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A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. [1] These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years.
Metamorphic rocks are formed when existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree. The importance of heating in the formation of metamorphic rock was first noted by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology ...
The prominent black diabase dike on Mount Moran is representative of similar dikes formed about 1.3 billion years ago. Sometime around 2.5 billion years ago, blobs of magma intruded into the older rock, forming plutons of granitic rock. [3] Extensive exposures of this rock are found in the central part of the range.
[1] [2] Metamorphic minerals are unstable upon changing P-T conditions. [1] [3] The original minerals are commonly destroyed during solid state metamorphism and react to grow into new minerals that are relatively stable. [1] [3] Water is generally involved in the reaction, either from the surroundings or generated by the reaction itself. [3]
[1] Monazite geochronology is a dating technique to study geological history using the mineral monazite. It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as igneous, sedimentary and hydrothermal rocks. [2] [3] The dating uses the radioactive processes in monazite as a clock.
The rocky cores of the mountain ranges are, in most places, formed of pieces of continental crust that are over one billion years old. In the south, an older mountain range was formed 300 million years ago, then eroded away. The rocks of that older range were reformed into the Rocky Mountains.
Metamorphic rock formed in these settings tends to shown well-developed foliation. [33] Foliation develops when a rock is being shortened along one axis during metamorphism. This causes crystals of platy minerals, such as mica and chlorite , to become rotated such that their short axes are parallel to the direction of shortening.
Nuummite is usually black in colour and opaque. It consists of two amphiboles, gedrite and anthophyllite, which form exsolution lamellae that give the rock its typical iridescence. Other common minerals in the rock are pyrite, pyrrhotite and chalcopyrite, which form shimmering yellow bands in polished specimens.