The chemical composition of the Hawaiian magmas plays a central role in determining the physical form of the Hawaiian Islands. All crustal rocks are formed of minerals composed of aluminum and silicon oxides with varying amounts of other elements, dominantly iron, magnesium, sodium, calcium and potassium, stirred in. In lava, silica tends to polymerize and greatly increase the viscosity of the melt—thus silica-rich lavas are fairly “sticky,” forming slow moving lava flows that pile-up into steep volcanoes which are typified by explosive eruptions. Silica-poor melts, on the other hand, because they are not so polymerized, form very fluid lavas that flow quickly and easily, forming gently-sloped volcanoes typified by relatively quiescent eruptions. Remember the key words here are: “relatively quiescent.”
One glance at the profile of the Hawaiian volcanoes demonstrates what chemical analysis tells us: the lavas are very silica-poor. The Hawaiian volcanoes take the form of very gently-sloped “shields” and the eruptions are generally, relatively peaceable--certainly most are civilized enough so that the casual viewer may approach them quite closely in relative safety. Compare these with the very steep-sided, highly explosive, violent eruptions of volcanoes like Mt. St. Helens, which is comprised of lava much more intermediate in silica composition, or the smoking hole in the ground that is the Yellowstone volcanic field, left by a massively explosive, very silica rich (and very, very viscous) lava erupting.
The rocks formed when the lava from Hawaiian volcanoes hardens are called “basalt.” Although you hear the term “lava rock” used by laymen, it is a nonsensical word geologically and the registered trademark of a commercial product. Commonly misapplied, it makes even less sense than the equivalent of using the term “water solid” instead of “ice.” The name “basalt” denotes both the silica-poor composition, as well as the fact that it was erupted onto, and it has cooled at, the Earth’s surface. When lavas cool within a magma chamber before being erupted onto the surface, they are called an “intrusive volcanic” rock; intrusive volcanic rocks formed from lavas of basalt-like composition are called “gabbros”. There are a few rocks in the Hawaiian islands of differing composition and texture than basalt and gabbro, but they are unimportant to this discussion.
The angle of repose, which is controlled by the viscosity of the melt, is about 6 degrees for Hawaiian basalts. Melt viscosity is a function of its temperature, silica content and fluid composition. Hawaiian basalts are very fluid because of the low silica content and high eruptive temperature (in excess of 1100 degrees C); this low viscosity accounts for low angle, “shield shape” of Hawaiian volcanoes.
As soon as the Hawaiian volcano forms, other forces begin to act upon it, affecting the general “shield shape” of the volcano: tides and waves attack the edges, rivers and streams begin to dissect the shield into ridges. As time goes on, erosion from surface water flow produces fluting along those ridges. Great landslides produce enormous cliffs; faulting produces huge valleys and collapse produces enormous craters and calderas. Further, the later stage of Hawaiian volcanism is typified by smaller eruptions of increasing violence, which makes deep explosion craters, steep-sided cinder cones and steeply sloped, but easily eroded, ash deposits.
To look at the volcanoes of Hawaii Island starting in the southeast at Kilauea and Mauna Loa and moving northwest, one can see this evolution quite clearly. Kilauea and Mauna Loa, being quite young, are still broad and shallow-sloped, basically the classic shield-shape. Mauna Kea and Hualalai are obviously more mature, showing steeper slopes, a reflection of greater ashfall and greater erosion, as well as pocked with explosion craters and topped with cinder cones. Moving father along, Kohala Mountain is so deeply eroded and festooned with long, fluted ridges as well as cut by enormous canyons and cliffs, that it is almost unrecognizable as a shield volcano. And so it continues as one proceeds north through the older islands and the more highly eroded mountains; the original shield shape of the volcanoes become more and more obscured by deeper and deeper erosion. By the time one gets to the famous and spectacular Na Pali Cliffs of Kaua'i, all that is left of the original volcano are the deeply fluted, now almost knife-like, ridges.
