Sunday, March 24

The fragility of our Earth’s crust


A plume of ash rises as Anak Krakatau erupts in Indonesia on Dec 23, 2018. – Susi Air via Reuters

WITH over 400 active volcanos spewing out lava, ash, and gaseous substances daily, it is not surprising that several, each year, will erupt in explosive events thus causing loss of lives and structural damage to buildings. In the Christmas week of 2018, two active volcanos wreaked their havoc in both Indonesia and Italy caused by earthquakes and the shuddering of tectonic plates.

In Indonesia, Anak Krakatau exploded and in Sicilian Italy, a few days later, Mount Etna caused havoc. I stress that the timings of these both events are unrelated and merely coincidental. Both the very mention of Krakatau and Etna conjures up explosive events in volcanologists minds. Even types of fireworks have such labels.

Resultant tsunami

A tsunami is not unlike throwing a large stone into the still waters of a small pond with subsequent ripple effects to the very edges of the pond as the waves gain energy. The Indian Ocean tsunami on Boxing Day 2004, which swept ashore at Banda Aceh in Sumatra, and the Sulawesi tsunami that hit Palu late last September still linger in our minds because of the tragic loss of thousands of lives and the displacement of people occasioned by these huge waves.

On Dec 22, a massive collapse of a flank of Anak Krakatau, triggered by a mighty implosion at 9.06pm, developed into a landslide as a flank of the cone slid into the sea thus displacing seawater. Some have reported this as an underwater slide and of course the dense material went underwater. The resultant tsunami hit the coastline of Western Java, from Serang to Pandeglang in the region of Bantem Province, ripping apart the coastal resort of Carita Beach, which took the full force only 20km east of Anak Krakatau.

All this happened in the peak tourist season. This tsunami occurred in darkness and penetrated inland for 30 metres or more where, there, it was at a height of one metre having lost energy in ripping houses apart and in dragging them seaward on the backwash under gravity. All this was exacerbated by an exceptionally high tide occasioned by a full moon.

As we now know, nearly 500 people were swept to their deaths with 100 or more people still missing. A massive military operation was immediately launched by the Indonesian Armed Forces to sweep beaches and clear inland buildings searching for any survivors. Worldwide help was offered by rescue teams in the European Union and from Malaysia and Australia. The Indonesian government is now taking dramatic steps to update its system of alarms to alert all residents in the Indonesian Archipelago of such future likely events. There is still a fear of future eruptions from Anak Krakatau and air flights from Djakarta, only 150km away, are being diverted.

Origins of Anak Krakatau

For two centuries, before its awakening in May 1883, the old volcanic wreck of Krakatau had been dormant with a series of parasitic volcanic cones within its crater walls known as Rakata, Danan, and Perboewatan. By August, many new vents opened around Danan with ever increasing violence. A climax was reached in the last week of that month with formidable detonations every 10 minutes. Dense volcanic clouds reached an altitude of 27,000 metres and ashes were converted into a stifling mud by the incessant rain plunging Djakarta into day time darkness.

On the morning of Aug 27, four stupendous explosions occurred, the greatest of which was heard 5,000km away in northern Australia, and a vast glowing cloud of incandescent pumice and ashes rose 80,000 metres into the air. Fortunately, Krakatau was uninhabited. The sides of the volcanic cone were blasted outwards and with total collapse into the sea, thus creating a tsunami with a wave height of 37 metres sweeping over the adjacent low coasts of Sumatra and Java. At least 36,000 people were drowned.

When Krakatau again became visible, it was found that 66 per cent of the island had disappeared. Later surveys revealed a submarine hollow measuring 2,072ha in area. The highly gas-charged underlying magma had blown Krakatau to bits and had left its superstructure without support by emptying its magma chamber. Krakatau suddenly collapsed and foundered into a submarine caldera of between 6km and 7km across with island rims. The energy expended in this eruption amounted to the equivalent of a 5,000 megaton atomic hydrogen bomb! The volcanic ash from this massive explosion was responsible for the vividly coloured worldwide sunsets seen in the following months.

Forty years later, after a period of superficial quiescence, submarine emissions broke through the caldera’s floor where the sea was 305 metres deep. Gradually an underwater cone grew and eventually broke the surface in 1937. Anak Krakatau was thus born, with eruptions occurring at intervals with major basaltic ejections in 1953 and 1959, each with a silicon dioxide composition of over 50 per cent.

Is Anak Krakatau a precursor of yet more explosive eruptions in 2019? The Indonesian government, awakened by the Lombok, Sulawesi, and Anak Krakatau eruptions and earthquakes last year, with inevitable loss of lives, is about to spend millions of dollars in updating their seismic equipment and alarm systems, thus alerting all people for imminent volcanic eruptions and tsunamis.

Mount Etna

Smoke rises over the city of Catania during an eruption of Mount Etna on Dec 24, 2018. – Photo by Giovanni Isolino – AFP

In taking a recent train journey from Messina to Catania in Sicily, when I returned from the island of Vulcano, Mount Etna’s dome dominated the skyline and reminded me of a similar ‘bullet train’ ride, in 2003, from Tokyo to Kyoto on Honshu island, Japan, where Fujiyama stood proud.

Volcanic eruptions vary widely in character and intensity according to the pressure of gas and the viscosity of the lava from and/or through the vent or fissure from which these are released. The lavas of a central volcano do not always issue from a central crater for they may find it easier to break through at lower levels, forming satellite or parasitic cones on the flanks of the main structure. Etna, the highest of all European volcanoes at 3,300 metres, has over 200 of these satellite vents within a radius of 30km from its summit, thus creating a dome.

Four months ago, I stayed briefly in Catania, the main port city in eastern Sicily and witnessed plumes of smoke and volcanic ash spewing out of Etna’s dome. Little then did I realise that such emissions were the precursors of a violent, magnitude 4.8 earthquake, which hit that region at 3.19am last Boxing Day. The Italian National Institute for Geophysics and Volcanology (INGV), located the epicentre only just north of Catania at a depth of a mere kilometre. Only three years before Etna spewed out huge amounts of black ash up to 13,000 metres in the sky, closing all airports and seriously affecting the maturation of citrus fruits. Etna’s eruptions in 1991 to 1992 were the biggest for 360 years!

Weathering of the lava flows over thousands of years has created fertile nutrient rich soils with vineyards, citrus and olive orchards, and cultivated fields spread along the lower slopes and scattered towns and villages sited half way up its dome some of which have been destroyed and rebuilt over the years. This recent big earthquake with subsequent tremors caused only 28 minor injuries to people but forced thousands to evacuate their homes to get away in their vehicles or sleep in the streets. Old buildings were severely damaged in Zafferana Etnea and parts of the coastal highway collapsed. In the main square of the small town of Pennisi, a statue of Sant’ Emidio, the earthquake protective patron saint, collapsed.

Subsequently, a bulge has developed on one flank of the dome near to the point where a BBC television crew had to be rescued by helicopter last March. Should this bulge develop further, a slope collapse could occur, thus releasing molten lava in downslope flows. This recent lateral eruption does not bode well.

Etna’s history

The first records of Etna’s eruptions were recorded nearly 2,820 years ago in the writings of the Greek poet, Homer, and a little later by the Roman poet Virgil in his ‘Aeneid’. In Medieval times, violent eruptions occurred, culminating in the 1669 blast, which destroyed the summit cone, leaving a giant crater and led to the total destruction of the city of Catania, only 15km away. The latest large eruptions were recorded in 2009 in the early months of 2017. Perhaps not unrelated to the 2018 rumblings of Etna, Mount Vesuvius, along the Italian Amalfi coast and south of Naples, witnessed an increase in seismic activity last month. Beneath the summit of that volcano three million people live and together with Java these areas are considered to be amongst the most densely populated parts of our world.

Our Earth is a fragile planet on which we, indeed, are fortunate to live. For despite its beauty, the realms of Vulcan have tragically cost so many human lives with tectonic plates shifting and shuddering daily for many millions of years to come. Much of our future monitoring of potential earthquakes, volcanic activity and tsunamis can be learned from the experimental work at INGV.