Erosion by volcanic base-surge density currents: U-shaped channels
Small U-shaped channels eroded by volcanic base surges occur on the steep outer slopes of some tuff cones but rarely on the gentle outer slopes of tuff rings. This suggests that their development is a function of velocity. Corroborating evidence from the 1952 phreatomagmatic eruption of Barcena Volcano is as follows: U-shaped furrows were cut by volcanic density currents on the steep slopes of the volcano, but at the base of the volcano, where the velocity decreased, dunes were deposited with long axes perpendicular to the furrows. These dunes were similar to those deposited by base surges on the low slopes surrounding Taal Volcano, Philippines, during its 1965 eruption. To be preserved, U-shaped base-surge channels must be quickly buried by deposits from penecontemporaneous eruptions; otherwise, as ready-made avenues for run-off, stream action soon destroys them. Even if preserved on the steep sides of a volcano, however, exposures in cross section are rare because (1) the channels are filled with pyroclastic deposits or (2) later stream dissection parallels them without revealing the cross-sectional profile or else completely destroys them. An explanation of the origin of U-shaped channels stems from (1) the parameters of base-surge flow deduced from descriptions of historic base surges and their deposits, (2) descriptions of prehistoric base-surge deposits, (3) the development of U-shaped furrows by base surges at Barcena Volcano, Mexico, and (4) descriptions herein of prehistoric U-shaped channels and their depositional fill at Koko Crater, Hawaii. Fortunate circumstances of erosion on the side of Koko Crater provide excellent cross-sectional exposures along a short stretch of the shoreline. U-shaped channels eroded by base surges superficially resemble equilibrium semicircular channels cut by streams and mud-flows, but the profiles develop by a different mechanism. The fronts of advancing volcanic base surges develop a cleft and lobe pattern; the lobes appear to be individual turbulent cells that splay outward from the source. To carve a smooth U-shaped profile, the concentration of particles must increase gradually (perhaps exponentially) from the edges of the lobes to their central part, where the boundary effects are least and forward velocity is greatest. Small channels that are cut into erodible material by turbidity currents also have rounded cross-sectional profiles and may be cut by a mechanism similar to that ascribed here to base-surge flow.