Lake Superior Agates
Forged by Fire
Liberated by Ice
Jim Miller
Minnesota Geological Survey
University of Minnesota Duluth
North America
(Laurentia)
in the Late
Precambrian
(1,100,000,000 years
ago)
_______________
THE
MIDCONTINENT
RIFT
_______________
An unsuccessful
attempt at
continental
separation 1.1
billion years ago
Plume
Head
200 Ma
Laurasia
Gondwanaland
125 Ma
Recent Example of
Continental Rifting
The Break-up
of Pangea
75 Ma
A. Rift Magmatism
Basalt
Flows
Gabbro
Crust
Mantle
Stages of the
Midcontinent Rift
1,109-1,086 Ma
Mantle
Plume
B. Sediment Infilling
Sandstone
1,090-900 Ma
C. Compression
1,000-900 Ma
Flood Basalts of the
North Shore
NORTH SHORE
ICELAND
HAWAII
Massive
Basalt
Amygdaloidal
Basalt
Sugarloaf
Cove
Amygdules – Gas bubble fillings
Chlorite & Zeolite
Epidote & Feldspar
Zeolite
Thompsonite
The Other Amygdule
Filling –
LAKE SUPERIOR
AGATE
Formation of
Lake Superior
Agates
Glaciers: Nature’s Bulldozer
Sculpting the Landscape of Minnesota
Deposits from Ice
Till
Deposits from
Meltwater
Outwash
Liberation of Agates Erosion of Weathered
Basalt by Glaciers
D. Surface Weathering
1,100,000,000 – 2,000,000 yrs
E. Glacial Erosion
2,000,000 –
12,000 yrs
F. Lake Superior
NE Minnesota
Today
Isle Royale
Keweenaw
Peninsula
Advance
of Glacial
Ice Lobes
over the
past
75,000
years
Where to
Find Agates?
Superior
Lobe Till
For More Information
John Green
Ron Morton
Scott Wolter