On August 20, 2016, Dr. Tim Chowns explained how rocks melt to form magma and volcanoes. Unlike water, which melts and then crystallizes (at 32o F, Oo C) as a single mineral (ice), magmas crystallize numerous silicate minerals all with different melting/crystallization temperatures. In general light silicate minerals (rich in light elements) melt and crystallize at lower temperatures than heavy silicate minerals (especially rich in iron). Apart from weight, it is easy to tell the silicate minerals apart on the basis of color. Low density silicates are light in color (colorless, white, pink) while heavier silicates are darker (green, brown, black) due to the presence of iron. This difference in melting point has important implications both for volcanoes and the planet as a whole.
When temperatures and pressures inside the Earth’s mantle reach critical values, melting may occur. This melting is usually partial; low density minerals begin to melt before high density minerals. As a result, a fractionation (separation) occurs and low density magma begins to rise towards the surface while heavier minerals resist melting and lag behind in the mantle. Thus mantle rock with a density 5.5 times greater than water may produce a lava 3 times greater than water together with gasses that are lighter than water. Based on density the lava will flow out at the surface while the gasses accumulate in the atmosphere. We call this kind of lava basalt. Basalt covers about 70 percent of the Earth’s surface and floors the oceans. It is a relatively fluid lava with low gas content and not especially explosive.
Around the borders of the Pacific Ocean there are numerous places where slabs of oceanic crust made of basalt are colliding, and one slab is forced to override another. The descending slab is heated as it is pushed down into the mantle and starts to melt. Again the process of fractionation takes place, lighter gas-rich lava rises while heavier unmelted material remains behind. The parent basalt undergoes partial melting to produce the much more explosive lava, andesite, named for the classic volcanic cones of the Andes.
Anywhere that partial melting occurs, this process of fractionation takes place and the more times the crust is subject to melting the lower the density of the lava and the more explosive the volcanoes. In addition the lightest material, which we call granite, accumulates at the surface and cannot be pushed back into the mantle. In effect it ‘floats’ on the mantle at a level determined by its thickness and density. These rafts of granite make up 30 percent of the Earth’s surface and we call them continents.
Much altered, deformed by mountain building and highly eroded, Macintosh Reserve includes outcrops of both light colored granitic gneiss, and dark colored metamorphosed basalt.
After the talk we examined the resistant light-colored gneiss that makes up Council Bluff and then walked across the field to the old railroad cuts along Beaver Pond Trail where dark-green metamorphosed basalt is visible. This ancient lava originated on the sea floor and was pushed up on the continent during the building of the Appalachian Mountains. In the accompanying pictures, participants examine various soils and rocks to see first hand evidence of the volcanic forces that have shaped our world. As always on walks at McIntosh, we also benefited from “collateral observations” of summer blooming flowers, beetles, grasshoppers and snakes.
Please visit the Photos to view pictures of this event.
