Glossarybase surge A turbulent density current that flows outward from the base of a partially collapsing vertical eruption column derived from a hydrovolcanic (phreatomagmatic) eruption; a type of pyroclastic surge.
bed form (or bedform) The surface configuration of a bed; also the three-dimensional configuration of groups of strata having geometric shapes that repeatedly occur in nature such as ripples, dunes, and plane beds.
bedset (or bedding set; also bed set) A sequence of beds with distinct internal structures, textures, colors or compositions that sets them apart from other sequences, usually bounded by unconformities, or by fallout layers.
blast A sudden, violent, overpressured explosion projected laterally or vertically. At Mt. St. Helens the blast was directed laterally and produced a high-velocity dilute pyroclastic density current, or pyroclastic surge.
blocking Deflection of the lower, denser parts of a pyroclastic density current (PDC) by a topographic barrier while the low-density upper parts of the PDC continue to travel over the barrier.
flow regime Hydraulic conditions of noncohesive flow of sand and silt that develop ripples, dunes, plane parallel beds, and antidunes. Progressive changes in bed forms occur as flow regime increases. Low-flow regime conditions form small scale ripples that progress to dunes; high flow regime conditions form plane beds and then antidunes.
flow transformation Reversible changes (in sediment gravity flows) between turbulent and steady flow related chiefly to particle concentration, thickness of flow, and flow velocity.
hydrovolcanic All volcanic activity resulting from the interaction between meteoric or connate water and lava, magmatic heat, or gases at or near the earth’s surface (also phreatomagmatic).
pyroclastic surge Low concentration, turbulent pyroclastic density currents (PDCs). Two kinds of pyroclastic surges are wet surges, having temperatures less than 100*C where steam condenses and the surge is a three-phase system with water drops, solid particles, and gas; and dry surges, which have mean temperatures greater than 100* C and form by (1) hydrovolcanic eruptions with a low water/ magma ratio, or (2) by magmatic eruptions that are driven solely by volatiles. Also see base surge.
transport and depositional system The transport system carries particles from source to area of sedimentation. The depositional system deposits particles and includes local movement of particles, such as movement of a mass of particles down a steep slope, after being carried to a particular locality by the transport system.