THE ROCK SPIRAL: THE ORIGINAL WORLD WIDE WEB

A traditional presentation of the 2-D rock cycle diagram consists of boxes representing materials (sediments, sedimentary rocks, metamorphic rocks, magma and igneous rocks) and arrows identifying processes (lithification, metamorphism, partial melting, crystallization, and weathering, erosion, transportation and deposition (WETD)). The basic concept conveyed to the student is that Earth materials are transformed as they are repeatedly cycled. A distinct weakness of this approach is the absence of a time axis, the addition of which converts the cycle to a spiral.

A second weakness inherent in traditional presentations of the rock cycle is the lack of clarification as to how Earth materials are “fractionated” at the elemental or mineralogical levels. There is enormous variability in the extent to which the content of a rock stays in the same volume as it is transformed to another Earth material. The complexity of the fractionation of this material as it resides in and moves between reservoirs defines a limitless, web-like network of potential pathways along which atoms move through time.

More often than not, as matter spirals its way down the time line moving from one “box” to the next in the cycle, it fails to stay together. It would appear as a blur of spiraling pathways if we could watch it coming toward us through time. Therefore, after presenting the pathways of a typical rock cycle diagram, we need to emphasize, for example, that 1) diagenesis commonly results in material being added and removed from sediments and sedimentary, 2) fluid migration during metamorphism transports components in and out, 3) partial melting preferentially leaves refractory elements behind, 4) crystallization allows extensive fractionation, and 5) WETD systematically separate and spread materials across the face of the globe. We can go full circle, but we are definitely not where we started.