A sill forms when basalt magma is injected between existing rock layers, spreading laterally. A buried lava flow forms when basalt erupts onto the surface as a lava flow that is later covered by overlying sedimentary deposits. The presence of igneous layering or contacts cutting across sedimentary layers indicates a sill, while uniform basalt lacking contacts with surrounding rock indicates a buried lava flow.
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Sill vs lava flow slides
1. Question 23 on study guide:
How can you tell if a layer of
basalt in sedimentary rocks is a
sill or a buried lava flow?
CLUES:
