HAWAIIAN ERUPTIONS

As the name implies hawaiian eruptions are common on Hawaiian volcanoes, but they are also the major type of eruptive behavior at almost all basaltic shield volcanoes around the world. A hawaiian eruption involves the steady supply of fluid, relatively gas-poor magma to the vent. The most common type of magma erupted during hawaiian eruptions is basalt. Remember from the explanation of how gas behaves in magma, that as pressure decreases, gas bubbles form and then expand.

So...in a Hawaiian eruption there is magma moving from deep in the volcano towards the surface, and as it does, bubbles are first nucleating and then expanding. The bubbles begin to nucleate at depths of about 2-3 km, and as the magma rises the bubbles grow to comprise an ever-larger % of the magma volume. At a few hundred meters below the surface the bubbles have expanded to the point that they constitute about 75% of the total volume, and at this point the magma breaks apart. You can think of this in a couple of ways: first is that at this depth the bubbles are all bursting; second is that at this 75% volume point there is a change from bubbles surrounded by rising magma to blobs of magma surrounded by expanding gas.

For this discussion we will assume that a vent has already been established, meaning that there is an opening to the surface. It is pretty obvious that the easiest way for this expanding mixture of gas and magma blebs to go is up out of the vent. This it does, forming a lava fountain. As you can see, however, the actual base of the fountain (where there is a change from bubbles suspended in magma to blebs of magma suspended in gas) is below the surface.

Because we are dealing with hot, low-viscosity magma, there is very little resistance to bubble growth and the bubbles can pretty much expand at will. This means that when they finally do burst (after they've achieved 75% of the total volume), there isn't a whole lot of pent-up pressure in them. Thus even though a hawaiian lava fountain is spectacular, it isn't very violent (as eruptions go). One result of this is that the blebs of magma that are thrown out (the pyroclasts) tend to be relatively large (you'll see why this is important when we get to Plinian eruptions).

There is a range in heights of hawaiian lava fountains, and most of this range is due to variations in the initial amount of gas available and the amount that may have escaped while the magma is migrating through the volcano. The highest observed hawaiian fountains IN HAWAI`I were just over 500 m in height (during the 1959 Kilauea Iki and 1969 Mauna Ulu eruptions). The record for the highest hawaiian fountain elsewhere probably belongs to the 1986 eruption of Izu Oshima volcano in Japan, which produced lava fountains about 1500 m high!! In most cases, however, hawaiian fountains are a few meters high to a few hundred meters high.

The products of a hawaiian fountaining eruption depend a great deal on how high the fountains are. If the fountains are more than about 50 m high, the pyroclasts have a chance to cool before they come back to land, meaning they don't stick together. Instead they form loose piles of material with outer slopes of about 33, called cinder cones. On the other hand, if the fountains are less than about 50 meters high, the pyroclasts are still fluid when they land and they can stick together. This results in much steeper vent structures called spatter cones or spatter ramparts. In either case there is almost always somewhere that the pyroclastic material is accumulating fast enough to form lava flows and these flows usually move rapidly away from the vent and downslope to produce `a`a.

One of the important characteristics of a hawaiian fountain is that there is a constant supply of magma up the conduit so that this formation of a spray of gas and magma blebs is also constant - there are no discreet explosions. Keeping a hawaiian eruption going requires a balance between the supply of gas-rich magma to the shallow part of the conduit. One way to think about this is that there is a competition between the rate at which magma is supplied upwards and the rate at which the disruption level (where bubbles become 75% of the volume) is trying to migrate downwards. If the two rates are balanced, the "base" of the fountain remains at a constant depth.

If, on the other hand, the magma supply rate cannot keep up (for example as supply from the main magma chamber gets less and less), the disruption level begins to migrate downward. Eventually it can reach the bottom of the conduit or small chamber beneath the vent, effectively ending the eruption:


THE OTHER TYPE OF HAWAIIAN ERUPTION

There is another type of eruption that is observed on basaltic shield volcanoes. In fact, the current eruption of Kilauea fits into this category. In these eruptions there is essentially no fountaining at all, and the lava pretty much just wells up out of the vent and flows slowly away. Without fountaining there are no pyroclastic products produced, and the usual structure of the vent is a gradual-sloped mound. Because these mounds look like miniature versions of shield volcanoes (but are actually on the flanks of big shield volcanoes) they are called satellitic shields. While active, satellitic shields often have lava ponds at their summits.

This slow, non-violent eruption of fluid lava produces the other common type of basalt lava - pahoehoe. Why would one volcano produce both the high-fountaining style of hawaiian eruption and the slow satellitic-shield style? What causes them to be different if they're all being fed by the same magma plumbing system? The answer is in how much gas is left in the magma at the point where it reaches the surface. If the magma is moving quickly through the plumbing system, even though bubbles try to rise out of the magma and escape, they are dragged along even faster, meaning that they are still in the magma when it reaches the vent and therefore still there able to expand and drive high fountains. On the other hand, if the magma is flowing slowly through the plumbing system, many gas bubbles are able to rise to the top of the conduit and escape, leaving the erupting magma so devoid of bubbles that there aren't enough left to drive fountains. The gas that escapes along the way may just percolate up through the volcano or it may come out through a pre-existing or earlier-active vent. This was the case during the Kupa`ianaha stage of the current Kilauea eruption. Pu`u `O`o, which had been the site of eruptions for the previous 4 years, became the escape point for gas bubbles so that at Kupa`ianaha (3 km downrift), the erupting magma was degassed.