Abstract
Wherever effusive volcanism has occurred, there is usually also evidence of explosive volcanism. The boundaries between these two kinds of eruption are blurred, because even the sources of lava flows, regarded as the classic effusive landform, may exhibit explosive activity. In the absence of an atmosphere, the expansion of gas (derived from volatiles either dissolved in or encountered by the magma) is uninhibited once any bubbles have burst, and explosively ejected particles of all sizes follow ballistic trajectories once they are clear of any gas jet. An atmosphere impedes bubble expansion, decelerates smaller ballistic particles preferentially compared with larger ones, and introduces the possibility of a convective plume (i.e., an eruption column) able to loft fine particles to much greater heights than would be possible ballistically. Atmospheres also enable the formation of ground-hugging pyroclastic density currents that have no equivalents on airless bodies.
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