DEFINING SUPER-ERUPTIONS AND EXPLORING THE LIMITS OF SUPER-ERUPTION SIZE

A super-eruption is defined as one with a magma yield > 10¹⁵ kg (Magnitude (M) 8). The term has mainly been applied to large-scale caldera and ignimbrite-forming explosive eruptions. The present suggestion is that it can be applied to all eruptions that released > 10¹⁵ kg magma. For effusive volcanism, present evidence suggests that individual eruptions of this size ( > ~ 370 km³ of typical basalt or 440 km³ of rhyolite lava) arise only during periods of LIP formation. This implies that lava super-eruptions have been confined to brief (c. 1 Ma-long) pulses during Earth history separated by many 10’s of millions of years, the most recent (c. 15 Ma) being the Columbia River basalts of the Pacific Northwest, USA. For all super-eruptions, we have only a rough idea of the true volume of individual eruptions, but the situation may be better for effusive than explosive products. Reasonably well-established lava-flow-field magnitudes up to ~ 7 x 10¹⁵ kg (M8.8; 2500 km³) are known from the Columbia River province; there may have been many eruptions > M9 (10¹⁶ kg) during formation of provinces such as the Deccan. Some LIPs also include lava flows of prodigious volume.

Explosive super-eruptions have probably occurred throughout Earth history, perhaps with a (poorly-defined) repeat period of ~ 100,000 to 1 million years, yet clusters of events only a few thousand years apart are recognized. One possible M9 example is known, which may be an approximate upper size limit. We have, however, only very rough idea of the true size of erupted magma batches in explosive super-eruptions due to complexities such as preservation of widespread outflow ignimbrite deposits (O), the amount of inter-caldera deposits (I), and the difficulty of recognizing co-ignimbrite ashes (A). Deposits are so widespread that considerable deposition of O or A often occurs over the sea, and the main products erode rapidly on land. The magnitude of such eruptions may be determined if O=I=A is a general rule. Test cases (Toba, Bandelier Tuff, Atana) suggest that this may often be the case. Effusive and explosive super-eruptions are so unusual compared with the magnitude range experienced during historic volcanism that the term ‘super’ may be reasonably applied. Both types raise interesting questions about the genesis and storage of magmas that feed these vast eruptions.