Hydromagmatic eruptions

This special type of volcanic eruption happens when large quantities of hot magma and water come into contact with each other, with explosive results. This interaction can take a variety of forms, including eruptions within crater lakes, under ice or snow, or eruptions in shallow water at sea. All these are somewhat alike

Hydromagmatic Eruptions

Fig. 3.7. Sketch showing the islands of the Krakatau group before the eruption of 1883 (A), after the eruption (B) and in 1960 (C), showing Anak Krakatau which is still active today. The dashed line indicates the approximate location of the submerged edge of the caldera. (Modified after MacDonald, 1972.)

Fig. 3.7. Sketch showing the islands of the Krakatau group before the eruption of 1883 (A), after the eruption (B) and in 1960 (C), showing Anak Krakatau which is still active today. The dashed line indicates the approximate location of the submerged edge of the caldera. (Modified after MacDonald, 1972.)

and are generally referred to as hydromagmatic, hydro-volcanic, or phreatomagmatic eruptions (phreato is a Greek word meaning "water well"). Eruptions that are referred to simply as phreatic are those in which no new magma was involved, meaning that the eruption was triggered by still-hot magma from a previous eruption.

These eruptions can be highly explosive and, therefore, quite dangerous. When hot magma comes into contact with water, the heat of the magma turns the water to steam and the steam expands explosively, tearing the magma apart and producing great quantities of ash. Magma underground can also heat up nearby groundwater until the water flashes to steam, resulting in a violent blast. Most sensible people would stay away from hydromagmatic eruptions.

However, now and then some volcano produces an eruption of this type that is suitable for watching from an appropriate distance.

One example was the Poas volcano in Costa Rica during the 1970s. For several years, Poas delighted tourists with small eruptions that took place under the murky waters of its shallow crater lake. The explosions shot plumes of muddy water into the air, sometimes spraying acid water droplets on tourists gathered at the crater rim to watch. Though the steady eruptions no longer happen, the crater lake at Poas still fumes away and small eruptions of mud and sulfur happen occasionally and are viewed by a lucky few.

Some of the most dangerous examples of hydromagmatic eruptions involve the interaction between magma and ice or snow. The effects can be quite devastating because the hot magma can melt large quantities of the frozen water, which then rushes downslope, mixing with ash and debris to produce mudflows. One of the most tragic examples was the 1985 eruption from the Colombian volcano Nevado del Ruiz. A relatively small summit eruption from this white-capped volcano was enough to melt large quantities of snow and ice and to generate a series of mudflows. These were channeled down narrow valleys on the flanks of the volcano and came down during the night. At the mouth of one of the valleys lay the town of Armero. Most of its 22,000 inhabitants never saw daylight again. The few survivors reported that waves of mud overtook the town, carrying away whole houses, cars, and people.

Mudflows are one of the major sources of devastation and tragedy caused by volcanoes. They can be produced by a variety of volcanoes and can be triggered even by minor eruptions. They are most likely to form on volcanoes that are steep-sided and snow-capped and which have large quantities of loose ash and debris on their flanks. Many of the volcanoes in the Cascade range in the USA are capable of producing devastating mudflows: Mt. Rainier has produced enormous mudflows in the past and the danger of repeat performances is very real. The glacier atop Mt. Rainier makes this volcano particularly hazardous, because torrents of meltwater could be released by an eruption.

The dangers of eruptions which cause glaciers and ice-caps to melt is well known to the people of Iceland, who have introduced the term jokulhlaup to volcanol-ogy (jokul means "ice-cap" and hlaup "deluge"). Jokulhlaups caused by eruptions from Grimsvotn volcano have released floods of water at outflow rates approaching 40,000 m3 (about 1.5 million cubic feet) per second - higher than many of the world's greatest rivers. Luckily the glacier-bursts have been confined to the southeastern coast which is largely uninhabited, perhaps because the volcano-wise Icelanders had the good sense not to settle there.

Among Iceland's many volcanic wonders is the island of Surtsey (Fig. 3.8) (see also Chapter 11), another example of water and magma at work. The island emerged from the sea in 1963, a superb example of volcanic eruption under water creating new land. Surtsey's formation was a spectacle hard to duplicate. The eruption was first seen on November 14, 1963, by local fishermen out in a boat. They thought that the dark column of smoke might be a ship on fire. Luckily, they realized their mistake before getting too

Surtsey Birth
Fig. 3.8. The eruption of Surtsey, Iceland. This photo, taken in 1963, shows clouds of white steam and black volcanic ash formed when hot lava meets seawater. Note the fire-fountain in the island, partly obscured by the clouds. (Photograph courtesy of John Guest.)

close for safety, as they saw tall jets of steam and ash rising above the surface of the sea. As is typical for shallow-water eruptions, powerful explosions rip the magma apart, sending ash and fragments hundreds of meters into the air, in a curious pattern known as a cock's-tail plume because of its arcuate shape.

Surtsey's growth was rapid: about 24 hours later the new island projected its ashy head above the surface and by November 19 the new elongated volcano was 600 m (2,000 feet) long and had reached 43 m (140 feet) in height. Once the volcano was above sea level, its activity changed character, becoming a typical Strombolian eruption. Five months after its birth, Surtsey was a sizeable cone over 150 m (500 feet) above sea level and 1,700 m (5,600 feet) long. Despite the size of the island, the loose ash could have been easily eaten up by the continuously pounding waves. However, in 1964 a lava lake filled the island's crater and soon lava begun to flow towards the sea. For about

Table 3.1. General relationships between volcano types, lava composition, eruption style, and common eruption characteristics

Volcano type Predominant lava

Eruption style

Common eruption characteristics


Basaltic, fluid

Generally non-explosive to weakly Lava fountains, long lava flows, lava lakes, explosive lava pools

Composite Andesitic, less fluid Generally explosive but sometimes Shorter lava flows, pyroclastic falls, tephra non-explosive falls, pyroclastic flows and surges

Composite Dacitic to rhyolitic, Typically highly explosive, but can Pyroclastic falls, tephra falls, pyroclastic viscous to very viscous be non-explosive flows and surges, short lava flows, lava domes

Source: Tilling (1989).

a year, lava flowed out of the crater, covering Surtsey's loose ash slopes and assuring the island's survival. When all activity stopped in 1967, Surtsey had an area of 2.8 km2 (1 square mile), all of which was to be carefully protected for a unique study to document the evolution of its geology and ecology. The studies go on to this day.

Can you plan to view a hydromagmatic eruption? In general, no. Because of their potential violence, these eruptions are not really suitable for watching. However, opportunities may strike occasionally. Visitors to Poas may be lucky enough to see some manifestations of the volcanic power underneath the lake. Surtsey-type eruptions are rare but, when they happen, they can sometimes be seen from a boat at a safe distance. Indeed, some Icelanders not only saw Surtsey in eruption but even took advantage of the late-stage activity to bathe in warm waters heated up by the lava flows!

The products of hydromagmatic eruptions are rather fascinating to see. Surtsey island still lives and can be viewed from a boat or plane. Iceland has a host of other spectacular features related to hydromagmatic eruptions, such as the products of glacier-bursts. On a smaller scale, visitors to the Big Island of Hawaii can often see a lava flow entering the sea and marvel at the notion that the island is growing before their eyes.

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