Photo by Tim Lincoln
Lava. It’s one of the most unstoppable forces on Earth – an intense, gluttonous mass that devours anything in its path. Magma – lava that has yet to surface – is the main source of the earth’s crust. But just what is lava?
On Hawaii, we are concerned with basaltic magma. Basalt is a fine-grained, mafic (Mg- and Fe-enriched) rock that primarily lacks silica-rich minerals. While basalt is the main constituent of oceanic crust, it also forms the Hawaiian Islands.
For a more detailed look at basalt, visit the Volcanic Petrology page.
Hawaii is located over a hot spot. Hot solid rocks rise to the hot spot from greater depths within the Earth. At shallower depths, there is less pressure applied to the solid rock and it begins to melt, forming magma. The hot spot acts as a common magma source, and as magma rises through the hot spot, it eventually reaches the surface. Since basalt is far less viscous than other types of magma, it rises to the surface far more rapidly than other magma types. This is the same magma that forms the oceanic crust, but since the magma source is concentrated, a small volcano begins to form at the bottom of the ocean. Over time, the hot spot continues feeding the volcano magma, and if this trend continues, eventually an island forms. The Hawaiian islands form in this manner, and as time progresses, the oceanic plate they are located on moves away from the hot spot source. Time and again, this process repeats, creating the long chain of islands that is today known as Hawaii.
This is all fine and good, but what about the lava we can see on the surface? Just what is this aa people are talking about?
When basaltic magma erupts on the island, it can behave in one of two ways:
If the lava has low viscosity, it tends to move faster. This type of lava is characterized by having a smooth, billowy, or ropy surface, and is known as pahoehoe. These characteristic ropes develop when a thick skin of lava at the surface of the flow is compressed into folds by the faster moving lava beneath it. Pahoehoe flows are relatively thin and can form lava tubes, paths of fast moving lava that are insulated by a hardened crust of lava at the surface. When the eruption stops, the lava drains from the tubes, leaving behind shelly pahoehoe.
Billy and Jen expertly showing us the marvelous pahoehoe ropes
Photo by Thom Wilch
Aa lavas have high viscosity which cause them to move very slowly. This type of lava is characterized by rough, jagged, spinose, and a generally clinkery surface. Aa flows tend to be thicker and slowly move over themselves, much like the treads on a bulldozer.
Pahoehoe and Aa flows are not mutually exclusive and can occur in the same flow with no significant chemical difference between the two forms. Pahoehoe can change to aa if it is subjected to a great deal of internal disturbance, but aa can never change to pahoehoe. Pahoehoe flows are more viscous and have higher temperatures than aa flows, however, molten lava of approximately the same viscosity can form either type of flow.
– Source: class notes