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Indonesia's Sinabung volcano has been erupting regularly for nearly a decade, and last week it produced another large ash column and pyroclastic flows. Much of the surrounding area has been previously evacuated/abandoned because of the ongoing eruptions.
Deuxième destination conseillée de Sumatra : Berastagi! Ville où l'on cultive le choux pour exportation à travers tout le pays grâce à sa terre volcanique. Nous escaladons donc notre premier volcan d'Indonésie, pays situé à la convergence de plaques tectoniques et de failles sismiques. En effet, il y existe au moins 150 volcans toujours actifs aujourd'hui. Nous voici sur le Mont Sibayak, stratovolcan culminant à 2181m d'altitude. Habituellement, il y a un lac dans le cratère mais lors de notre passage il était asséché par manque de mousson. Les puissantes fumeroles étaient quant à elles bien présentes !
An island expands and shrinks
On December 25, 2004, the island at the center of this photograph was only as big as the area covered by trees.
The next day, the island was this size.
This island sits off the coast of Sumatra above the Sumatran-Andaman subduction zone. On December 26, 2004 that fault broke, leading to the disastrous 2004 tsunami and producing one of the largest earthquakes in recorded history.
The energy released in an earthquake is stored in rocks over hundreds of years. As tectonic plates move, they grind against each other and the rocks begin bending. In a subduction zone, these forces bend some parts of the plate downward and push other parts upward.
When the fault finally breaks, all the land that was bent downward suddenly pops loose and snaps back. That snapping releases the energy we feel as an earthquake and releases the land back to its original position. When the December 26 earthquake happened, this island and many around it popped up, suddenly exposing areas that were formerly submerged, often damaging or killing coral heads that grew in the shallow water during the hundreds of years between earthquakes. This island more than doubled in area in an instant.
If you came back to this island today, plant life would probably have started growing in the areas that were exposed in 2004, but the island would also be slightly smaller. It’s been a decade since the 2004 earthquake and the relentless motion of tectonic plates has continued. It will take centuries for the island to submerge to the same degree as in 2004, but every single day the plates move just a little and the land exposed in 2004 will sink again in response.
-JBB
Church in the abandoned village of Simacem, North Sumatra, Indonesia
Waited around for ages for the clouds to clear around this infernal lake. Was it worth the effort? My host outright refused to drive down the 40-something switchbacks to the bottom in her ancient truck with dubious suspension (which, given my delicate carsickness-prone self, was absolutely the right choice), so I feel like something was missing. But hey. Lake Maninjau! Yes, that is the ocean beyond the hills.
Real time view of a Sinabung Volcano eruption. You can also see the old channels created by many pyroclastic flows traveling down the slope.
marvloke
This unexpected eruption happening in front of me will always be a highlight in my time here.
Explosive eruptions and lava flowing down the slopes at Anak Krakatau volcano, Sunda Straits, last week. Really nice shot where you can see the ocean steaming from lava entry with the ashy cone in the background.
Steaming Sibayak volcano and crater lake, Sumatra
What are pyroclastic flows?
Mt. Sinabung in Indonesia has been up to its old tricks again this year, repeatedly sending pyroclastic flows down its slopes (see here for video: http://tinyurl.com/pd4u9ed)
Naturally, that drives the question “what are pyroclastic flows”. You can see one of them in this image from Mt. Unzen. A pyroclastic flow, or more accurately a pyroclastic density current, is a peculiar mixture of rock and hot gases.
Pyroclastic flows are similar to the concept of “suspension” in fluids. You might remember this idea from chemistry – even when matter doesn’t dissolve in water, you can often mix the 2 components together mechanically. If you throw mud into water…the mud doesn’t dissolve, but you create muddy water.
The properties of suspensions can differ from the constituents making them up. For example, muddy water is denser than fresh water, so if you pour muddy water into a lake, or throw mud onto the surface of the lake, the mud will sink to the bottom and take some of the water with it.
A pyroclastic flow is a suspension like those examples, except in this case the mixture is made of rock fragments and air.
The term pyroclast includes the roots for fire (pyro) and broken into pieces (clast or klastós) and that’s what these are made of. When volcanoes explode, they shatter rocks and magma into fine pieces of superheated ash and pumice.
Those shards are fine enough to be carried by the wind like dust, and the heat from the grains can even drive winds. But, when the mixture of ash and becomes too thick or dense, it can start to behave distinct from the surrounding air, flowing as a single mixture.
The ash-gas mixture can be formed a number of ways – it can collapse out of the bottom of a volcanic column in the sky or it can come out of a volcano directly if mixed in the vent.
Pyroclastic flows move across the terrain like floods. They will follow canyons, they pick up speed where slopes are steeper, and they can even turn. They’re able to travel at hundreds of kilometers per hour, making them incredibly dangerous as they can outrun vehicles.
The air above a pyroclastic flow also gets stirred up and mixed with hot ash. Those areas can be lower density than the flow below, but are still superheated and deadly. Those parts are termed “pyroclastic surges” and because of their lower density, they can jump right over topography, hopping over ridges and obstacles and leaving no safe high ground.
Pyroclastic flows have been incredibly deadly throughout history. Pyroclastic flows from Vesuvius buried the cities of Pompeii and Herculaneum. A pyroclastic flow in 1902 hit the town of St. Pierre on the island of Martinique and killed about 30,000 people. Volcanoes can send of enormous numbers of pyroclastic flows during a single eruption, making them an even greater risk to life.
-JBB
Image credit: Wikimedia commons http://commons.wikimedia.org/wiki/File:Mayon_pyroclastic_flow.jpg
Lake Toba
This image shows the site of one of the largest volcanic eruptions on Earth in the last several million years, Lake Toba on the island of Sumatra.
Sometime between 69,000 and 77,000 years ago the land you’re looking at was much higher up. It was sitting on top of a magma chamber and held as high ground from the pressure in the crust.
That magma chamber then exploded, pouring out hundreds of cubic kilometers worth of material. A portion of the magma chamber emptied, leaving a gap behind in the earth.
The overriding rocks collapsed downwards, falling to fill in the space. The wall you see in the distance is a fault scarp along the edge of the caldera produced during the Toba eruption.
The giant gap left behind when the volcano exploded has since filled with water to produce this lake.
-JBB
Image credit: Marc Veraart (Creative commons licensed) http://www.fotopedia.com/items/flickr-310613213
Driving through the ashscape Mt Sinabung has been erupting ash for several years (see our last at http://tinyurl.com/lrlf34p), prompting the evacuation of over 20,000 people from around the volcano, and dusting a much wider area with its grey tephra deposits. Here a brightly coloured bus is driving through a normally verdant jungle growing on the fertile volcanic soils. It's easy to get lost in beautiful pictures of eruptions, and the press reports casualties (so far mercifully few in this case) and ignore the countless daily changes that living through a long term eruption entails (as the inhabitants of Monserrat found out in the 90's). Loz Image credit: Yt Haryono/Reuters
Steam erupting from Sinabung, with trails from recent pyroclastic flows on the slopes
Sinabung’s Blast Sinabung volcano on the island of Sumatra had its first eruptions in recorded human history in the year 2010. Since then, it has been almost constantly erupting, producing a combination of explosive eruptions and lava domes that collapse once they become too steep. A week ago, on Monday the 18th of February, Sinabung produced its largest eruption of this cycle. It sent an ash cloud over 15,000 meters into the air and portions of this ash cloud collapsed under their own density, sending pyroclastic flows down the slopes. The eruption was large enough to be readily seen by satellites as shown in the second shot. Images shared by Planetlabs after the blast showed that the ashcloud itself was blown off to the west, while much of the volcano’s eastern slope was covered by the newly erupted pyroclastic flows -JBB Image credits: Agence France -Presse/Anto Sembiring http://bit.ly/2EOy3I7 https://earthobservatory.nasa.gov/IOTD/view.php?id=91753 https://twitter.com/simoncarn/status/966011654633873408 Timelapse video of the eruption: https://mobile.twitter.com/janinekrippner/status/965755599551156226
