The 6th century eruption likely started with widespread tumescence (enlargement) of the ground
over an area of a thousand square kilometers around Proto Krakatau, occurring over a period of years and reaching a magnitude
of several meters or more near Proto Krakatau. This rising of the ground was likely to have proceeded so slowly that people
in the area might not have noticed it, but they would have noticed the ever increasing occurrence of small earthquakes that
perhaps were felt every few weeks in the year before the giant eruptions and reached nearly continuous shaking in the weeks
before the eruption.
The following illustrations were generated by the volcanic eruption simulator, Erupt3, and they depict cross-sectional views of the Sumatra-Java island arc structure, built up by volcanism over thousands of
years, consisting of layers of lava, pumice, and ash (red, magenta, and green colors). Proto-Krakatau is shown as a cone-like
structure forming a land area connecting Sumatra to the west and Java to the east.
FIG 7.1: Location of the volcano.
The next series of pictures are snap shots of a computer simulation of eruptive events. FIG 7.2: The first
eruptions were of a type known as “Phreatic,” involving no fresh magma, but rather dust and rocks blown out of
the crater of Proto Krakatau by up-rushing steam, which formed by boiling of groundwater below the volcano by the heat of
magma as it approached the surface.
FIG 7.2:
FIG 7.3: The start of the truly giant eruptions were marked by appearance of stiff, viscous, 900
degrees C magma at the surface that was so highly gas-charged that it exploded out of the volcano with the force of hundreds
of nuclear bombs. This explosion, known as an “Ultra Plinian” eruption (after Pliny the elder who witnessed much
smaller but similar in appearance phenomena during the AD79 eruption of Vesuvius), drives pumice, ash, and gas as a jet out
of the volcano, the jet rising to about 50 km into stratosphere. The speed of up-rushing gas, pumice, and ash likely exceeded
the speed of sound, and the jet pushed a shock wave into the atmosphere that raced ahead of the jet at a speed of nearly twice
that of sound (~650 m/s). The jet emplaced pumice and ash into the atmosphere at a rate of 100 to 1000 million kg per second,
much of it falling from high in the sky to blanket surrounding lands under meters of pumice and ash (light blue layer in illustration
below). The jet of pumice, ash, and gas was so hot that it continued to rise as a buoyant plume high into the stratosphere,
where it was entrained in stratospheric winds circling the earth from west to east.
FIG 7.3:
FIG 7.4: As the Ultra Plinian eruption continued the land under the volcano began to subside, gradually
falling into the evacuated part of the magma chamber several km below. With this subsidence, fractures opened around the volcano,
allowing sea water to rush into the magma conduit that connected the magma chamber and volcano. This sea water was instantaneously
vaporized with additional explosive force, resulting in an eruption called “Phreatoplinian,” which connotes the
combination of Plinian eruption with phreatic (near surface) water. The addition of vast quantities of water vapor to the
eruption column caused it to collapse on itself, spilling out clouds of ash, pumice, and steam that raced across the surrounding
country side at speeds of up to hundreds of meters per second, slowing down to tens of meters per second after traveling about
10 km from the volcano, but continuing to engulf the country side for up to 50 or more km from the volcano. These “pyroclastic
flows” completely wiped out all vegetation and any dwelling structures, leaving nothing alive in their wake.
FIG 7.4:
After days to weeks of these Phreatoplinian eruptions, the volcanic fury subsided, leaving a collapse caldera
filled by the sea, separating Sumatra from Java, with vestiges of Proto Krakatau barely peeking above the newly formed Sunda
Straits
FIG 8: The following before and after cross sections show how the earth’s surface collapsed downward
during the 6th century caldera eruption, producing the Sunda Straits, where the Proto Krakatau volcano collapsed
below sea level. Note how rock layers and units, shown in various colors, have been displaced downward after the eruption,
and a gray-colored layer blankets the topography denoting the pyroclastic flow deposits left by the eruption. The simulated
caldera is over 40 km in diameter.
FIG 8
FIG 9: The next illustration shows the 3-D plan view of the simulation results, a view from high altitude
above Sumatra and Java. The simulation shows a rough portrayal of the Sumatra-Java island land mass extending from west to
east, with the volcanic cone of Proto Krakatau near the center. This island was constructed by the simulated growth of overlapping
volcanic cones and lava plateaus, but does not exactly match the coastline of pre-6th century Sumatra and Java.
Note that after the 6th century eruption, Proto Krakatau has mostly disappeared, falling below sea level, replaced
by a body of water separating Sumatra and Java called the Sunda Straits. From the cross section views above, one can see that
the straits are shallow, just covering the sunken volcano. Below, parts of the volcano still protrude above sea level, leaving
islands in the straits that are remnants of Proto Krakatau. Perhaps the southern island in the view below is where modern
Krakatau will form. Since the 6th century erosion has likely caused some of these islands to fall below sea level,
while new volcanic extrusions along the caldera margins have produced new islands, including the present-day Krakatau, which
has grown along the southern margin of the caldera. One may also notice a slight change in the coastlines of Sumatra and Java
outside of the caldera limits (dotted curve); this change reflects a regional subsidence after the caldera eruption. Although
these simulation results are not an exact portrayal of the Sunda Straits, they serve to illustrate the general development
of land masses.
FIG 9:
Proto-Krakatau - Refers to the volcano before it destroyed itself in the eruption of 1883. The new volcano
was created later in around 1927, and was named Anak Krakatoa, meaning ' The Child of Krakatoa.'
NB: I refered to Krakatoa as Proto-Krakatoa as that is how the volcano is labled in the diagrams. It would
have been confusing if i had referred to it as Krakatoa.
