The diagrams below show the life stages of a Hawaiian volcano. Note that because of subsidence about half of the volume of the volcano is below the level of the ocean floor. Within the main SE Hawaiian chain, Lo'ihi is in stage 2. Kilauea is a young stage 4 and Mauna Loa is a mature stage 4. Hualalai and Mauna Kea are both in stage 5. Kohala is in stage 6. Moving to Maui, East Maui is in stage 7 whereas West Maui is between stages 7 and 8. East Moloka'i is also between stages 7 and 8. Lana'i (which skipped stage 5), Kaho'olawe, and West Moloka'i are in stage 6, perhaps never to go through stage 7. On O'ahu, Ko'olau is in (still in?) stage 7 (but also skipped stage 5), and Wai'anae is probably between stages 7 and 8. Kaua'i and Ni'ihau are between stages 7 and 8. As you can see, even though a lot of work has gone into figuring out this sequence, the volcanoes themselves have a good deal of variation and haven't perfectly "followed" the sequence. The diagram is adapted from Peterson & Moore (1987).
The Lo'ihi seamount off the southeast coast of Hawai'i was known from bathymetric surveys (see below), and thought to be a large slump. However, dredging in the 1970's recovered fresh lava samples, and the growing HVO seismic network began to record earthquake swarms centered on Lo'ihi. Submersible investigations have confirmed that Lo'ihi is actually the youngest Hawaiian volcano, with its summit some 975 m below sea level (~4000 m above the adjacent sea floor). Geochemical analyses of Lo'ihi samples show many of them to be alkalic basalt. Lo'ihi has a flat top that may be an almost-filled caldera, and as we'll see later this means it probably also has a magma chamber. Many of the pillow lavas observed on Lo'ihi have little or no sediment on them, a good indication of their recent formation.
The map on the right shows bathymetry of Lo'ihi volcano, off the SE coast of the big island of Hawai'i. The colors go from deep blue (~4500 m depth) to red (~1000 m depth). Note the relatively flat summit that is pocked with a couple of pit craters. This summit is probably a filled caldera. A seismic swarm in July of 1996 was accompanied by the formation of a new pit crater near the SW edge of the summit. This diagram was kindly provided by John Smith and Terri Dunnebier of the Hawai'i Mapping Research Group.
Below is a photograph of pillow lavas near the summit of Lo'ihi taken from the research submarine ALVIN, in 1987.
The prominent landmarks around the city of Honolulu (Diamond Head, Punchbowl crater, etc., collectively called the Honolulu volcanic series) started erupting some 1 million years after the last eruption of the tholeiite lavas (Ko'olau apparently never went through a post-shield alkalic stage).
Air photo to the right is of the Koko rift section of O'ahu (the E and of Ko'olau volcano). This prominent line of vents built up after erosion had carved deep valleys into this southeastern end of the Ko'olau volcano. The dashed pink lines mark the rims of phreatomagmatic craters, and the two pink dots mark non-phreatomagmatic "dry" vents. The green line is the crest of the Ko'olau mountains, which in this part of the volcano, roughly marks the top of the giant landslide scarp. Hanauma Bay is obvious with its white sand beach, Koko Crater is the tallest cone, and Manana Island marks the northeast end of the rift.
The other fine example is the East Maui volcano, commonly (but incorrectly) called Haleakala. The "crater" at the summit (which is properly called Haleakala) actually formed from the coalescence of two very large valleys that in the 400,000 years after the cessation of the post-shield alkalic stage, were able to eat out the heart of the volcano. After this long eruptive repose the rejuvenation stage filled these valleys with lava flows and cinder cones, providing the spectacular scenery found today.
The lava erupted in these rejuvenation-stage eruptions is highly alkalic and geochemically indicates that it came from a great depth (Chen & Frey 1983); a few of the deposits contain garnet-bearing xenoliths. These are indicative of a rapid and violent journey from the zone of magma generation. The volume contribution from these rejuvenation-stage volcanics is <<1% of the total for a Hawaiian volcano, and they form monogenetic fields - each vent only erupting once. Like monogenetic fields elsewhere in the world the overall eruption rate during this stage is very small (Walker 1990).
It is not known why this stage of volcanism occurs. There have been numerous explanations put forth, the most popular one today suggests that the lithosphere rebounds upward after having been depressed while directly over the hotspot. This rebounding is because the lithosphere is no longer being thermally weakened and because the overlying volcanoes are eroding. Depressurization due to this uplift would then lead to melting and magma generation. It is not clear that the lithosphere does indeed rebound in that way, however. Perhaps batches of magma attempt to make it to the surface all over under the Pacific plate, and only where the plate has been fractured and weakened by hotspot volcano formation are they able to make it to the surface. On the Ko'olau volcano, many of the rejuvenation stage vents are found to lie along rifts that are perpendicular to the trend of the old Ko'olau volcanic structure. Many of the Honolulu volcanic series vents happened to erupt into shallow seawater, and the eruptions were phreatomagmatic.
It is also during this stage that a magma chamber fully develops to serve as a way-station for ascending magma. A magma chamber migrates upward as the volcano grows, and the magma chambers of Mauna Loa and Kilauea are both 2-3 km below the summits. Although usually depicted as giant balloons, magma chambers are most probably a complex of smaller interconnected voids (more like a magma chamber complex). This idea has been confirmed at by geodetic measurements that show the center of deformation moving around during periods of inflation and deflation (Fiske & Kinoshita 1969).
The end results of all these processes are that lavas erupted during this main shield stage of volcanic life are: 1) hot and fluid because they have an efficient pre-heated plumbing system to get them to the surface; 2) have already lost some of their gas when they eventually erupt because it escaped while the magma was resting in the magma chamber; 3) possibly olivine-rich if the eruption taps the lower part of the magma chamber; and 4) unlikely to include xenoliths because the xenoliths sank to the bottom of the magma chamber.
Another important consequence of the development of a magma chamber is that it can lead to the formation of a caldera. Calderas result from collapse and/or subsidence into the magma chamber, thus a caldera is a sign that an active magma chamber is or once was present, and this in turn implies a high supply to the volcano. Calderas are very dynamic features, however, and at times they can be completely filled in, only to re-form again later.
The lower magma production rate during the post-shield alkalic stage decreases the thermal efficiency of the plumbing system and eventually the main magma chamber solidifies due to the lack of replenishment. This lack of a shallow magma chamber has a great effect on the nature of eruptions during the post-shield alkalic stage. Without the shallow magma chamber to rest in, only large batches of magma are able to make it to the surface, and they have to make the trip from the source region to the surface quickly to avoid solidifying along the way. This means that their xenoliths and large crystals don't have a chance to settle out, nor does gas have a chance to escape.
The consequences of all this are that post-shield alkalic stage eruptions usually consist of large volumes of cooler, gas-rich, xenolith-rich lava, but they are infrequent. The greater gas content means higher fountains and consequently larger cinder cones such as on Mauna Kea (right). Alkalic lava theoretically has a lower viscosity due to its lower silica contents. However, because during this stage the lavas also tend to be cooler, the viscosity increase due to this lower temperature usually outweighs the viscosity decrease due to the lower silica. Post-shield alkalic activity tends to be more concentrated at the summits than during the main shield activity.
This combination of lots of big cinder cones and lots of thicker viscous flows all concentrated near the summits causes Hawaiian volcanoes in this alkalic stage to be noticeably steeper and bumpier than in the tholeiite stage (see below). Mauna Kea and Hualalai are in this stage of development. Hualalai last erupted in 1800 and 1801, and Mauna Kea about 3600 years ago.
The air photo below, aiming towards the SW, shows Mauna Loa, Mauna Kea, and Hualalai. Note that Mauna Loa (ML), which is in its tholeiite shield stage, has a more gradual and smooth profile compared to Mauna Kea (MK) and Hualalai (H), both of which are in their post-shield alkalic stages.
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.