Ten years ago I wrote a series of articles about Rocky, a gneiss (pronounced “nice”) from a quarry near Morrison, Colorado. Two of those articles were written by Rocky himself!
I still listen to stories the rocks tell. The exit from the Interstate to our neighbourhood cuts through a rocky knob and exposes a very interesting sequence of rocks. Cars zoom along the exit ramp, so I have never stopped to closely study the rocks. Even so, the rocks tell me their story as I drive by.
The rocks are Miocene age – in this area probably 15 to 25 million years old. The rocks tell me that back in the Miocene the landscape looked like a lush East African savannah. Various mammals including prehistoric dogs, camels, horses, deer, and giant rhino-like critters wandered the landscape, and a variety of snakes, turtles and lizards slithered around.
{{image2-a:r-w:300}}The light-coloured rocks at the bottom of the road cut tell me they are volcanic rocks referred to in general as felsic rocks. They said that more specifically they prefer to be called tuff. (To explain, saying “felsic” is like saying “fruit”; saying “tuff” is like saying “apple” or “banana”.) The tuff rocks were made from magma (molten rock) ejected from a volcano as ash – similar to the debris from Mount St Helens in 1980.
The dark coloured volcanic rocks overlying the tuff said they are referred to in general as mafic rocks. They said that more specifically they are basalt made from magma ejected as lava like that currently flowing out of the Kilauea Volcano in Hawaii.
In addition to colour, the words “felsic” and “mafic” describe the minerals in the rock. Felsic rocks contain light-coloured and lightweight minerals rich in feldspar (fel) and silica (si); hence the name felsic. Mafic magmas are rich in dark-coloured, heavy minerals containing magnesium (Ma) and iron (Fe); hence the name mafic.
Viscosity, a liquid’s resistance to flow, determines how magma will behave during an eruption. The more silica in the magma, the higher its viscosity. Felsic (silica-rich) magma is viscous and does not flow easily. When it rises into a magma chamber it may be too viscous to move and plugs the chamber vent. Dissolved gases become trapped by the magma and pressure builds up. Eventually the pressure becomes so great that the magma chamber explodes. Magma, rock, and ash erupt, creating a cloud of volcanic ash. The ash falls to the ground and cools into a rock called tuff. That is how the white rock in my road cut said it was formed.
In contrast to felsic magma, mafic magma is not viscous and will flow easily to the surface through volcanic vents. From there the lava flows across the land surface and cools to become basalt.
Therefore, the story the rocks tell me is that back in the Miocene, lighter weight, viscous felsic magma floated to the top of a magma chamber. Pressure built up and the magma exploded, spewing ash into the air. That ash fell to the earth and cooled into tuff. With the lightweight felsic magma being gone, the heavier mafic magma rose to the surface, oozed out as lava, and cooled into basalt.
Some day you should listen to the rocks in your pit, quarry, or nearby road cut. Even if what you hear might be wrong, there’s no harm in story-telling!