Category Archives: Catastrophe

THE LOMBOK EARTHQUAKE August 2018

THE LOMBOK EARTHQUAKE
OR
BEWARE THE BEAST THAT LURKS BENEATH

BACKGROUND
Current ABC ‘breathless’ reporting on the Lombok earthquake negligently omits information on the situation lurking beneath the surface. This article is a plea for balanced reporting.


PLANET EARTH 

Rim of Fire
PACIFIC RIM OF FIRE & SUBDUCTION ZONES (en-wikipedia.org)

The equatorial Indonesian archipelago is part of the Pacific Rim of Fire with its associated subduction zones, earthquakes and volcanos. A subduction zone is formed where a continental plate collides with and is forced beneath another continent. The Australian continental plate is moving north a few centimetres a year beneath the Indonesian islands which form the southern edge of the Eurasian plate. As rocks are driven deeper they melt and then form lines of volcanos: earthquakes are generated as these molten rocks rise to the surface and create volcanos.

Subduction
SUBDUCTION ZONE & VOLCANOS (hhgs.edu.uk)

As the Australian plate moves deeper into the earth’s crust, melting rocks form magma and produce gas and steam which start rising to the surface. This process will generate earthquakes. This could be the situation on Lombok this August. Volcanos along the Indonesian archipelago can produce and destroy islands.

LOMBOK, SAMALAS-RINJANI VOLCANIC COMPLEX
Lombok is an island comprised mainly of volcanic deposits; the island has a geological history of strong earthquakes and catastrophic volcanic eruptions. The geological record proves there were major eruptions around 4000 and 500 BC. In 1257 AD there was a catastrophic eruption (Volcanic Index 7) which vented more than thirty cubic kilometres of rock into the air to be laid down as pyroclastics  and ash over four hundred square kilometres. Dust in the air caused the Little Ice Age in Europe and Asia which altered the climate causing mass starvation.

In 1257, as the magma rose to the surface, it would have caused strong earthquakes and the fractured rocks around the volcano would have facilitated percolation of water towards the volcano. The geological record indicates the Samalas-Rinjani eruption commenced with a massive (phreatic) steam-laden explosion. (Vidal. CM, 10 October 2016, The 1257 Samalas Eruption, Lombok.)

During a ‘breathless’ press interview recently, a witness noted that ‘water levels had dropped’. Certainly the current earthquake would have opened up old fractures and water will be available to generate superheated steam and set the scene for a phreatic explosion over the rising magma. It is reported there have been over 500 after- shocks. Are all these after-shocks, or are some shocks due to rising magma? Has anyone asked the question?

SUMBAWA, TAMBORA STRATOVOLCANO
Sumbawa island is close to Lombok island. It contains the fearsome stratovolcano, Tambora, which lies on the same subduction zone as Rinjani. A stratovolcano is a large volcano that has a long history of strong eruptions.

Geological records indicate Tambora has erupted several times in the last 11,000 years – the Holocene. Major eruptions occurred in 740 and 1816. The 1816 eruption was one of the most devastating in recorded history with an EI 7. The eruption blew out thirty cubic kilometres of ash and pyroclastics and produced a caldera seven kilometres across. More than 100,000 people died from the volcanic activity and starvation.

In recent years there has been earthquake activity in the vicinity of Tambora. The volcano is still active, small eruptions  occurring in 1967 and 2011: earthquakes and steam venting accompanied this activity. (Smithsonian Institute, 28 August 2014, Tambora Explosive History)

Other active volcanos, among many, are Agung on Bali and Krakatoa in the Sunda Strait. The map below shows the amazing number of volcanos along the subduction zone of the Indonesian archipelago.

Volcanos
INDONESIAN VOLCANOS ABOVE SUBDUCTION ZONE. RINJANI & TAMBORA NAMED.
(geology.sdsu.edu)

FOREWARNED
ABC reporting is inadequate. There is no commentary on wider issues associated with subduction volcanism and its disastrous effects. The listening public deserves better. In 1990, the Soviet Union imploded and it was put about that this event heralded ‘the end of history’ – the reverse is true, international affairs are more complex. So too, the eruptions in 1257 and 1816 did not herald the end of subduction volcanism.   20 August.  Two magnitude 6.9 earthquakes have again shaken Lombok on the 19th and 20th August. Since the end of July Lombok has been shaken by four strong earthquakes, 29/7, 5/8, 19/8, 20/8. The epicentre on the 5th August was shallow, taken in conjunction with a report on falling surface water levels this phenomena could indicate  lava is rising beneath Mt Rinjani and water percolating through fractured rocks towards the volcano could cause a dangerous phreatic explosion.
There will be more strong volcanic eruptions.

 

JOHN HUGH HILL
Current Affairs Flash Points.   towardsthefinalhour.com
lurgashall@westnet.com.au