VI. Coastal Erosion
Because central California is a tectonically active area with extensive
coastal uplift, coastal erosion rates are high. Erosion rates vary along
the MBNMS coast, depending on the coastal morphology, lithologies and degree
of rock fragmentation (i.e. faulting and jointing). Along the northern and
north central segments, erosion rates are moderate to high (long term rates
of about 30 cm/yr) due to the exposure of fairly erodable Tertiary sedimentary
rocks. However, erosion near Capitola, where the Purisima Formation is located
in the surf zone, has been recorded as high as 1.5 m/yr (Plant and Griggs 1991).
Along the south central and southern MBNMS segments, coastal erosion is
slower due to the resistant nature of coastal rocks. Exposed granitic basement
and well lithofied Cretaceous sedimentary rocks withstand wave attack better
than the more northerly coastal cliffs. However, erosion rates are higher
in locations where Franciscan rocks are exposed and granitic rocks are highly
fractured from faulting. Point Sur stands out as a resistant gabbro body
within the more erodable Franciscan mass. Unfortunately, no good erosion
rates exist for this part of the MBNMS.
Although coastal erosion is a continuous process, the rate of erosion is
accelerated during times of severe storm activity (Griggs and Johnson 1979,
1983). In addition, human-made structures placed in the coastal zone have
increased erosion locally (Plant and Griggs 1991; also see Coastal Dunes section).
Much of the eroded materials are transported along the coast to canyon heads
where they are lost to the sedimentary system. Material transport can be
seen as coarse sand bands on the continental shelf (Arnal et al. 1973, Chin
1984, Chin et al. 1988, Hunter et al. 1988, Yancy 1968). Much of this transported
material travels down canyons as turbidity currents, especially during earthquakes
(Garfield et al. 1994). These sediments are deposited as overbank features
and sand waves at the base of the continental slope (Normark et al. 1980).
Numerous major rivers and streams also carry considerable eroded products
to the sea; these are eventually deposited on the MBNMS seafloor. In addition
to natural eroded detritus, anthropogenic material and waste products are
transported through these drainages, and often end up concentrating along
the MBNMS coast and the nearshore seafloor. Some may be carried down canyons
via mass wasting events, and may ultimately rest on the abyssal plain.
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