Characteristics of basalt lava · PDF fileCharacteristics of basalt lava flows Pahoehoe or...
Transcript of Characteristics of basalt lava · PDF fileCharacteristics of basalt lava flows Pahoehoe or...
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