Australia: The Land Where Time Began

A biography of the Australian continent 

Volcanoes – Behavioural Differences Exhibited by Different Types of Volcano

It is shown by the examples of Mt Vesuvius and Mount Pelée that there are important differences between eruptions at different types of volcano. Eruptions at Hawaiian volcanoes are relatively quiet and have killed relatively few people, while the Pelée eruption demonstrated that in this type of volcano the lava is not diverted by river valleys as occurs with other types of eruption, and therefore it cannot be assumed that river valleys protect against superheated clouds of gases and volcanic ash that can travel for many kilometres, even across obstacles.

This is the result of differences in the chemical composition of the lavas erupted by different types of volcano. In the case of volcanoes such as those in Hawaii and Iceland the magma beneath these volcanoes is sourced from hot mantle plumes rising up from the mantle or when the oceanic crust is pulled apart at mid-ocean ridges on the sea floor. Magmas derived from the mantle are rich in such elements as magnesium and iron. And the black rocks that result from the cooling lava from these eruptions are basalt. The melting that forms these magmas that are rich in magnesium and iron occurs at very high temperatures of 1,600^oC (2,900^oF), resulting in magmas that are very hot and fluid. The lava erupted from these volcanoes flows along the landscape with the viscosity of water. Such magmas don’t often explode, though they can throw blobs of lava into the air (lava bombs) during eruptions. In most cases it is not difficult to move out of the way of such lava flows, which do not move very fast. At Kilauea, on the big island of Hawaii, e.g., the inhabitants are rarely in danger from them, though they do engulf objects in their way with black lava.

Magmas that are richer in silicon, aluminium, sodium and potassium are at the other extreme. These elements are minerals that melt at lower temperatures than basaltic minerals, sometimes melting at temperatures as low as 600^oC (1,000^oF). Depending on the exact chemistry of these minerals, these magmas are known as andesites (named after the Andes Mountains, South America), dacites, or rhyolites. They have a tendency to be very viscous, and flow like molasses. This is especially the case with rhyolite; they are too thick and sticky to generate any lava flows at all. The volcanoes that produce these magmas get plugged by the magma until the pressure inside the volcano builds high enough to eventually explode the top of the volcano. A wide range of shattered volcanic rock, known as pyroclastics, are produced which range from tiny glass shards of volcanic ash to the pebble-sized pieces of pumice that are known as lapilli. Cobble-sized and boulder-sized magma that cool as they travel through the air, are volcanic bombs.

Magmas of rhyolite, dacite and andesite start out as basaltic magmas but are transformed into magmas with new chemical composition in subduction zones: Locations at which a tectonic plate plunges beneath another. A subducting plate slides beneath an adjacent plate until it reaches a depth at which the material in the plate, as well as the base of the overlying plate, has heated to a temperature that it is high enough to melt with the result that it forms the magma. Sodium, aluminium, potassium, and silica enrich the melts because the minerals that are composed of those elements melt at lower temperatures than do the basaltic minerals. The minerals composed of the above mentioned elements that melt at lower temperatures than the basaltic minerals melt first to produce those distinctive magmas. Blobs of the magmas then rise to the surface, and sometimes absorb even more of the low melting point minerals that formed those magmas, as they melt the overlying crustal rocks, which are composed of granites and metamorphic rocks, that are rich in those elements. When these magmas finally reach the surface they form a chain of volcanoes on the surface as the tectonic plates continue moving. Subduction zone volcanoes are almost always explosive because of the chemistry of their magmas, and produce few, if any, lava flows.

Most of the subduction zones of the Earth are located around the rim of the Pacific Ocean, roughly in a circle, the Ring of Fire. Most of the deadliest volcanoes as well as biggest earthquakes are located on the ring. The earthquakes occur when a subducting plate that has been slowed in its descent by friction then suddenly slips as it grinds past an overlying slab. Long chains of arcs of volcanoes can be seen on a map of the Ring of Fire. Some appear as volcanic islands in the middle of the ocean, like those in Japan, the Aleutians, the Philippians, and Indonesia. Others are located on the edges of continents, in locations where an oceanic plate is subducting beneath a continental plate, which is thicker. Included among these land-based volcanoes are the Andes, the Central American Volcanic Arc, and the Cascade Mountains of Northern California, Oregon, Washington, and British Columbia.

Sources & Further reading

Prothero, 2018, When Humans Nearly Vanished, Smithsonian