The main rock type of unit P18 is mudstone. This photo shows another rock type which occurs in minor amounts within unit P18.
The ovoid shapes in this photograph are pillows of basalt. Pillow lavas form when basic magmas are erupted under water. As the pillows are piled up on each other, the base of each fills in the gaps between the underlying pillows - thus most pillows have a pointed base and a convex top. This can be used as a way-up indicator: here the pillows indicate a younging direction towards the right.
There is also a close-up of this rock type.
- Look at the rounded shapes in this outcrop. Here we see them in cross-section, but in long section they have a roughly cylindrical shape
- If we could see them more closely, the rock is finer grained towards the edges of the round bodies.
- How exactly did these structures form? Some of the ideas on this in textbooks are outdated. For a good modern explanation, go to Volcano World, or the USGS. Also, watch these video clips of 'live' pillow lavas.
Scale: hammer head is 200 mm long.
Grid reference: 980
275