or mud flows represent the exogenic hazards.

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