Volcanic structures
Modified dome, Tick, Scalloped margin dome, Fluted dome
After the first example recognised, they are also called ticks. They are steep-sided domes whose shape has changed for some reason.
More than half of all steep-sided domes have lost their regular shape. Landslides have given them a scalloped, sometimes star-like outline. Domes with the following features count as “modified” compared with the original form:
– a crater with a large central vent; a gently convex surface, but several sharp fractures on the margin;
– a less steep margin but a flat surface; unusually flat, round or oval domes; a surface that sinks inwards, with a star-like outline formed by merging landslide scars. The example on the plate is of this kind. It is the type example of the so-called ticks, in Alpha Regio.
The debris heap of the slid material is often found near the domes (see the figure). Bulmer (1994) identified more than a hundred such debris aprons or debris heaps. The study showed that many of these debris deposits did not come from landslides at all. They resemble either debris avalanches on Earth or volcanic pyroclastic flows.
Domes can be renewed as they evolve, and the debris heaps show exactly this: old debris lies next to a young lava dome. So the old dome was eroded but later renewed.
