Our excursion takes us to a viewpoint at Mount Penanjakanat, an elevation of approximately 2646 meters abou sea level. It is reknowned for offering stunning panoramic views of the Tengger Caldera, including the iconic Mount Bromo, Mount Batok, and Mount Semeru. The viewpoint is accessible by 4x4 vehicles, which are typically used by tour operators to transport visitors from nearby villages such as Cemoro Lawang. Visitors can witness breathtaking sunrise views over the Tengger Caldera. The sunrise illimunates the landscape, casting dramatic shadows and highlighting the features of the volcanoes.
Fig. 1: Sunrise over the Tengger Caldera
The Tengger Caldera was formed through a series of geologial processes primarily involving volcanic activity. The region where the Caldera is situated is part of the Ring of Fire, an area known for its intense volcanic activity due to tectonic plate movements. The movement of the Indo-Australian Plate and the Eurasian Plate contributes to the tectonic activity in the region, providing the necessary conditions for volcanic activity. Over millions of years, repeated eruptions from a large stratovolcano in this region built up layers of volcanic material.
A caldera forms when a massive volcanic eruption empties the magma chamber beneath a volcano. With the support of the underlying magma, the surface collapses, creating a large, bowl-shaped depression. This is the process that led to the formation of the Tengger caldera.
The Caldera likely underwent several cycles of eruption and collapse, contributing to its current size and shape. Each major eruption would have emptied the magma chamber significantly, causing further collapses and enlarging the caldera. Within the Tengger Caldera, new volcanic cones formed from subsequent eruptions. The most notable of this are Mount Bromo and Mount Batok. The first is one of the most active volcanoes in Indonesia, continues to erupt periodically, contributing to the ongoing geological evolution of the caldera. These and the surrounding volcanic cones are composed of layers of volcanic ash, pumice, and other pyroclastic materials deposited by eruptions. The ongoing erosion and sedimentation processes also play a role in shaping the caldera's landscape.
Summary: The Tengger Caldera was built through a combination of massive volcanic eruptions and subsequent collapses of the magma chamber, followed by ongoing volcanic activity that formed new cones within the caldera. The interplay of tectonic movements and volcanic processes over millions of years shaped this impressive geological structure.
The Tengger Caldera was built primarily from stratovolcanoes. They are also known as composite volcanoes, characterized by their steep profiles and layered structures. These layers include:
1. Lava Flows
Basaltic to Andesitic Lava: The lava flows in stratovolcanoes can range from basaltic, which is relatively low in silica and flows easily, to andesitic which has a higher silica content and flows more slowly.
Fig. 2: Porous lava
Fig. 3: Lava with flow structures
Rhyolitic Lava: Occasionally, stratovolcanoes may produce rhyolitic lava, which is high in silica and very viscous, resulting in thick, slow moving flows.
Fig. 4: Different Tephra Layers
2. Tephra Layers (not solidified)
Ash: Fine particles under 2mm of volcanic glass and rock. They are ejected into the air during explosive eruptions and settle in layers.
Lapilli: Pebble-sized volcanic fragments between 2 - 64 mm, that fall to the ground and accumulate in layers around the volcano.
Fig. 5: Lapilli
Volcanic Bombs and Blocks: Fragments of rock over 64 mm, which are ejected during eruptions and can form layers of coarser material.
3. Pyroclastic Flows and Deposits
Nuee Ardentes: Hot, fast moving clouds of gas and volvanic matter that flow down the slopes of the volcano, depositing layers of mixed material.
Ignimbrites: Deposits from pyroclastic flows that solidify into rock, forming extensive sheets of welded tuff.
4. Tuff and Breccia
Tuff: Consolidated volcanic ash that has been compacted and cemented into rock, often found in layered deposits within stratovolcanoes.
Breccia: Rock composed of broken volcanic fragments cemented together by fine-grained volcanic ash or other materials.
Fig. 6: Volcanic stratifications, different colors and composition
5. Sedimentary Layers
Reworked Volcanic Material: Layers of volcanic material that have been transported and redeposited by water or wind, forming sedimentary deposits within or around the stratovolcano.
6. Volcaniclastic Deposits:
Lahar Deposits: Volcanic mudflows of a slurry of pyroclastic material, rocky debris, and water that flow down the slopes of the volcano and deposit layers of mixed materials.
These layers are built up over many eruptions, with alternating sequences of lava flows and pyroclastic materials. This stratified structure gives atratovolcanoes their characteristic steep, conical shape and makes them capable of producing both effusive (lava flow) and explosive (pyroclastic) eruptions. The interplay of these various materials and eruption styles contributes to the complexity and potential hazards associated with stratovolcanoes.
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Fig. 7: Rock behind the viewpoint (about 8 meters high)
Behind the viewpoint you find the rock shown in Fig. 7. There is a large number of volcanic deposits visible.
Task 1: Different deposits are hidden behind the yellow and blue areas. Explain their origins in general terms.
Task 2: In which area can tephra-layers (see listing) be identified ? Which ones are it ? What is their maximum size ?
Task 3: Where can sedimentary layers be seen ? Describe them according to their composition (color, thickness of the layers). How are the individual layers arranged ? Horizontally or from the left (Caldera side) to the right? Sloping or rising ? Can this observation be interpreted with regard to the previous state of the "original volcano" ?
Sources:
All Figures: Own