When geologists reconstruct the geologic history of an area, why is it imporant for them to defferentiate?

Question

between a sill and a lava flow? How could you tell the difference between a sill and a lava flow at on outcrop if both structures consisted of bassalt? What features would you look for in an outcrop to positively identify the structure as either a sill or a lava flow?

Answer 1

there is usually evidence of overlying sediment found near the sill, although little to none is found near surface flows. sills often have metamorphic rocks found near them as they form deep underground, as the heat would have changed the structure of the sediment. surface flows are covered by 'cold' sediment (unchanged rock).

Answer 2

If the basalt was transformed into metamorphic rock on BOTH sides of the hardened magma, it was a sill because it broke in between the layers of deposited rock.

If the rock was transformed into metamorphic rock ONLY under the hardened magma, it was a lava flow because the lava would have cooled before another layer of rock was deposited.

Hope I helped. :)