Petrology and Structure of the Hawaiian Islands
GEOL205 - Lecture Notes
Life Stages
The first lava erupted on the sea floor in the formation of a new volcano of the Hawaiian chain is a more alkalic form of basalt than that forming the vast bulk of the edifice. Alkalic basalts have slightly enhanced levels of the elements Na (sodium) and K (potassium). Many petrologists attribute the more alkalic nature nature of this material to a somewhat lesser degree of partial melting, in the range of 5% to 8% as might be expected around the margins of the "hot spot". Recently some very alkalic lava flows were found along the Hawaiian Arch, helping to confirm this theory. During the "Submarine Stage", lava is erupted in contact with cold seawater, forming pillow basalts - about the size and shape of watermelons. As the nascent volcano grows from the sea floor, these pillows are stacked on upon the other, eventually forming a pile high enough to reach the surface of the sea (about 5km). Since there is little lava to cement the pile together, one might imagine a submarine volcano to be somewhat unstable. Indeed, during the growth of a submarine volcano, it appears that it collapses many times before finally reaching the sea surface. The only example we have of an actual volcano in this first alkalic phase comes from Loihi, which is now undergoing a change to the tholeitic or less alkaline form of basalt. If other volcanoes in the Hawaiian chain had such a phase, the evidence lies buried deep below millions of tons of tholeite. It was once thought that calderas did not form until late in the life of Hawaiian volcanoes (as shown in the diagrams in the text that differ somewhat from those shown here). With the discovery of small summit calderas on Loihi, it is now known that summit calderas form, refill, and reform throughout the life of the volcano as the summit magma chamber is pressurized and depressurized during eruptions, leaving the summit unsupported. During the Fall of 1996 a swarm of many thousand earthquakes shook the summit of Loihi for several weeks (many up to magnitude 5). Submersible dives to the summit area after the earthquake swarm confirmed that a new collapse pit had formed near the summit. It is still unclear if a major eruption accompanied this event, but it does seem likely to have happened. Before the summit of the volcano reaches the surface of the sea, the chemistry of the lava shifts completely to tholeite as the volcano moves directly over the center of the hot spot where partial melting fractions of the mantle may reach a value of about 20%. Tholeitic basalt makes up about 90% to 95% of the volume of the edifice, and continues to erupt until the volcano moves off of the hotspot (see below) and melt fractions again drop producing eruptions of alkalic basalts as might be expected.
As a young volcano reaches the surface of the sea, eruptions must become more explosive because of the reduced pressure. Pyroclastic debris together with lava that is fragmented into black sand as it is chilled in shallow water begins to form a coating over the pile of pillow basalt that made up the submarine edifice. This black sand is known to geologists as hyaloclastite, or loosely, broken glass. The result is a pile of "watermellons" with an expanding shell of hyaloclastites as shown in the accompanying diagram. There are no contemporaneous examples of this stage as it lasts only a short portion of a volcanoes history. The submarine and emergent parts of the volcano are much steeper (10-15 degrees) than the subaerial shield (5-8 degrees) that forms next.
As the edifice grows above the sea, lava cooling more slowing in contact with the air forms a dense disk of pahoehoe, perched precariously on a pedestal of pillow basalts and hyaloclastites. The disk of dense material supported so weakly is one reason that huge chunks of island periodically break off forming tsunamis with wave heights exceeding 350 meters (1000 feet. This is the main shield building stage produced by the voluminous eruption of tholeite. The subaeral tholeiite lavas form very fluid pahoehoe flows and only slightly stickier a`a flows. These are very thin flows and thousands of them are stacked up to build the shield shapes (named after warriors shields, these could have just have easily been called "wok" volcanoes had the Chinese named them). Mauna Loa and Kilauea are examples of this stage, although Mauna Loa may be reaching the end and beginning its entry into the declining stage discussed next. The shield volcanoes are fed lava continuously as they sit over the active hotspot and erupt very frequently. They are characterized by large shallow magma chambers some 3-4 km beneath their summits. The shields tend to be preferentially elongated and have narrower extensions of the summit magma chamber that feed eruptions along linear fracture zones called rift zones.
During the declining stage, eruption volumes decrease and the summit magma chamber solidifies (apparently because the input of lava is too infrequent and insufficient to keep the chamber molten). On Hualalai, eruptions of lava from the mostly crystalline chamber produced some very evolved sticky lavas called trachytes, a unique occurrence on the Hawaiian Islands. Following the tholeiite stage, eruptions of more viscous alkalic basalt produce the steep, hummocky cap on many of the shield volcanoes. Unlike the more fluid tholeiitic eruptions, alkali basalt eruptions are very short lived and build up clusters of steep sided cinder cones. Most of the lava flows are short, but some can be very extensive. Though these eruptions tend to exploit the pre-existing fractures of the rift zones, they are not confined to the rift zones. Alkalic cones are scattered all over declining stage volcanoes, though the highest concentrations of cones occur near the summit and along old rift zones. Mauna Kea is presently in the declining stage, with no more than about 8 eruptions during the last 40 thousand years or so. From birth through the declining stage, the life of an Hawaiian volcano lasts somewhat less than 1 million years. Not all Hawaiian volcanos go through this post-tholeitic stage, although the reason for this is not known. The transition to the post-tholeitic stage is gradual, lasting as long as 100 thousand years. The chemistry slowly changes (from petrological analysis of sections) with occasional interbedding of tholeitic and alkalic flows depending upon vent location.
As the declining stage wanes, erosion becomes the chief geological force shaping the surface of the edifice. Large gullies form as erosion outpaces lava production, like the gulleys on the north slopes of Mauna Kea, and the surfaces begin to weather into deeper soils. Erosion of the coastal areas by wave action is no longer held at bay by advancing lava, and steep coastal cliffs develop as the island begins to shrink due to erosion and subside.
After the decline of a Hawaiian volcano there is sometimes a brief hiatus before a rejuvenated stage begins. Kohala volcano is currently thought to be in this resting stage as it has been about 60,000 years since its last eruption. Most Hawaiian volcanoes to not manifest this rejuvenated stage, and when they do the interval varies greatly and seems to depend upon whether or not a large shield volcano is being built several hundred kilometers down the chain. The lavas erupted during this stage are strange highly alkalic rocks called basanites, very depleted in silica, and forming cones in Haleakala crater and tuff rings like Diamond Head and Coco Head on the island of Oahu. The latest eruption of Haleakela volcano on the island of of Maui around 1790 represents an early form of the rejuvenated phase, in this case with more fluid lava forming an a'a delta on the western coast of Maui. In some cases, such as Oahu, the delay can be several million years and the late stage alkalic lavas are seperated from the rejuvenatated flows by a thick soil layer. In other cases, as with Haleakela, there is no time for soil to form, and the rejuvenated lavas lie in direct contact with the late stage alkalics beneath them. The cause of the rejuvenated stage is thought to be related to the remelting of still hot rocks at depth in the volcano as a result of depressurization caused by the erosion of the edifice. These rocks result from even smaller amounts of partial melting than the alkalic basalts of the declining phase.
Eventually the increasing weight of the cooling sea floor drags the volcano beneath the surface of the sea. For a time, coral form surrounding the island, growing towards the surface as the island continues to sink. These corals, as discussed in a previous lecture, form thick bands near the onset of ice ages as the down draw of the oceans by forming glaciers keeps pace with the subsidence of the island. As the island continues to subside, the coral reefs grow further and further from the edifice, forming broad, lagoon like planes in the coastal zone.
Eventually, though as the island moves north and sinks with the cooling oceanic plate into colder waters, the coral dies. The island continues to sink, leaving coral reefs as much as a kilometer beneath the surface of the sea - far too deep for corals to grow. Drill holes in submerged islands along the emperor chain first encounter this coraline cap, followed by late stage alkalic lavas and then massive tholeite. There appears to be very little variation in this pattern going back to the oldest islands in the chain about 65 millions years ago. The fact that there is so little change in this pattern points to the rather incredible stability of the "hot spot", and suggests that such phenomenon are extremely long-lived.
Examination Questions
- What are the four petrologically distinct phases in the evolution of
Hawaiian type volcanoes, and how are they related?
- Describe the evolution of a typical volcanic edifice from its emergence on
the sea floor through its stage as a submerged guyot.
- How far does a volcano in the Hawaiian-Emperor sea mount chain travel
during its active stage and is this consistant with the distance
along the chain of currently active volcanoes.
Extra Reading
Moore, J. G., and Clague, D. A., 1992, Volcano growth and evolution of the island of Hawaii: Geological Society of America Bulletin, volume 104, p. 1471-1484.
If you have comments or suggestions, email me at kenhon@hawaii.edu
The first lava erupted on the sea floor in the formation of a new volcano of the Hawaiian chain is a more alkalic form of basalt than that forming the vast bulk of the edifice. Alkalic basalts have slightly enhanced levels of the elements Na (sodium) and K (potassium). Many petrologists attribute the more alkalic nature nature of this material to a somewhat lesser degree of partial melting, in the range of 5% to 8% as might be expected around the margins of the "hot spot". Recently some very alkalic lava flows were found along the Hawaiian Arch, helping to confirm this theory. During the "Submarine Stage", lava is erupted in contact with cold seawater, forming pillow basalts - about the size and shape of watermelons. As the nascent volcano grows from the sea floor, these pillows are stacked on upon the other, eventually forming a pile high enough to reach the surface of the sea (about 5km). Since there is little lava to cement the pile together, one might imagine a submarine volcano to be somewhat unstable. Indeed, during the growth of a submarine volcano, it appears that it collapses many times before finally reaching the sea surface. The only example we have of an actual volcano in this first alkalic phase comes from Loihi, which is now undergoing a change to the tholeitic or less alkaline form of basalt. If other volcanoes in the Hawaiian chain had such a phase, the evidence lies buried deep below millions of tons of tholeite. It was once thought that calderas did not form until late in the life of Hawaiian volcanoes (as shown in the diagrams in the text that differ somewhat from those shown here). With the discovery of small summit calderas on Loihi, it is now known that summit calderas form, refill, and reform throughout the life of the volcano as the summit magma chamber is pressurized and depressurized during eruptions, leaving the summit unsupported. During the Fall of 1996 a swarm of many thousand earthquakes shook the summit of Loihi for several weeks (many up to magnitude 5). Submersible dives to the summit area after the earthquake swarm confirmed that a new collapse pit had formed near the summit. It is still unclear if a major eruption accompanied this event, but it does seem likely to have happened. Before the summit of the volcano reaches the surface of the sea, the chemistry of the lava shifts completely to tholeite as the volcano moves directly over the center of the hot spot where partial melting fractions of the mantle may reach a value of about 20%. Tholeitic basalt makes up about 90% to 95% of the volume of the edifice, and continues to erupt until the volcano moves off of the hotspot (see below) and melt fractions again drop producing eruptions of alkalic basalts as might be expected.
