Like a cannonball: origin of dense spherical basaltic ejecta

Cannonballs are rare spherical to sub-spherical eruptive products associated with basaltic explosive activity. The origin of cannonballs is still debated and subjected to a wide spectrum of different interpretations. In order to better understand the physicochemical mechanisms controlling the formation of these explosive products, we investigated the textural and chemical features of cannonballs from the Cerro Chopo monogenetic volcano (Costa Rica). These explosive products ubiquitously show a core domain with coalesced bubbles (30–36% porosity) wrapped in a dense rim domain with small, isolated bubbles (20–27% porosity). Both domains are identical in terms of bulk rock composition and mineral chemistry and are portions of the same magma batch. Results from combined petrological and thermodynamic modeling indicate that a low-viscosity (~20 Pa s) melt containing early-formed olivine phenocrysts (~9 vol.%) ascended from storage at a decompression rate of 0.5 MPa s−1 until it reached a depth of 4.5 km (equivalent to a pressure of ~150 MPa). While rising from depth to 4.5 km, the melt underwent rapid decompression (0.5–2.6 MPa s−1) and H2O exsolution, driving late-stage crystallization of the groundmass. The fast ascent velocity (21–110 m s−1) while rising between 4.5 km and the surface induced turbulent (Re >103), annular flow development in the uppermost region of the conduit. We propose that cannonballs represent blebs of fluid magmas that underwent shear-driven detachment from the annulus of magma lining the conduit walls at depths lower than 4.5 km. The formation of such cannonballs is dictated by magma transport dynamics of low-viscosity, phenocryst-poor, and volatile-rich melts that rapidly accelerate within the shallow conduit.