Stone Insight: Volcanic rock, such as lava or tuff, with its many pores is very suitable infiltration trenches (tree swales), which have recently been used in cities to store rainwater

Basalt lava from Mendiger Basalt company.

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In the main article “Adapting to climate change: The city of Stein near Nuremberg designs new development areas according to the ’Sponge City’ concept’ we described the concept of rainwater management with infiltration trenches (tree swales) Here are a few details about the natural stones that are suitable for this purpose:

Typically, volcanic rock is used in infiltration trenches because it is very porous. In the case of basalt lava – which is a type of cooled, formerly glowing, and liquid magma – the porosity is around 20%. This means that one-fifth of the material consists of tiny air-filled cavities in the material.

They are large enough to allow water to penetrate quickly, despite its surface tension, and fill the stone pores before running out again.

In the case of tuff, the porosity can even be as high as 35%, as Rainer Krings of the Mendiger Basalt company says: “We are talking about a blotter effect.“ In material tests, his material has shown a water permeability of more than 450 mm per minute. This means that water in the stone advances by about half a meter per minute (about 30 m per hour).

The special nature of volcanic rock results from the way it cools: when the magma in the volcanic vent comes to the surface, the gases it carries escape. The cavities for the gases later form the pores in the stone.

If there is even an eruption, that is, if the gases expand suddenly because they are no longer under the pressure of the earth’s crust above them in the volcanic vent or near the earth’s surface, the lava explodes and is ejected from the mountain like fire.

In such a case, tuff is formed after cooling and solidification.

The important thing with infiltration trenches is to find the right grain size for the stones. This is because the rainwater should not simply run down between the grains of stone but be held up and penetrate the pores.

To prevent the interstices from becoming clogged with sand or dust, a cleaning manhole catches the water at the edge of the road or lawn. These filters, commonly referred to as gullies, are emptied at regular intervals.

Rainer Krings points out two other special features of volcanic rock:
* “Tuff has a high ion exchange capacity: it absorbs heavy metals and overfertilization and releases nutrients, so it can serve to improve the soil or strengthen trees.

* “There are attempts in Iceland, for example, to inject CO2 from the atmosphere into basalt lava in the sea, thus pulling the climate gas out of the material cycle. “

Two more words about the glowing lava that cools:
* It can also become pumice – which is so porous that it even floats. Spectacular are the so-called pumice rafts, which after eruptions of submarine volcanoes cover huge areas on the water surface and drift over long distances;
* when the glowing lava is quenched, so to speak, i.e., flows into the cold seawater at the surface, obsidian can be formed. This is a pitch-black, glassy material that can be easily cleaved to form razor-sharp edges. For many centuries, it was used for blades; however, its major disadvantage compared to steel is that it cannot be bent and breaks easily.

Mendiger Basalt (German)

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(04.09.2023, USA: 09.04.2023)