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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.
For instance, the image to the left is Kohala volcano viewed from the north. Note that the original volcano surface (outlined by the dashed white line) is well-preserved in the highlands. Stream capture has allowed Waipi'o and Waimanu valleys to become huge at the expense of their neighbors, and Waipi'o has captured the headwaters of the left-hand branch of Waimanu. The green arrow indicates a "wind gap" between the two large valleys. The dotted yellow line roughly corresponds to the extent of (younger) Mauna Kea Lavas, and the town of Waimea is in the middle distance. Kohala last erupted ~60,000 years ago.
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.
Bathymetry map (right) showing a set of slope breaks (gradual to steep heading offshore), corresponding to the now-submerged tholeiite-stage coastal shelves of Mauna Kea (magenta arrow), Kohala (yellow arrows), E Maui, W Maui, Kaho'olawe, Lana'i (cyan arrows), E Moloka'i, and W Moloka'i (green arrows). The diagram is adapted from Moore (1987), and the contour interval is 100 fathoms (= 600 feet or 183 meters).
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).
To the left is a map of the first two mapped giant landslides (north of O'ahu and Moloka'i). These landslides are the windward halves of the Ko'olau and E Moloka'i volcanoes. Note that they traveled across the Hawaiian Deep and climbed up the inner face of the Hawaiian Arch. Counting contours indicates that the slide from O'ahu climbed 2400 feet (~ 750 meters), a good indication that it was moving quickly rather than slowly creeping along (adapted from Macdonald et al. 1983).
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 the right is an image of the deposit of coral and basalt rubble attributed to a large tsunami that washed up the southern flanks of Lana'i approximately 100,000 to 150,000 years ago due to the most recent giant avalanche (probably off the W flank of Mauna Loa). The geologist's right foot is on the contact between underlying lava of Lana'i volcano and the tsunami deposit.
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. Below left is an air photo of the north coast of Moloka'i. The 600-1000 m high cliffs are the headwalls of a giant landslide that carried away half of the E Moloka'i volcano. The dotted red line outlines the original shield surface, into which the large valleys of Wailau, Pelekunu, Waikolu, and Wai'ale'ia Valley have cut. Oloku'i is a high point between Wailau and Pelekunu; because of its isolation by steep cliffs, Oloku'i boasts a fine complement of rare native plants.
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 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.
A series of diagrams to the right show how Hawaiian volcanoes have an inherent weakness that can lead to giant landslides (from ideas presented by Dave Clague, USGS). In A, a young volcano erupts pillow lavas on the seafloor; explosive activity is prevented by deep water pressure. In B, the volcano nears the surface; the water pressure no longer prevents explosive mixing of hot lava and seawater, and phreatomagmatic explosions produce a layer of hyaloclastite (yellow). In C, the volcano has grown above sea level so that eruptions no longer encounter seawater; they are not explosive, however, lava flowing into the ocean breaks up and occasionally produces littoral explosions, both of which also generate hyaloclastite. In D, the volcano is continuing to build subaerially; the layers of hyaloclastite are an inherent weakness that may promote giant landslides.