7. Semester, ETH Zürich


Julian Holz


Anne Holtrop



a relation of void and structure

The pumice stone is a pyroclastite formed by explosive, acidic to alkaline volcanism. The stone is not heavy and massive like other stones, but so light and porous, that it even floats on the water. The gas that emerges from the excavation forms interesting spatial structures, a relationship between void and structure. This relationship is translated into walls, ceilings and floor elements and in turn forms again a space. Threw this different architectural structures we can move physically or mentally. The bubbles form connections between the interior and outside. They give an indication of what awaits us inside and give views of the surrounding mountains and rising steam. The voids expand the space, form niches and in the cavities in the ground you can take a healthy Onsen bath. Furthermore they question the massiness of the elements and let themselves appear fine and light. The massive and at the same time light ceiling closes the room to the air and forms a protected exterior space. The light diffuses through the walls and creates a cozy, homely atmosphere in which you can rest.

Formation of pumice

The Pumice is a pyroclastite (questioned rock) formed by explosive, acidic to alkaline volcanism. For example, during a Plinian eruption where the high smoke towers are formed. The fragmenta-tion takes place at several hundred meters depth of the volcanic vent and at eruption velocities of 100 to 500 m/s. The volcanic eruptions are carried out at a depth of several hundred meters. The magma experiences a pressure loss in the vent of 15 to 20 Mpa to atmospheric pressure, 110 Pa, which causes it to expand strongly.

The sudden pressure loss releases stored gases such as carbon dioxide or hydrogen, which cause the magma to foam. Due to the rapid eruption, the magma in the volcanic vent does not cool down and therefore remains liquid. As soon as the magma leaves the opening, however, the spread stops and it cools rapidly from 750 – 850 Celcius to 25 – 125 Celcius.

This process produces extremely porous, glassy lava fragments of angular and sharp shape. In the dense eruption clouds, these angular scraps are rounded off by abrasion, resulting in a very fine pumice ash as well as the pumice stones.

The viscosity and the drop rate of the internal pressure are the main parameters for the size of the bubbles in the pumice. The pumice then deposits itself over 4 ways on the ground: by ash fall, ash stream, ground cloud and lahare, i.e. by mud or debris stream.

The technique of foaming is also used in the development of ultra-light materials. For example in the production of foamed gypsum or aluminium. This is done on the one hand using a blowing agent, such as titanium hydride in the case of foamed aluminium, and on the other hand by moulding materials that can be dissolved again. These materials have various advantages. They require less material, which makes them lighter and allows them to be used in a resource-saving manner. In addition, they are nevertheless strong, so that they can also be used as a supporting structure. The project investigated how concrete can be made lighter by moulding balloon-like structures. The round shapes of the cavities support the flow of forces through the structure and make it stable. Concrete can be saved and still contains strong structures with a special aesthetic.


The Japanese island of Kyushu is known for its volcanoes and geothermal fields. The Takigami geothermal field is located between the volcano Aso, one of the most active volcanoes on the island and the geothermal field near Beppu. In this field lies the Kyushu Electric Power Co. Hachobaru Geothermal Power Station, which uses the escaping steam and water to drive the turbine.
There are also many onsen and mud baths in these areas. The strong mineral water is said to have healing powers. Regular bathing is healthy for the body and lets you relax wonderfully.

Next to the Takagami Powerplant is the excursion destination „Komatsu Jigoku Hell“. A stony path leads to a tree clearing of different mud fields and escaping fumes. In some fields you can prepare the famous Onsen egg with a basket. There is also a part of Kyushu University in the neighbour-hood, which is investigating the existing geology in this area. The building joins a path in the „Komatsu Jigoku Hell“.