Dikes radiating out from the Spanish Peaks
I am NOT a geologist, but I did pay attention in middle school. What we are looking at here is a dike. This dike was formed when molten rock, deep within the earth, pushed up into cracks in the crust of the earth and cooled forming extremely hard igneous rock. In this case the crust was layers and layers of sedimentary rock laid down over millions and millions of years. Now, millions of years later, some (make that a lot) of the soft sedimentary rock has eroded away exposing the much harder igneous rock that intruded into the cracks and cooled. Several things to emphasize and keep in mind, molten rock cools to form igneous rock which is HARD, much harder than the softer sedimentary rocks that the molten rock intruded into. Over millions of years in this case over 25-million years, the sedimentary rock has eroded away exposing the much harder igneous rock that intruded and hardened in those cracks deep underground.
Let's start at the beginning and bring things forward in an effort to explain these dikes since it began many, many millions of years ago. For eons inland seas washed the area of the Great Plains and the Rocky Mountains and thousands of feet of sand, mud, silt, clay and marine animals were deposited. As time went on these deposits emerged from the water as layers of sedimentary rock.
Then from 35 to 75 million years ago the sediments in the area were subjected to stresses which caused cracks and seams to form. Again we are talking about a LONG time ago and over a LONG period of time. All those layers of sediments deposited in that inland see over eons were turning into sedimentary rocks and emerging from that inland see while at the same time being subjected to stresses that we know cause earthquakes -- where the plates of the earth shift. In any event when this happens cracks formed in those layers of sedimentary rock.
Then around 35 million years ago pressures increased, the sediments cracked more and at the same time molten rock began to surge upward into the cracked/fractured sedimentary rock. Great extremely hot fluid masses of molten rock invaded the relatively soft sedimentary rock by melting the sediments and assimilating them into the hot magma. Meanwhile the thick molten rocks entered the cracks. The molten rock squeezed its way into the cracks like mud between your toes. In the 25-plus million years since this event rain, frost and wind have eroded the much softer overlying sedimentary rock exposing the much harder igneous rock that filled those cracks and cooled creating these dikes.
This dike is mostly comprised of igneous rock but some metamorphic rock can also be seen. In this case the metamorphic rock is the sedimentary rock that was change via extreme pressure and head when the molten rock intruded into cracks over 25-million years ago. So what once was sedimentary rock was changed into much harder metamorphic rock. The metamorphic rock in this picture is the lighter colored rock or tan colored rock.
Like in the above picture most of this exposure is igneous rock (molten magma that slowly cooled underground) however some is metamorphic rock. Again the metamorphic rock is the lighter colored (tan) on the left edge of this outcropping. Keep in mind that metamorphic rock in this instance is sedimentary rock that was changed by extreme heat and pressure. Also remember that metamorphic rock is much harder than the original sedimentary rock and that is why is has not eroded like the surrounding sedimentary rock.
Some sources indicate that the igneous rock is granite while others label it as rhyolite while yet others label them with more technical variations of igneous rock. Some may be one type of igneous rock while others may be another type of igneous rock. Whatever, that is above my pay-grade. The metamorphic rock was actually "baked" by the intense heat and pressures. Geologist indicate that the metamorphic rocks are generally gneisses & schists.
The length of the exposed portion of this dike is impressive to say the least. These dikes vary from 1 to 100 feet wide and some are up to 14 miles long. I wonder if geologist even know how long they may be considering some of the dike may not yet be exposed by erosion. Does anyone know if geologist know how far these dikes stretch when they go underground?
Joyce took this spectacular dike exposure as we traveled through the small community of Stonewall, Colorado on the Highway of Legends. This picture is only a very small view of a dike that stretches for miles and miles on both sides of Stonewall. As you can see the highway makes a sweeping turn before finding a gap in the "stone-wall" or dike.
Until next time remember how good life is.
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