Characteristics of basalt lava flows

Pahoehoe or ropy lava surface- lava cools to form a surface skin. As the lava continues to flow below this surface,the skin is wrinked.

Aa or blocky lava- the thick, cooled surface breaks up with continued sub-surface flow, breaking it up into massive blocks.

vesicular lava

columnarjointing

pipe vesicles

water rising as steaminto molten lava

lavaadvance

Columnar structures

cooling centre

joint at right anglesto contractiontension

joints most commonlyform hexagonalcolumns

Cooling causes contraction and contraction producesjoints.Joints tend to form at rightangles to cooling centresevenly distributed over thecooling surface.Under pressure of continued lava movement the columns may be bent over.

Pillow lavas

Where basalt lava surfaces below water it congealsvery rapidly to form ‘pillow’ or ‘bolster’ forms. These will have solidified in less than 5m, but may spawnsecondary smaller flows. New ‘pillows’ pour out on to the top of older ones creating characteristic crosssections which can be used as a way-up criterion

cuspate pointed base to flow

convexupper surfaceof flow

Basalt glass on surface fine grained exterior

concentric joints

some may have a central tube or jasper infill

coarserinterior

radial joints

n.b. most are 15 to 30cm across

contraction towardsa cooling centre