Eventually, the volcano moves so far off the hotspot that magma
is unable to be supplied; erosion takes over as the dominant geological
process. This erosion is both gradual and catastrophic. Hawaiian
lavas are very permeable. Many are vesicular, and lava tubes,
clinker layers, and flow boundaries all provide easy pathways
for percolating water. For this reason, even in many of the wettest
areas of Mauna Loa and Kilauea, erosion is minimal. During the
post-shield alkalic stage, however, the greater explosivity of
the eruptions deposits many ash and cinder layers. These pyroclastic
layers are much less permeable, and they allow
streams to form
readily.
Another consequence of the lower eruption rate is that the coastal
plain that formed from lava deltas during the tholeiite stage
is not re-surfaced fast enough to avoid being submerged below
sea level (the volcano continues to sink even though the eruption
rate has decreased). The submerged (once coastal)
plains can be
identified offshore by bathymetric surveys, and if submerged coral
reefs on these can be dated it is possible to determine roughly
how long ago that particular volcano finished its tholeiite shield
stage (Moore 1987).
For example, corals on the shelf off Mauna Kea are about 500,000
years old. The shelf is at a depth of about 1000 meters, yielding
a subsidence rate of 2 mm/year.
There is another form of volcano degradation that has only recently
been recognized as a significant process on Hawaiian volcanoes.
Bathymetric surveys in the early 1960's showed two
tongues of
rough under-sea terrain extending ~180 km offshore from the eastern
parts of O'ahu and Moloka'i
(Moore 1964).
Considerable debate raged over whether or not these were landslides.
Using GLORIA sidescan sonar, 17 of these
giant deposits have since
been identified off the 8 main islands, and the general opinion
is that they are indeed landslides. Many of them flowed out and
sloshed up the island-facing slope of the Hawaiian trough, and
must therefore have been moving quickly. Of course, if one of
these events were to take place today the results would be devastating.
A deposit of beach cobbles has been identified on
Lana'i extending
up to an elevation of ~100 m, and it has been attributed to the
tsunami generated by the most recent of these giant landslides.
(Moore & Moore 1988).
On land, the headwalls of these giant landslides are indicated
by steep ocean-facing scarps and slopes. The north coasts of
E
Moloka'i, and Kohala are prime examples. The western slope of
Mauna Loa is very steep, and has been the source of many giant
landslides
(Normark et al. 1987;
Lipman et al. 1988),
however, because Mauna Loa is still active, any scarps that may
have formed have been mostly mantled by lava flows. The
Hilina
fault system of appears to be a different type of mass-wasting
structure. Here, large fault blocks tend to move in small increments
rather than in huge catastrophic slides. During the large M7.2
1975 Kalapana earthquake, these blocks subsided up to 8 meters,
and a small tsunami was generated
(Tilling et al. 1976;
Lipman et al. 1985).
The Hilina fault scarps are continually resurfaced by lava flows
which spread out when they reach the coastal plain.
The giant landslides were at first thought to be problematic because
unlike steep strato volcanoes (where landsliding is expected),
Hawaiian shields have very gradual slopes and very little ash.
When further consideration is made of the structure of the Hawaiian
shields, however, the mechanism of catastrophic failure becomes
evident. When lava flows into, or is erupted in, shallow seawater,
explosions occur. This happens as the volcano first grows through
sea level and also when an already-subaerial volcano sends lava
flows to the coast. These explosions fragment the lava into sand-sized
particles consisting mostly of glass. Additionally, flows break
up when tumbling down offshore slopes or being beaten by ocean
waves. Lava flows extend the island offshore on top of all this
loose material. The result of these processes is that much of
the submarine component of all the Hawaiian volcanoes consists
of very weak and unconsolidated easily-weathered
material.
Lava flows on land are mechanically strong but because they are underlain
by these deposits of junk, the volcanoes as a whole are weak.
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