Eruptions of gas or steam and non-juvenile debris are common in volcanic and hydrothermal areas. From reports of non-juvenile eruptions or eruptive sequences worldwide, at least three types (or end-members) can be identified: (1) those involving rock and liquid water initially at boiling-point temperatures ("boiling-point eruptions"); (2) those powered by gas (primarily water vapor) at initial temperatures approaching magmatic ("gas eruptions"); and (3) those caused by rapid mixing of hot rock and ground- or surface water ("mixing eruptions"). For these eruption types, I compare the mechanical energy released, final temperatures, liquid water contents, and maximum theoretical velocities by assuming that the erupting mixtures of rock and fluid thermally equilibrate, then decompress isentropically from initial, near-surface pressures (<10 MPa) to atmospheric pressure. Maximum mechanical energy release is by far greatest for gas eruptions (<~1.3 MJ/kg of fluid-rock mixture)--about one half that of an equivalent mass of gunpowder, and one fourth that of TNT. It is somewhat less for mixing eruptions (<~0.4 MJ/kg), and least for boiling-point eruptions (<~0.25 MJ/kg). Final water contents of erupted boiling-point mixtures are usually high, producing wet, sloppy deposits. Final erupted mixtures from gas eruptions are nearly always dry, while those from mixing eruptions vary from wet to dry. If all enthalpy released in the eruptions were converted to kinetic energy, the final velocity (vmax) of these mixtures could range up to 670 m/s for boiling-point eruptions and 1820 m/s for gas eruptions (highest for high initial pressure and mass fractions of rock (mr) near zero). For mixing eruptions, vmax ranges up to 1150 m/s. All observed eruption velocities are less than 400 m/s, largely because (1) most solid material is expelled when mr is high, hence vmax is low; (2) observations are made of large blocks whose velocities may be less than the average for the mixture; (3) heat from solid particles is not efficiently transferred to the fluid during the eruptions; and (4) maximum velocities are reduced by choked flow or friction in the conduit.
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