All Starting with Basalt
Long, very long time ago, before Paleogene epoch, the subduction of Eurasian and Australian plate has been started. This made a series of volcanoes in Bangka, Malaysia, and Riau Islands, according to Pupili (1973). The remnants of the volcano is a belt of tin-bearing granite which is mined extensively in some parts of Bangka, Belitung and Riau Islands. However, at the Paleogene (about 60 million years ago), something new was about to happen, south of the old contestant - Borneo.
Basalt lava made its way to the sea bottom, partly because of it is pushed by the subducting Australian Plate, and in another part it may be pushed out by convective flow of magma. The earliest known form of volcanism is Watuadeg pillow lava which crystallized deep in the sea about 56 mya. Basalts tends to crystallize at their time of extrusion because of their very hot temperature (1200 to 1400 K) , which does not allow magma to crystallize before it is extruded. More felsic (acidic) an igneous rock, more time passes between its crystallization and extrusion.
How is Watuadeg pillow lava looks like?
That's the basalt. Looks impressive, isn't it? With cracks in its surface caused by fast cooling, fragile glassy minerals in its surface and fast movement of lava flow. When it is cut latitudinally, we would see concentric rings which mark the changing of rocks composition ; glassy mineral abundance nears the surface and subhedral to anhedral minerals visible in inner part. Subhedral mineral is a mineral in which some of its sides is not perfectly shaped. Anhedral mineral is a mineral in which all of its sides is not perfect. Silicate minerals present in this rock are pyroxene and plagioclase. Pyroxene is black, and plagioclase is slightly transparent. The abundance of pyroxene and plagioclase is 5% and 15%, respectively. Olivine in this rock has been oxidated into Ferric Oxyhydroxide.
The picture below shows pyroxene and plagioclase in basalt rock specimen from Watuadeg. The big, shiny crystal near the left-center of image is plagioclase, more specifically labradorite. Labradorite tends to be more shiny than pyroxene, especially at the cleavage. Labradorite cleaves in good quality (smooth surface) at {001} but not do at {010}. Compared to pyroxene, pyroxene cleaves more clearly and better at that sides. {001} and {010} is Bravais - Miller indices, in which {001} corresponds to crystal face facing in z-direction, and {010} is crystal face facing in y-direction. In Bravais - Miller indices, the x-y axis is rotated 90 degrees counterclockwise.
The picture below shows pyroxene and plagioclase in basalt rock specimen from Watuadeg. The big, shiny crystal near the left-center of image is plagioclase, more specifically labradorite. Labradorite tends to be more shiny than pyroxene, especially at the cleavage. Labradorite cleaves in good quality (smooth surface) at {001} but not do at {010}. Compared to pyroxene, pyroxene cleaves more clearly and better at that sides. {001} and {010} is Bravais - Miller indices, in which {001} corresponds to crystal face facing in z-direction, and {010} is crystal face facing in y-direction. In Bravais - Miller indices, the x-y axis is rotated 90 degrees counterclockwise.
Geochemical and Tectonic Aspects
In Java Island alone, there are many places in which basaltic pillow lava exposed. I will divide them into two categories, the eastern and western part. The eastern part, comprising Watuadeg, Nampurejo, and some areas to the east of it are the place where pillow lavas exposed without significant deformation. The western part like Karang Sambung and other areas to its west are the place where pillow lavas exposed with significant deformation. The geochemical aspects of western and eastern part is also different.
The main geochemical difference between western and eastern part is the amount of Potassium Oxide of basaltic rock. This Watuadeg pillow lava contains 0.64% Potassium Oxide, while the Karang Sambung one contains 0.28%. Based on the amount of Potassium Oxide, scientists like Kuno (1960), Bronto (1994), Suparka and Soeria-Atmadja (1991) can determine that Watuadeg pillow lava is a back arc/subduction zone-related volcanism and Karang Sambung is an oceanic ridge-related volcanism.
Back arc volcanism is an evidence of calc-alkaline magma series. Quite a jargon? Yeah, I will explain here. Calc-alkaline magma series is a series of magma evolution ; the first volcanoes give us basaltic magma, then another volcano that erupts much-much later (million years afterwards) give us a more acidic magma, call it andesite basalt. Much later another volcano born and give us andesite. The latter volcanoes , but before the more recent Merapi or Merbabu, gives us diorite. Merapi and Merbabu alone gives us rhyolit. The changing of magma composition from basalt to rhyolit is a sign of silicate enrichment - caused by melting of subducted Australian Plate. The melting itself is promoted not only by heat, but also by seawater that contained in the subducted sediments or seeping in rock cracks/faults. Seawater also carry ions like Potassium to the mantle, that's why subduction related-volcanism typically have higher Potassium Oxide than the ridge-related one.
By being a back arc volcano, this Watuadeg pillow lava is relatively undisturbed by tectonic deformation. The three thing that may disturb him is magmatic intrusion, faulting and acidic water. But knowing that glassy minerals is mostly composed of stable and resistant silica, acidic water can only oxidize and dissolve the olivine inside by means of seeping into rock cracks - creating an interconnected void which later cause some part of rock to detach and lost into sediments around. This mechanism may explain why there is basaltic inclusion in pumiceous sandstone ~5 meters east of it.
The main geochemical difference between western and eastern part is the amount of Potassium Oxide of basaltic rock. This Watuadeg pillow lava contains 0.64% Potassium Oxide, while the Karang Sambung one contains 0.28%. Based on the amount of Potassium Oxide, scientists like Kuno (1960), Bronto (1994), Suparka and Soeria-Atmadja (1991) can determine that Watuadeg pillow lava is a back arc/subduction zone-related volcanism and Karang Sambung is an oceanic ridge-related volcanism.
Back arc volcanism is an evidence of calc-alkaline magma series. Quite a jargon? Yeah, I will explain here. Calc-alkaline magma series is a series of magma evolution ; the first volcanoes give us basaltic magma, then another volcano that erupts much-much later (million years afterwards) give us a more acidic magma, call it andesite basalt. Much later another volcano born and give us andesite. The latter volcanoes , but before the more recent Merapi or Merbabu, gives us diorite. Merapi and Merbabu alone gives us rhyolit. The changing of magma composition from basalt to rhyolit is a sign of silicate enrichment - caused by melting of subducted Australian Plate. The melting itself is promoted not only by heat, but also by seawater that contained in the subducted sediments or seeping in rock cracks/faults. Seawater also carry ions like Potassium to the mantle, that's why subduction related-volcanism typically have higher Potassium Oxide than the ridge-related one.
By being a back arc volcano, this Watuadeg pillow lava is relatively undisturbed by tectonic deformation. The three thing that may disturb him is magmatic intrusion, faulting and acidic water. But knowing that glassy minerals is mostly composed of stable and resistant silica, acidic water can only oxidize and dissolve the olivine inside by means of seeping into rock cracks - creating an interconnected void which later cause some part of rock to detach and lost into sediments around. This mechanism may explain why there is basaltic inclusion in pumiceous sandstone ~5 meters east of it.
What Happened Afterwards
After this extrusion of pillow lava, this pillow lava may be raised up well above his hometown in 2000 meters-deep ocean. This thrusting activity works for so long until this pillow lava is exposed above sea level, where acidic water is abundant. Acidic water carve this pillow lava and deposited the sandstone containing pumice and tuff. The pumice and tuff might be resulted from the slopes of a group of andesitic volcanoes 20 to 50 kilometers SE. The andesitic volcano might have erupted colosally due to very thick volcanic debris deposits. The colossal eruption is due to accumulation of gases during transition from basaltic to andesitic volcanism, which is also the quiet era of Javan Volcanoes. That colossal eruption was happened in Middle Miocene (around 16 to 11 mya).
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