Myrtle’s Turtle and the Beehive are two of the most spectacular rock exposures in the Selkirk Mountains. They are the Half Domes of the Selkirk Mountains and rightly so because they, including Half Dome in Yosemite, are masses of granitic rock.
Myrtle’s Turtle is a huge dome of exposed granitic rock
Last week, I discussed the composition of granitic rocks but didn’t delve into the formation. Going deep beneath the surface reveals the variations of the granitic rocks in the Selkirk Mountains.
As mentioned last week, igneous rock forms by the crystallization of molten rock. If the molten rock is below the Earth’s surface it is called magma and if it reaches the surface it is called lava.
Igneous rock forms from the crystallization or “freezing” of magma. As hard as it is to imagine hot magma freezing it does but at temperatures way hotter than water. Magma typically freezes at temperatures ranging from 650 C to 1,100 C (1,202 F to 2,012 F).
Slow cooling created the large crystals
The variation in crystallization temperatures results from the minerals present. Magma infused with silica-rich minerals (like quartz) crystallizes at lower temperatures whereas mafic-rich minerals (like hornblende) crystallize at higher temperatures.
In the case of the Selkirk Mountains, the exposed rock is part of a pluton. Plutons are blob-like intrusions of magma that cooled underground in sizes upwards of tens of kilometers.
Not all magma within a pluton is of the same composition. As the magma moves upward it melts surrounding rock which can vary in composition along the way. This disparity in composition contributes to the variations seen across the Selkirk Mountains.
A porphyritic rock
As a body of magma cools, not all areas cool at the same rate. The differences in cooling results in different crystal sizes. When the magma cools slowly, the minerals have a chance to form large crystals. If the magma cools quickly, the minerals don’t have a chance to grow big before all the space is filled.
Granite and granodiorite are both coarse-grained rocks which indicates the body of magma cooled slowly.
Some rocks have mineral grains that are two sizes, usually large crystals within a matrix of smaller crystals. These rocks are called porphyritic and are a result of the magma cooling in two stages.
The large mineral grains, called phenocrysts, crystallized first while the magma cooled very slowly allowing for sizable growth. Then the magma, for one reason or another, cooled more quickly and the remaining minerals crystallized. Phenocrysts are commonly plagioclase (a mineral) because it is one of the first minerals to crystallize.
A possible xenolith because the dark rock is unlike anything surrounding it
On a larger scale are chunks of rock that don’t match the surrounding rock at all, like the really dark-colored rock I found along the Hidden Lake trail.
One possibility for these “odd” rocks is they are xenoliths. Xenoliths are rock fragments that are foreign to the body of igneous rock in which they occur. They may have fallen from the edge of the magma chamber and didn’t melt completely before the magma solidified.
Another possibility is that it is part of a vein that cut through the igneous rock after it solidified. Fractures, sometimes from fault movement, fill with magma of varying compositions, cool and crystallize into veins. Sometimes the veins are mainly quartz.
The possibilities are really quite numerous on how igneous rocks form and I’ve only covered a few. Finding the different types of granite, granodiorite, mineral veins and xenoliths throughout the Selkirk Mountains is like a scavenger hunt without a list because you never know what variation you’re going to stumble upon next.
