Náttúrufræðingurinn

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Náttúrufræðingurinn - 1987, Qupperneq 59

Náttúrufræðingurinn - 1987, Qupperneq 59
evolves into a magma chamber. If the sill is to expand and evolve into a chamber, magma must continue to flow into it (through dikes) from the reservoir below at sufficiently high volume rate. Ideal sites for high horizontal compressive stresses within dike swarms, hence for magma chambers, are rock formations where indi- vidual layers have different elastic prop- erties. The rocks formed during the Pleis- tocene have notably different elastic prop- erties and, when buried in the volcanic zones, they form more promising sites for magma chambers than the Tertiary rocks. This may explain why the number of cen- tral volcanoes (most of which are fed by shallow chambers) during the late Pleis- tocene is proportionally greater than dur- ing Tertiary time. 5) It is estimated that during individual volcanic events the volume of dike and eruptive material is, on average, 0.02% of the total volume of the corresponding magma reservoir but 0.05% of the total volume of a magma chamber. Provided a reservoir does not supply magma to a chamber during an eruption, or at a negligible volume rate, the volume of dike and eruptive material from a chamber in individual volcanic events is less than 0.25 km3 and usually less than 0.1 km3. 6) It is argued that many historical vol- canic events in Iceland, considered to be single eruptions, were in fact composed of many smaller eruptions. In such volcanic sequences the chamber is reloaded between events (eruption and/or intru- sion). The estimated volumes of historical eruptions in Iceland thus cannot be used to calculate the volumes of the corresponding chambers or reservoirs. It is concluded that, from the point of view of volume, a typical historical eruption could have been fed by a shallow magma chamber. 53
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