Unconventional filling dynamics of a pit crater

The rise and fall of magma columns is a process commonly observed in volcanoes hosting lava lakes and serves as proxy of magmatic reservoir pressure and warning for potential eruptions. This dynamic process typically involves the filling and emptying of a “pit crater” - a cylindrical depression usually formed by one or more collapses of a structure lying above an emptied surficial reservoir. Between 2016 and 2020, the 300 m deep pit crater located within Nyamulagira volcano (Democratic Republic of Congo) underwent an unusual filling dynamic. In May 2019 while filled to about 90% the crater floor collapsed by 90 m within two months, followed by the rapid emergence of a sub-circular solidified block overlooking the crater rim in less than 6 months. Using numerical simulations of models based on thermal energy and constrained by multiparametric data, we account for the incremental filling of the crater by successive intra-crateric lava flows and the subsequent collapse of the crater floor. We further characterize this unconventional filling mode based on thermal budget considerations.