ALBERT

Description: Pressure ridges are surface features on basaltic lava flows and, as with other surface features, they may be related to the emplacement of a flow and the rheological properties of the lava. Since many ridges are of sufficient size to be detected on high resolution orbital images, an understanding of pressure ridges could provide a means for interpreting volcanic flows on other terrestrial planets. Some proposed formation mechanisms are reviewed and three different types of pressure ridges are identified on the basis of morphology. Type 1 ridges are the most common and are associated with multiple flow unit pahoehoe in which the ridges are embayed by secondary toe fed lava. They tend to be restricted to wider sections or margins of the flow and to be oriented longitudinal to flow direction; however, oblique or transverse orientation is not uncommon. Bulbous squeeze ups are common within cracks and may reflect relative timing of crack formation. The interior structure of type 1 ridges consists of an upper slab section which generally contains columnar joints and a lower massive section with an irregular surface. This basic distinction may mark the thickness of the surface crust when ridge formation was initiated. Type 2 ridges occur in association with type 1 and are very similar with the exception of the secondary squeeze out material. Instead of only filling cracks, the secondary material on these ridges originated from underneath a thin crust and flowed as toes or channels from the top and sides of the ridge. Type 3 ridges have much steeper sides (almost vertical at the top) than the other types. Medial cracks are very wide and the crack walls are convex upward. No squeeze ups are present. The main difference between type 3 and the others may be reflection of viscosity.