Planet Earth/6h. Bowen’s Reaction Series

The Distribution of Rocks on Earth
Norman L. Bowen navigated his canoe across the densely forested Larder Lake, near the border between the Canadian Provinces of Quebec and Ontario, a sparsely populated wilderness south of the Hudson Bay in 1907. The rocks in this region are some of the oldest rocks in North America, part of the craton of North America, peppered by more recent volcanic igneous rocks leading to the possibility of rich deposits of gold and silver. Bowen was hired to explore this area, as a young student by the Ontario Bureau of Mines. During the summer alone in the field, Bowen learned to read the rocks by identification of minerals, the classification of rock names, and the quest to find regions for mining in the area. His experience in the field observing the way minerals were distributed in the rocks, underlay the rest of his life, even when he later dedicated it to laboratory experiments involved in the heating and melting of minerals to understand how they turned to liquid and how they cooled into crystals. The studies and experiments lead to a great insight into how rocks become enriched and depleted regarding different mineral compositions. His life-long work clearly explains why the oceanic crust laying near mid-ocean regions are mafic, while continental crust is felsic.

Bowen's Reaction Series
Today, Bowen’s reaction series that he came up with is a chart that illustrates how various minerals melt at different temperatures. If we think of the Earth’s rock cycle as the continue process of melting and freezing of solid matter on the planet, overtime, the minerals that melt at lower temperatures will “float” near the surface, while minerals that only melt at very high temperatures will “sink” into the Earth. This differentiation of the Earth’s outer crust results in the observed felsic minerals found in continental crust (quartz, orthoclase, albite and muscovite), while mafic minerals are resigned to oceanic crust (anorthite, biotite, amphibole, pyroxene, olivine). The solution is that felsic minerals melt at lower temperatures, while mafic minerals will remain solid, and hence sink deeper into the Earth’s magma. Over time, with the continued process of the rock cycle, Earth’s continents will become enriched in these felsic minerals, particularly quartz, while the deeper interior of Earth will become enriched in mafic minerals, particularly olivine. The remarkably young rocks that lay on the world’s ocean floors continually produced by volcanic activity along the mid-ocean ridges, the magma is enriched in mafic minerals that requires high temperatures to melt. Often students mistakenly view the ocean crust as being that crust that solely exists beneath the ocean, but this mafic rock is chemically and physically very different than the majority of continental rock found on continents, which is unusually enriched in felsic minerals. It is odd to think that the rocks that we daily pick up on the continents of Earth are fundamentally very different in their mineralogy and chemistry than the rocks found on the ocean floor.

Plagioclase Feldspar
One of the important discoveries made by Bowen was the process of how the mineral plagioclase melts with increasing temperature. Plagioclase is actually a series of minerals in the feldspar group that exhibit a white to blue color and includes two end members, albite (NaAlSi3O8) containing sodium, and anorthite (CaAlSi3O8) containing calcium. Albite melts at a lower temperature than anorthite, hence the percentages of albite and anorthite in a rock can reveal the history of the partial melting of the two minerals, and how the rock became enriched in either mineral over time. Rocks deeper in hotter magma will be enriched in anorthite, while rocks in shallow crust and cooler magma will be enriched in albite. The ratio between the two minerals can reveal the melting temperatures within the magma.

The Differentiation of Earth's Crust
Bowen’s reaction series is an important view into how felsic and mafic minerals have over Earth’s long history become differentiated forming solid felsic rich continental cratons that float over magma of increasing mafic composition. The process in building Earth’s continents is a long history of accreting felsic rich crust onto these floating solid islands in a sea of mafic oceanic crust continually being formed at mid-ocean ridges and destroyed in subduction zones. A rock cycle that continually moves these floating felsic rich continents around the Earth throughout its long history, as a result of plate tectonics.