North Shore Volcanic Group - Ely's Peak Basalts

Information of the geolgy on this page is from the 2005 MN DNR Field Trip Guide

Authors Heather Arends, Chuck Corwin, John Heine, Richard Patelke, Rick Ruhanen and Mark Severson

These photos are from the Ulland Brothers Aggregate Quarry about 1.5 miles south of Interstate 35. Exposed in the quarry are four basalt flows from the lower part of the North Shore Volcanic Group called Ely's Peak Basalts. The rock is being mined for landscape and erosion control along the North Shore of Lake Superior as well as asphalt filler. Some of the rocks have hints of copper. Other rocks show amygdaloidal basalt ("air holes" in the basaltic rock) that have been filled with other minerals, mostly epidote, chlorite and feldspar. At the tops of some of the lava flows are pahoehoe (ropy) basalt rock structures.

In these photos we see evidence of earthquake activity with a break or fault in the rock. The rock has been displaced from the original position

In the photos right and below the shear face of the fault is visible. The formation of quartz, calcite and altered basalt give evidence to the change because of the movement of the adjoining rock faces. The contact pressure during the movement caused a change in the composition of the basalt and left a glossy surface marked with scratces indicating the direction of the rock movement.

The rocks here are part of the layerd series fo the Duluth Complex. It is in the 57th Avenue Quaary of West Duluth and is now part of the Duluth Park land. The quarry is accesible from the end of 57th Avenue by following a trail about 100 yards through some brush that is beginning to grow at the base of the quarry.

The quarry was mined in the early part of the 1900s for pier construction in Duluth Harbor. The rock (samples of which I have in my collection) is a variation of gabbro that contains more plagioclase than normal gabbro (normal gabbro should contain 65-70% plagioclase feldspar). A rock composed entirely of plagioclase is called anorthosite. This rock contains about 80% plagioclase so it is called gabbroic anorthosite. It is often the roof (top set of rocks) of most Duluth Complex intrusions (rocks from lava that came up and pushed into the existing rock) composing what is called the anorthositic series. The gabbroic anorthosite commonly display a foliated (folded) texture which is defined by the parallel alignment of the plagioclase crystals. Dark minerals of pyroxene, olivine and iron oxide are common as clots up to 10 cm across.


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