Lavas

Origins of lava

Magmas with a low viscosity and volatile contents (e.g. water vapour, carbon dioxide) are erupted quietly as effusive lava flows (as seen almost daily at Mauna Loa, Hawaii) whereas lavas with a high viscosity and/or excess volatile content are erupted explosively and generally result in fragmental deposits rather than lava flows (e.g. Mount St Helens, United States of America(USA)). Basaltic magma (low silica content) is usually erupted as lava flows whereas rhyolitic magmas (high silica content) are commonly erupted as pyroclastic deposits or short flows.

Lava flows

The flow of a lava from a vent depends on its viscosity (thickness), which in turn relates to the composition and volatile (explosive) content of the erupting magma. Many lavas are fluid enough to flow away from a vent under gravity (e.g. basalts and some andesites). However, more viscous lavas, such as rhyolites and most dacites, well up into lava domes (when low in volatiles) or explode in pyroclastic fall out (when rich in volatiles).

Basaltic lava flows in three distinct forms.

• Pahoehoe flows have a smooth rounded undulating form, often with a ropy appearance.
• Aa flows have a very rough fragmented top and their surface is very sharp and abrasive.
• Block lava flows also have a fragmented top but the individual fragments are smooth-surfaced many-sided blocks.

Rhyolitic lava flows are usually short and thick and often travel down a very steep-sided slope. The Glass Mountain rhyolite flow of Mono Craters, California is 3.6 km long and 75 m thick but was gas-rich when it was first formed.

Highly alkaline basic lavas (low in silica) are extremely mobile. In the 1977 eruption of the Niyragongo volcano in Zaire, a 20 000 000 km3 lake of nephelinite lava flowed over an area of 20 km2 in under an hour. The lava was so low in viscosity that in places, it was only 1 mm thick.

Lava domes

The most viscous lavas grow into domes, which are forced upwards (simply by pressure from the underlying magma) through the vent to form a protruding plug or spine. The weight of the growing plug or spine starts a collapse. The spine loses material from its sides and top, and crashes down around the base. Some domes expand outwards as well as upwards by inflation of the hot underlying magma. The Tarawera Complex on the North Island of New Zealand is an excellent example. Lava domes grow relatively slowly, about 1 m - 2 m per day.

Submarine lavas

Many volcanic eruptions occur on the sea-floor. These eruptions often form pillow lavas. These oval-shaped masses and tubes usually less than 1 m in diameter form outer layers that are normally glassy (caused by the rapid quenching as cold sea-water hits hot lava) and they chill from the margins inward. Most pillow lavas are basaltic in composition.