OREGON STATE UNIVERSITY

Main Tholeite Shield Stage

 

As heating of the lithosphere continues, the degree of partial melting increases and the absolute volume of magma produced really increases. A higher degree of partial melting produces tholeiite basalt, which has a slightly higher % silica than alkalic basalt. It is during this stage that the plumbing systems within the volcanoes are the most efficient at transporting magma to the surface. >95% of each volcano consists of lava erupted during this main tholeiite stage (e.g. Clague 1987). Both Mauna Loa and Kilauea are in this stage of life, and have together erupted ~114 times since the arrival of Westerners. These tholeiite lavas are fluid and can build up only gradual slopes, producing the classic shield volcano shape.

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).

While stored in the magma chamber complex, magma cools and partially crystallizes. Olivine is usually the first mineral to crystallize out of Hawaiian tholeiite magma, and olivine crystals will settle out while the magma is sitting in the magma chamber complex. Olivine-rich lavas are thus expected if an eruption taps the lower part of the magma chamber complex. An additional thing that happens during storage is that gases can escape from the magma and migrate to the surface. The three main volatiles are water (H2O), sulfur dioxide (SO2), and carbon dioxide (CO2). CO2 exsolves at a greater depth so it escapes shortly after a batch of magma reaches the magma chamber complex. H2O and SO2 stay in solution within the magma for a longer period of time. This means that scientists can determine some of the processes going on down in the magma chamber complex even though they can't actually go there. For example, if gases collected at the summit show a high amount of CO2 relative to SO2; then a fresh batch of magma must have recently arrived from the mantle. If you are concerned with eruption prediction this might be something good to know about! On the other hand, if the ratio of CO2 to SO2 is relatively low, then you know that the magma that is giving off gases is not new--it is just slowly releasing the SO2. The final process that goes on while magma is resting in the magma chamber complex regards pieces of rock that are picked up while the magma is migrating from its initial source. Such pieces are called xenoliths ("foreign rock"), and they are almost always denser than the magma. As soon as the magma comes into the magma chamber and stops moving upward, these xenoliths can no longer be supported and they sink to the floor of the chamber.

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.

 Coastal Plain

The main products of Hawaiian eruptions are lava flows and pyroclastic deposits. When lava flows encounter the ocean they may spread along the coastline because the water provides a cooling barrier that slows the forward progress. This results in the formation of a relatively flat lava shelf, even if the slopes directly inland and offshore are steep. The coalescence of numerous shelves forms a coastal plain (see left). The frequent eruptions during the tholeiite stage mean that construction of the coastal plain is able to keep pace with the subsidence of the volcano, and indeed the parts of Kilauea and Mauna Loa that have the most young flows are characterized by well-developed coastal terraces.