Four Billion Years and Counting: Canada’s Geological Heritage. Produced by the Canadian Federation of Earth Sciences, by seven editors and dozens of authors. 2014.
November-December, 2024.
I am reading this with CJS. It is a nice overview of regional geology, and it is nice that all the examples come from Canada, and at least some of the discussion is relevant to Minnesota Geology. The book is notable for its beautifully done pictures and diagrams.
The first part of the book, Foundations, is an introduction to general geological concepts. For CJS and I this will be largely review. Here I will preterit any summary, and simply list some of the points that stood out because they filled in a gap, or provided a different perspective.
FOUNDATIONS
C1: Miscellaneous Points
- Polygonal jointing occurs when basalt flows stop moving before they cool.
- Granitic magma forms at 600-900°, sometimes with water contributing to lowering the melting point, where silica minerals tend to melt, but more magic minerals remain solid. The melt tends to move upward, either because it is less dense than surrounding rock or because of tectonic pressures. As it forms a mass, chunks of surrounding “country rock” fall into it in a process called “stoping,” making the melt more silicacious and also persisting in solid chunks that will eventually become xenoliths.
- Ripples (and their large scale cousins, dunes) are straight and symmetric if they are formed by currents moving back and forth, or curved if they are created by a unidirectional flow.
- Mud cracks form as a result of repeated drying and wetting, as occurs in mudflats with seasonal rain, or intertidal areas.
- A seam of coal that is a meter thick was originally 5-10 meters thick and took on the order of 2500 years to accumulate.
- Paleosols, fossil soil surfaces/horizons, are generally rock-like with a characteristic disrupted knobby appearance.
- Metamorphic rocks develop cleavage planes perpendicular to the direction force or pressure is being applied; metaphorphic rocks split along cleavage planes, not their original bedding surfaces.
- Schist has a lot of mica; gneiss has little. Both are coarsely crystalline and so highly-altered that it is difficult to tell what the source rock was. The light and dark banding in gneiss is the result of recrystallization, and has nothing to do with the original bedding plane.
At lower metamorphic grades, platy crystals of chlorite and mica are common. As higher metamorphic grades are reached, minerals such as garnet, staurolite, and sillimanite may form. Such high-grade metamorphic rocks form at depths of 15 to 25 kilometres within the crust. If pressure (usually the result of deep burial) is a major factor during metamorphism, minerals such as kyanite and glaucophane may grow. The blue colour of glaucophane gives rise to the name blueschist, a rock formed under conditions of low temperature and high pressure.
Igneous rocks also show interesting metamorphic changes.When basalt is metamorphosed at low pressures and temperatures, some of its constituent minerals convert to the green minerals chlorite, actinolite, and epidote, producing a type of rock called greenstone or greenschist. At higher metamorphic grades, greenstone becomes amphibolite, a dark green to black rock made up of interlocking amphibole crystals.
C2: Miscellaneous Points
- Reverse and thrust faults are different. They are created by the same array of forces, but reverse faults are steeper (closer to vertical) than thrust faults. …The text doesn’t say where the line is between them…
- Fault breccia. Rock formed of a jumbled mix of sharp rock fragments embedded in lithified rock flour.
- Nice review of minerals: pages 19-21.
- The Greenville orogen underlies Quebec, the midwest US, and stretches into mexico.
All modern oceans contain areas where the lithosphere is thicker than regular oceanic litho-sphere. These areas include island arcs, oceanic plateaus perhaps bearing atolls, and isolated fragments of continental lithosphere such as present-day Madagascar. If subduction continues and these within-ocean features are swept toward the continent, they will ultimately collide with it. Because high-standing islands or plateaus are more buoyant than regular oceanic lithosphere, they will be scraped off the sub-ducting plate and will stick, or accrete, to the overriding plate rather than be subducted. Many mountain belts contain remnants of such former within-ocean features; such remnants are called terranes (a term not to be confused with terrain, which denotes topography). Terranes thus have a variety of origins: they may be continental fragments (microcontinents); former island arcs; or former pieces of thickened oceanic lithosphere such as Hawaii may become if it is accreted to a continent. Many terranes are a mixture of these elements. The convergence and collision of terranes with continental margins commonly leads to the rise of mountains.
Remnants of former deep oceanic lithosphere can be preserved within an ancient mountain system. Such remnants are known as ophiolite suites or ophiolites…
- Rift Shoulder Highlands are mountains produced as a side effect of continental rifting — when the floor of the rift is bulging upwards.
- Basins. Fore arc basins are in front of island arcs; back arc basins are behind them. Fore land basins can form on continents, due to the weight of accretionary material pushing down that part of the continental plate. …Is the Owens Valley in California such a basin?
- The next supercontinent has already been named: Amasia.
C3: Miscellaneous Points
C4: Miscellaneous Points
- xxx
Part II: The Evolution of Canada
Cx: xxxx
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