What is this Rock, 3

I just returned from a weekend up on the North Shore. K and I went up and stayed at Cove Point Lodge, while I took a 3-day course on North Shore Geology, focusing on the Beaver Bay area. The course was taught by Jim Miller, a retired Minnesota Geological Survey person, and an emeritus professor from UMD: he was a great person to teach the course, both because he was a good and enthusiastic instructor, and because he has spent his career focused on the northern Minnesota in general, and the North Shore in particular. Many of the geological maps of the north shore include his name as cartographer.

This is a laundry list of what I learned — however, there are problems with the images which I have yet to fix.

Rock-related

Gabbro. Gabbro consists of olivine, plagioclase and pyroxene and has visible crystals (pheoncrysts) due to its slow cooling (10,000 to 50,000) years in the crust. It is strong and relatively erosion resistant, perhaps because of its interlocking crystals and lack of vesicles. When gabbro is formed from magma that comes straight from the crust, it is relatively primitive or undeveloped: it consists of olivine and plagioclase crystals (which have a higher melting point) in a matrix of pyroxene; when the gabbro is at the surface and weathers, it displays gryy-white splotches (that can flash in the sunlight when the light catches the crystal faces) as the pyroxene weathers.

 In looking at weathered gabbro, the brown material is altered olivine, the white is plagioclase (altering into kaolin), and the black is pyroxene 

Here is gabbro with laminated foliation due to aligned pyroxene crystals… I find this hard to see:    

Basalt. Basalt flows have an anatomy: a vesicular top; a massive interior; and a bottom, often with pipe vesicles. Basalt can be any color from black to brown to red to grey, depending on its degree of weathering and factors like amount of iron. When its vesicles are filled with minerals to become amygdale, as they are exposed to the surface freeze-thaw will pop out the amygdules. If the basalt is more viscous and flowing, there may be stretched amygules – these will have circular cross sections but with otherwise be ellipses, all oriented in the direction of flow. Basalt may also exhibit oxidation banding – very thin parallel cracks that indicate direction of flow. 

Ophitic textures in basalt and gabro indicate that the gabro is quite ‘primitive’ – i.e. it came straight from the mantle, and did not undergo much in the way of fractional crystallization, crustal melting, or interaction with other magmas. The ophitic ‘spots’ are weathered plagioclase crystals that are turning white (into kaolin). Basalt and gabro may also exhibit  liesgang banding – a secondary sedimentary structure in which (presumably) precipitating bands or rings occur in repeating concentric patterns. 

Rhyolite is not a lava, per se. It explodes out of the earth, and goes from magma to particles, and forms a granular flow of hot particles. It is a sort of homogenized tuff that is subjected to more pressure than tuff due to greater thickness. It will lack vesicles, because it is never liquid and the gas has all escaped during the explosive phase. It may contain feldspar and quartz crystals, but these will be as crystals in the matrix, and not in vesicles. Weathered rhyolite may exhibit rectangular pits where the feldspar crystals have weathered away. 

Although rhyolite is in theory lighter in color than basalt/gabbro, in fact it can be any color.

Anorthosite. Anorthasite is gabbro but missing olivine and pyroxene — it is mainly (calcium) plagioclase.  Unweathered  Anorthasite is very dark, but the plagioclase weathers to a light color.  Plagioclase is about same density as erupting magma; so as magma ascends, olivine and pyroxene crystals can be left behind in the magma chamber and the mostly-plagioclase anorthite can accumulate at the mantle/crust boundary, forming a massive cap that was eventually broken up with its blocks incorporated into basaltic mantle-eruptions.  [Flesh out]

Felsite. At Ionas beach we saw felsite, which is an intrusive rock that under other circumstances would have erupted as rhyolite [not totally certain of this]. From Wikipedia: “Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. … Color is generally white through light gray, or red to tan and may include any color except dark gray, green or black (the colors of trap rock). This rock is typically of extrusive origin, formed by compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. Dendritic manganese oxides such as pyrolusite and/or iron oxides such as limonite may precipitate along rock crevices, giving some rock chunk surfaces multicolored or arborescent patterned textures. ”

Landforms, in general

• Points and Bays. Looking at the shore, every point is the interior of a lava flow; every bay is a weathered top-of-flow. The landforms of the North Shore as we see them today are recent: The glaciers rejuvenated the landscape, scraping away weathered basalt and other sediment, and most of the north shore rivers were cut by sediment-laden glacial melt thousands of years ago; since then relatively little erosion has occurred (except via freeze-thaw), as water-only has greatly reduced erosive power. 
• Glaciation in MN: Superior, Rainey and Des Moines (granbug, st louis, xxx sub-lobes) lobes.
• Glacial Chattermarks. In addition to glacial polish and striations, one can also see chattermarks, where the glacier was grating along the surface and creating a sort of torn-up or gravel-surfaced texture on the basalt. 

• Spheroidal Weathering

North Shore Locations

• The Beaver bay complex is very complicated: It is a chaotic mishmash of volcanics and gabbros. Going N and NW from north shore is going back in time, since the layers tilt towards the lake/MCR. 
• Silver creek. Tunnel is through gabbro. Just NE of the tunnel you can see diabase intrusion into gabbros along fault zone.  You can also see lighter colored Andesite flows showing convoluted banding; some of convolution could be due to metamorphosis due to heat from intrusion.

• xxx

Miscellaneous

• Greenstone is highly metamorphosed lava — in particular has chlorite.  Ely flows 2.7Ga. 
• Snowflake obsidian — the snowflakes are crystallizing feldspar
• Joints imply a regular pattern — fractures are all directions 
• Island Arcs. Oceanic-oceanic subduction produces island arcs. 
• Wilson Cycle. Supercontinents tend to cause mantle overheating and lead to new mantle plumes. The Wilson Cycle — supercontinent formation – mantle overheating — consequent fragmentation: 500 Ma. 
• Duluth complex is second largest gabro intrusion on earth.

Northern MN, etc., History

• 2.7 Ga – granite greenstone terraine produced by kenewa orogency [check – not sure on this]
• 1.11Ga: Mid-Continent Rift. Middle of North America (Laurentia, I believe) develops a rift and lava erupts for 250Ma over about 200 K. Overall there were more than a thousand lava flows, producing a mass of lava equal to about half the thickness of the crust. 
• The Greenville orogeny – the collision of Amazonia with the east coast of Laurentia –closes rift after 25 Ma at 1.086 Ga.
• The thick mass of basalt and gabbro caused ongoing downwarping of the crust, creating a depression that filled in with 15 K sediment over the next billion years. (Note that erosion in the absence of plant life would have been considerably more active than we experience today.)
• At 900 Ma: the faulting caused by the downwarping stopped; it had created a grabben with an elevation of about 5K. This is why you can see deep volcanics adjacent to shale at Amnicon falls. 
• The next billion years was geologically quiescent. Weathering and erosion caused the basin to fill in, and the landscape would have been mostly flat, the volcanic bedrock buried under regolith.
• About 2 Ma, the Pleistocene Ice Age began, and glaciers advanced and retreated more than 30 times, the last retreat being about 10Ka. The glaciers moving south encountered the sedimentary rock filling the rift basin, and as it was considerably softer, dug it out; as the glaciers melted, they generated sediment-ladder rivers that carved out the north shore gorges.