The rocks of the North Shore of Lake
Superior record the last period of volcanic activity in Minnesota.
This volcanism occurred 1.1 billion years ago when North America
continent began to rupture along a great rift valley, which
extended from the Lake Superior region southwest to Kansas. As this
rift valley opened, basaltic lavas erupted into it intermittently
for about 20 million years, accumulating to a thickness of up to 20
kilometers in the Lake Superior region.
With each eruption, red-hot lavas
fountained from kilometer-long fissures for up to decades at a
time, flooding over large areas of a barren landscapes. Flood
basalt eruptions typically followed one another in geologically
rapid succession, but at times there were significant intervals
(thousands to millions of years) without volcanic activity. During
such intervals, streams and rivers flowing over and eroding the
volcanic terrain would deposit sediments into lakes in low-lying
areas. When volcanic activity resumed, these sediments could in
turn be buried, heated, and compacted by lava flows and transformed
into sedimentary rocks.
An example of such a geological cycle of
eruption, sedimentation, and renewed volcanism appears in the cliff
face across the highway from this
marker. Beneath a dark-gray basalt flow is a reddish, thinly
bedded siltstone, sandstone, and shale formation. Beneath
these sedimentary rocks is another lava flow, which exposed in
the creek bed of Cut Face Creek just north and down the hill
from this road cut. This thickness indicates a prolonged lull
in volcanic activity, perhaps lasting several million years.
The broken-up and mineralized character of the basalt as the
left side of the cliff face resembles features observed when
lavas explosively encounter standing water. This and the fine
sediments beneath the lava suggest that a shallow lake may
have existed in the area at the time of renewed volcanism.
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2) What evidence suggests that the bottom layer is sedimentary in
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