Mount Merapi erupts again

Mount Merapi on a clear day,
photo from
http://uktodaynews.com/9910/indonesias-volcano-mount-merapi-erupts-again/

Mount Merapi in Indonesia began erupting on October 25 (2010), causing 38 deaths including the "gatekeeper" of 220 years.  Merapi is a dangerous and heavily monitored volcano, with a long history of eruptions. It builds domes which collapse, effuses lava flows, pyroclastic flows and lahars.  It has been argued that a major eruption in 1006 AD weakened the Mataram civilization of Central Java, causing it to move from Central to East Java (see summary of 10,000 years of Merapi history in Newhall et al., Journal of Volcanology and Geothermal Research, v. 100, Issues 1-4, pp. 9-50, 2000). A distinguishing feature of Merapi, shown in the photo to the left, is a "somma", an opening in the summit to the southwest.  Such "sommas" usually indicate that a flank of the volcano has collapsed; the "amphitheater" at Mount St. Helens that opens to the north is such a feature. Hard evidence for the collapse such as obvious avalanche deposits has not been found and so the somma itself is the best evidence for this process. Eruptions in the 20th-21st century have been rather mild, and less explosive than those for which the field evidence suggests occurred between the 7-19th centuries A.D.  Dome collapse producing pyroclastic flows is so characteristic of this volcano that this style of eruption has been called "Merapi-type" at volcanoes over the world.  Volcanologists suspect that the 20th century "mild" activity of Merapi is just an interlude between larger events typical of the past record.  Precursors of such an event are not known, resulting in a dangerous situation for the hundreds of thousands of people who now live around the volcano.  80,000-100,000 live inside"The Forbidden Zone", an area of about 10 km radius on the south west side.  Several hundred thousand more live just outside this zone.  Volcanologists work with the populace to come to an understanding that prediction is not an exact science, that there will be false alarms, but that risk management is a necessary public good.  A documentation of historical eruptions between 1768-1998 is available in Voight et al., Journal of Volcanology and Geothermal Research, volume 100, issues 1-4, pp. 69-138, 2000.

From the U.S. Geological Survey

Santorini volcano

Santorini calder in the Aegean, Greece
Photo by Aster aboard the Terra spacecraft, NASA

Santorini Volcano erupted about 30 cubic kilometers of magma in ~1650 B.C. The ash column is estimated to have risen to ~36 kilometers (~22 miles).  The eruption of such a large volume left a large caldera (the whole image is 18 x 18 km). There has been much speculation that this eruption is the source of the myth of the lost land of Atlantis.  The largest island is There, the next largest is Therasia, and the small islands in the center of the caldera are the Kameni Islands, which are the site of ongoing mild activity.  The most recent eruption was phreatomagmatic eruption in 1950, in which phreatic activity preceded the effusion of lavas.  There may have been a precursor to this island as early as 197 BC, but the current island appears to have started in 46 AD.  Interestingly, it was described by a number of the Roman historians, including Pliny the Elder, who would die 33 years later in the eruption of Vesuvius.

Typhoon Megi causes multiple landslides in Taiwan

Landslide in Taiwan as a result of Typhoon Megi
Photo from BBC news

Landslides in Taiwan have stranded 400 drivers, buried a Chinese tourist bus carrying 19 passengers, and inundated a Buddhist temple, killing three people and leaving six missing.  Megi dumped 45 inches of rain to one county on the north east tip of Taiwan in just 48 hours. As of today (October 23) Megi has existed for XXX days since it first formed off of the Philippines.  The record for cyclone duration belongs to Typhoon John, which lasted 31 days in 1994.  John formed in the northeast Pacific, travelled west across the international dateline, and then recurved back and crossed it again.  Since convention indicates that cyclones be named "hurricane" if they are east of the dateline and "typhoon" if they are west of it, John's name changed twice! Megi attained typhoon status on October 14, and thus has a long way to go to challenge the duration record.  It is, however, one of the strongest storms to make landfall anywhere in the world after attaining "supertyphoon" status on October 16 (see earlier post).

Volcanoes and atom bombs

Photo of Matua volcano, Siberia, taken by NASA astronauts

Detonation of a thermobaric (fuel-air) bomb by the Russians.
Believed to be the largest of its kind ever detonated.

The New York times recently had a photo documentary of development of the atomic bomb. The photo below is of a "fuel-air bomb", often called a "vacuum bomb". This type of explosive uses oxygen from the surrounding air to increase the duration, and destructiveness, of the shock waves/fireball. They were first developed by the Germans during WWII, and are in use by modern military and in guerilla warfare (1993 World Trade Center bombing, 2002 Bali bombings).

Some volcanoes have eruptions that are "explosive" enough to produce shock waves.  These were first recognized by Perret during explosive eruptions of Vesuvius in 1906, and were later observed and analyzed quantitatively at Ngauruhoe, New Zealand (Nairn, I.A., Nature 259 (5540, pp. 190-192, 1976). On June 12, 2010, NASA astronauts were able to capture the photo at the left of an eruption of Matua volcano, Siberia.  You can't see the shock wave directly, but can see the hole that it punched through the cloud deck.  The rising ash plume has also pushed up a layer of moist air forming a pileus "cap cloud".  A pyroclastic flow is visible at the base of the column, descending toward 5 o'clock on the flank of the volcano.  Volcanologists use some of the same basic concepts about shock waves that were developed to analyze shocks from atomic bombs, and although we talk about the energy released in eruptions in terms of "kilotons" or "megatons". For example, I analyzed the energetics of the lateral blast at Mount St. Helens in 1980, and concluded that about 24 megatons of energy was released during the blast (Kieffer, S.W., Nature, 291, 568-570, 1981).

Hotel Montana, Haiti, and amplified seismic waves

Hotel Montana, Haiti, after the January 12, 2010 earthquake
Photo credit

The 2010 Haiti earthquake killed over 230,000 people, and caused extensive damage in the capital, Port-au-Prince.  Three factors have generally been cited as causes of the extensive damage: (1) the proximity of the city to the earthquake; (2) poor construction; and (3) liquifaction and soft-sediment amplification. These factors do not, however, explain why the relatively well-constructed buildings, such as the Hotel Montana, two United Nations buildings, and a number of substantial private residences sitting on a relatively hard bedrock ridge also suffered extensive damage. In a Nature article published on-line recently, a fourth factor has been recognized. Hough et al. (Nature Geoscience, October 17, 2010 on-inline publication) installed portable seismometers to monitor after-shocks and found that the topographic shape of the ridge amplified the ground motions were strongly amplified at frequencies between ~0.5-20 Hz, a frequency range that corresponds to the fundamental periods of 1-5 story buildings.  By modeling the ridge as a wedge with an internal angle of 135 degrees, and a width of 400 m, they were able to provide an analytic solution that an amplification of 2.7 for frequencies of ~7 Hz, in good agreement with the observations.  This work suggests that topographic effects need to be incorporated into microzonation maps that characterize seismic hazards.

Hail, hail, the ?'s all here!

Hail damage to a windshield.

Hail causes nearly $1 billion damage in the US each year, mainly to crops, but buildings, vehicles, and people are not immune to hail damage.  On April 30, 1988, a hailstorm in India is believed to have killed 246 people and 1600 animals. More intriguingly, it is believed that a huge hail storm may have killed at least 200 nomads in the Himalayas during the 9th century. The nomads are believed to have been Hindu pilgrims, and more than 600 bodies may remain buried in the ice.  Their skeletal remains are being disgorged from ice high in the mountains.  The skulls of these people showed short, deep cracks caused by round objects about the size of cricket balls.  This event may have inspired a traditional song of the Himalayan women that describes a goddess "so enraged at outsiders who defiled her mountain sanctuary that she rained death upon them by flinging hailstones 'hard as iron.' " A hailstone the size of a baseball falls at something like 100 mph, about the velocity of the throw of a major league pitcher. Hail can damage airplanes because they fly at speeds of 200-300 mph, and on April 4, 1977 a DC-9 crashed in Georgia when both engines of the plane ingested hail.  The plane crashed an burned, killing two crew members, 60 of 81 passengers, and 8 on the ground. Hail does not reach even higher speeds because of several factors: the turbulence of the atmosphere prevents a straight-line path from cloud to ground, and hailstones also bump each other and raindrops.  Hailstones deform during their descent because of friction with the atmosphere, and cannot be modeled as perfect spheres.  Small hailstones are commonly nearly spherical, but large hailstones are almost never spherical.

Mount Etna, Italy--a new model for why it exists where it does

Mount Etna, Italy  (photo from NASA)

Mount Etna, the largest volcano in Europe, has been active for the last half-million years.  It is located near, but not above, the Ionian subducted slab.  A number of theories have been proposed to explain its location and existence: magma migrates up through complex fault systems, aesthenopheric melting from Africa, a deep mantle plume. In a recent paper, Schellart proposes that upper mantle material is flowing around the southern Ionian slab edge and upward (W.P. Schellart, Geology, 38, 691-694, October 19, 2010).  This model incorporates some elements of the older subduction models, and presents a new fluid dynamic model based on experimental results.  In the la experiments, two viscous layers are contained in a rectangular tank.  A high-viscosity upper layer rests on top of a lower layer made of low viscosity glucose syrup.  A subducting slab is placed on the upper layer. At the start of the experiment, subduction was initiated by bending the model slab downward, and the slab is moved to simulate the history of the Ionian slab.  Fluid motions in the surrounding material were documented by the use of sheet lighting.  Schellart observed upwelling at the (scaled) distance of Mount Etna (a few hundred kilometers), and proposes that the melt originates at about a few hundred kilometers depth. The most rapid upwelling is in the middle to upper mantle, but slow upwelling is observed all the way to 660 km depth.

Super Typhoon Megi--What's a super typhoon?

"Super typhoon" Megi hit the Philippines today (October 18, 2010).

Photo above from: NASA. Storm track below from here.  Megi is a Korean word for catfish.  

Moisture rising off the warm ocean waters in the tropics causes a mass of cloud buildup (unless there are strong winds aloft).  As the water vapor rises, incoming air flows in toward the rising column of moisture. The incoming air is deflected to the right,  and the Coriolis force starts the whole mass spinning, somewhat counterintuitively, counterclockwise in the northern hemisphere. An excellent simple graphic can be found here.  The clouds aloft become larger and more organized, at some point reaching 39-73 mph, at which time the storm is designated as a "tropical storm".  In satellite views, an indication of the tropical storm stage is the appearance of the spiral arms that spin off from the main mass.  Because the air is spinning, the rotation keeps the moist air from collapsing into the very center, the so-called "eye" of the hurricane. The winds spiral around the eye, up the wall of the eye (the "eye wall"), and down the eye itself.  Thunder and lightning can be intense in the spiraling arms outside the eye wall.

Typhoons in the Pacific (or hurricanes in the Atlantic) are storms that originate in the tropics.  Two conditions there favor the creation of large storms: warm water (27C or warmer) and the strong Coriolis effect near the equator (10-30 latitude). The main condition that prevents development of typhoons is strong winds in the troposphere. In the northern hemisphere, the Coriolis force causes anything moving in the northern hemisphere to be deflected to the right (to the left in the southern hemisphere).


In the Pacific, the typhoons originate  in Micronesia, and regularly hit the Philippines as is the case with Typhoon Megi. In the Atlantic, most hurricanes originate off the west coast of Africa and then veer northward toward the Caribbean, the Gulf of Mexico, southern U.S., the east coast, and ultimately, eastern Canada. An excellent simple graphic of the formation conditions for typhoons or hurricanes can be found here.


If sustained winds reach 74 mph, the storm is officially designated as a hurricane or typhoon. They are typically about 300 miles across, and travel at 15-60 mph until they encounter land, where they rapidly dissipate into "mere" strong rain storms.  When Megi made landfall in the Philippines, it was more than 370 miles across.  It was the most intense tropical cyclone of 2010 to date.

On the Saffir-Simpson Hurricane scale, Typhoon Megi is a catagory 5, with damage predicted to be "catastrophic".  Central pressure in the eye can be <920 mb (compared to >980 mb for smaller category 1 storms, wind speeds >155 mph, and storm surges >18 feet. Hurricane Mitch in October, 1998 left over 9,200 people dead in Honduras, destroyed over 150,000 homes, and caused ~$1 billion in crop damage. Peak sustained winds in Megi have been reported at 180 mph, with gusts estimated to 220 mph.  For those interested in aeronautics, these peak winds are Mach number 0.3!!

Katabatic winds on Mars

Figure: This view of the north polar region of Mars shows the icy polar cap, about 1,000 km across.  The large canyon (arc arc)  in the lower right is Chasma Boreale which is about as long as the Grand Canyon, and up to 2 km deep.  The dark spiraling bands are troughs. Credit: NASA

The north polar region of Mars contains spiraling troughs up to 10 km in width and 1 km depth.  Winds spiral out from the north pole and in many places cross the troughs at nearly right angles. (In other places, such as the large Chasma Boreale) they flow down the canyons.  By comparison with winds on earth that flow down off high terrain, the winds on Mars have been called katabatic winds. Simulations suggest that horizontal wind velocities in some places on Mars may reach 30 m/s.  In the second figure here, streaks descending the slopes of one of the spiraling canyons are taken to indicate winds pouring over the rim of the canyon. They are eroding grooves into the slope and entraining material, presumed to be a mixture of ice and dirt.  The grooves are being carved by longitudinal vortices in the boundary layer of the winds.  The spacing of the grooves--hundreds of meters--suggests that the boundary layer is hundreds of meters thick (approximately two times the spacing of the grooves). Near the base of the canyon, the winds decelerate--possibly through a hydraulic jump--and the organized structure of the vortices is disturbed.  The entrained material is being dumped out of suspension as indicated by the turbulent clouds.

Added on December 18: Here's a New York Times article about katabatic winds in the Antarctic.