College of Architecture and Landscape Architecture
University of Minnesota, Minneapolis, MN 2001-5

Site Visit to Mesabi Black Quarry, Lake County, MN, Nov., 2003

Notes from Image above (quoted from an e-mail recieved from Dr. Paul Weiblen, Professor Emeritus, UMN Geology Department):

Mesabi 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 when near horizontal. As magmas cooled in the deep chambers a variety of minerals 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 interbedded 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 colored 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.



Quarry Block Diagram; Survey Instruments; Quarry Excavation 1; Quarry Excavation 2

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