Dense-rock equivalent

The Dense-rock equivalent (DRE) is a measure of the ejection volume of a volcanic eruption , which is also translated as "bubble-free magma". The deposition products of different densities from explosive volcanic eruptions are traced back to the bubble-free starting magma in order to be able to better compare the ejection volume of different eruptions. The unit is usually cubic kilometers (km³).

calculation

The amount of material emitted during volcanic eruptions can only be determined by measuring and estimating the pyroclastics deposited . The material can be in a loose form ( tephra ) as pumice or volcanic ash , as a compact form such as tuff , or as a fused form such as ignimbrite .

All of these forms have their own density , ranging from 240 kg / m³ for pumice, over 1000 kg / m³ for volcanic ash and other tephra, to 2400 kg / m³ for ignimbrite. The respective DRE volume is calculated using the following formula:

${\displaystyle DRE(km^{3})={\frac {TephraVolumen(km^{3})*TephraDichte(kg/m^{3})}{MagmaDichte(kg/m^{3})}}}$

The magma density can range from 2200 to 2800 kg / m³ depending on the magma type. Since almost every explosive outbreak results in different forms of deposits, their DRE volumes must be determined separately and then added.

Magnitude

Up until now, the volume of the deposited material was used to derive the volcanic explosion index without taking into account the different density, which made it difficult to compare different eruptions.

Now the magnitude can be calculated from the mass or directly from the DRE:

${\displaystyle M=\log _{10}([eruptierteMasse(kg)]-7)\,.}$

${\displaystyle M=\log _{10}([DRE(m^{3})*MagmaDichte(kg/m^{3})]-7)\,.}$

Example Pinatubo

The eruption of Pinatubo in 1991 is quite well documented and should serve as an example here. The eruption left two large forms of deposits that form the basis of the calculation here. First, the fall deposits of pumice and volcanic ash, and second, the deposits of pyroclastic flows .

deposit	volume	density	DRE
Pumice	3.4 to 04.4 km³	1100 kg / m³	1.6 to 2.0 km³
Pyroclastic currents	5.0 to 06.0 km³	1000 to 1300 kg / m³	2.1 to 3.3 km³
total	8.4 to 10.4 km³		3.7 to 5.3 km³

The magma density of Dazit was set at 2400 kg / m³. The magnitude of 6.0 was calculated from the rounded sum of 5 km³ DRE.

Web links

• Calvin F. Miller and David A. Wark: SUPERVOLCANOES AND THEIR EXPLOSIVE SUPERERUPTIONS (English; PDF, 992 kB)

Individual evidence

1. ^ DM Pyle: "Sizes of volcanic eruptions." In Encyclopedia of Volcanoes. Academic Press, London 2000, ISBN 0-12-643140-X , pp. 263-269
2. ^ Tephra Falls of the 1991 Eruptions of Mount Pinatubo
3. ↑ Pyroclastic Flows of 1991 Eruption of Mount Pinatubo