Geologic description of rock from Mesabi Black Quarry - quoted from an e-mail recieved from Dr. Paul Weiblen, Professor
Emeritus, UMN Geology Department:
Black is one of the varieties of coarse-grained (>2mm)
igneous rocks that formed in magma chambers in the rift. These
chambers formed at different depths >1-2 km deep in the rift.
They were the source for the surface lavas. Feeders from the magma
chambers to the lavas are called dikes when near vertical and sills
near horizontal. As magmas cooled in the deep chambers a variety
crystallized and formed layers due to settling or flow giving rise
to “layered gabbros”. Because the Keweenawan rocks along
the North Shore are tilted to the east, erosion has exposed a cross
section of this system of rocks with lavas along the shore of Lake
Superior and the underlying intrusive feeders and magma chambers
away from the shore (the feeders are also found along the shore,
Silver Cliff is an example).
The exposed section also includes
sandstones that formed in shallow basins as rifting began,sandstones
with the lavas which represent lulls in the volcanism during which
sediments derived from the lavas were deposited, and sandstones
which filled in the rift after volcanism ended. The brown and buff
sandstones used for Pillsbury Hall across from CALA are an example
of the latter.
Separation of different minerals during settling and flow in the
magma chambers gave rise to a variety of igneous rocks iincluding:
anorthosites with >90% plagioclase and minor olivine, pyroxene,
and iron-titanium oxides (mafic minerals), anorthositic gabbros
with > 75% plagioclase and pyroxene dominated mafic minerals
(Mesabi Black), troctolite with < 75% plagioclase and olivine
dominated mafic minerals, gabbros with < 75% plagioclase and
pyroxene dominated mafic minerals.
As mentioned by Dr. Weiblen, the anorthositic gabbro from Mesabi Black Quarry is related to the Proterozoic rift system that extends beneath the College of Design site at the University of Minnesota and is the reason this rock was selected for use in SITE INDEX.