Upon deposition, surface tension of water cohesively bonds the particles and causes the deposit to behave plastically. Base surge deposits are moderately to poorly sorted, decreasing in grain size and thickness logarithmically away from the source, with local thickness controlled by topography. The deposits commonly display sedimentary structures resembling nonvolcanic structures but are unique to volcanic deposits. Hydroclastites are commonly composed of vitric shards, crystals and lithic fragments. Vitric shards from hydroclastic eruptions are characteristically blocky, with equant shapes and fracture-bounded surfaces transsecting sparse vesicles.
Vesicular shards are also present where vesiculating magma was quenched by water or steam.
These deposits may clasts intermediate between blocky fragments and those that are slightly vesicular with scalloped edges to highly vesicular clasts. Surfaces of many lapilli and bombs from hydroclastic eruptions have an irregular cracked structure variously described as cauliflower, crackled or bread crust. This texture resembles that of bread crust bombs, but forms by thermal contraction rather than from internal gas expansion. Cauliflower bombs are commonly dense. Accretionary lapilli are common in many hydroclastic layers due to abundant water and steam in the eruption column occuring in conjunction with abundant fine-grained tephra.