Geologic hazards resulting from sedimentary, oceanographic and tectonic pro
cesses affect more than one third of the offshore Gulf of Cadiz, and are id
entified by interpreting high-resolution seismic profiles and sonographs. H
azards of sedimentary origin include the occurrence of slope instability pr
ocesses in the form of single or multiple slumps occupying up to 147 km(2)
mainly concentrated in the steeper, upper slope area. Besides the presence
of steep slopes, the triggering of submarine landslides is probably due to
seismic activity and favoured by the presence of biogenic gas within the se
diment. Gassy sediments and associated seafloor pockmarks cover more than 2
40 km(2) in the upper slope. Hazards from oceanographic processes result fr
om the complex system of bottom currents created by the interaction of the
strong Mediterranean Undercurrent and the rough seafloor physiography. The
local intensification of bottom currents is responsible for erosive process
es along more than 1900 km(2) in the upper slope and in the canyons eroded
in the central area of the slope, undermining slopes and causing instabilit
y. The strong bottom currents also create a mobile seafloor containing bedf
orms in an area of the Gulf that extends more than 2500 km(2), mostly in th
e continental slope terraces. Hazards of tectonic origin are important beca
use the Gulf of Cadiz straddles two major tectonic regions, the Azores-Gibr
altar fracture zone and the Betic range, which results in diapir uplift ove
r an area of more than 1000 km(2), and in active seismicity with earthquake
s of moderate magnitude. Also, tsunamis produced by strong earthquakes occu
r in the Gulf of Cadiz, and are related to the tectonic activity along the
Azores-Gibraltar fracture zone. (C) 1999 Elsevier Science B.V. All rights r
eserved.