Magma, Metamorphism and Mountains

Traditionally, Historical Geology has focused its attention on sedimentary rocks, built up in layers and host to distinctive fossils. But this is only one dialect that Earth is capable of speaking. She is equally eloquent when she writes igneous and metamorphic stories or deforms her earlier works through later episodes of mountain building. In this text” book,” we embrace the full gamut of Earth’s works, giving equal value to the Earth stories we can decode from metamorphic and igneous rocks and tectonic structures too.

For example, the outcrop below shows a pattern that’s quite striking.

The pattern you observe above is a particularly regular system of fractures that developed in a cooling lava flow. The iron-rich lava erupted 50 million years ago in Northern Ireland, congealed, and cooled down, shrinking as it did so. (Cold rock takes up less space than warm rock.) The contraction induced tensional stresses across the surface of the lava flow. These stresses were greater than the strength of the freshly-solidified rock, and it cracked. The fractures met each other in a “honeycomb” like pattern on the surface of the flow and then propagated down into the flow’s interior, dividing the warm young rock into a series of cold polygonal pillars.

The locals were so impressed with the precise regularity of this pattern they were convinced a conscious mind must have constructed it. They spun a legend to explain it – a tempestuous giant named Finn MacCool (Fionn mac Cumhaill), trying to pick a fight with a Scottish giant named Benandonner. Benandonner had been so bold as to claim Ireland as his own, and Finn wouldn’t stand for it! Finn started pitching rocks toward his rival and realized he could build out a causeway to connect the two landmasses. He journeyed to Scotland (specifically, to the island of Staffa) to confront the usurper but got scared by Benandonner’s size and rushed back across the causeway to Ireland. The Scot pursued but was tricked into a hasty retreat thanks to some clever subterfuge by Finn’s wife Sadhbh (pronounced “Siive”). She dressed Finn as an infant and told Benandonner that he was, in fact, merely Finn’s baby and that big daddy Finn would be home soon! Benandonner beat his hasty retreat, knocking the causeway to pieces as he fled. The site is therefore known as the Giant’s Causeway.

Geologists have a different way of explaining the Giant’s Causeway, of course.  Because they speak Rock rather than Giant, they note that on a landscape scale, this lava flow is one of many that erupted in a vast region between 60 and 50 million years ago, smothering the local landscape as they flowed and pooled. The volcanic activity was not limited to Ireland and Scotland. It also produced basalt in the Faroe Islands, Norway, and Greenland. All these sites are in the North Atlantic Igneous Province (NAIP). The Paleogene-aged lava flows link them all to be in the same spot around 60 million years ago, marking them all with a common stamp before the various landmasses separated from each other, with the Atlantic Ocean opening up in between.

The cause of this eruption may have been the Icelandic hotspot, which continues erupting similar lava today at the Mid-Atlantic Ridge. The now-widely-dispersed basalts of the NAIP help link together landmasses that experienced divergent tectonics during the Paleogene and have been moving apart ever since.

Furthermore, the timing of the NAIP’s eruption overlaps with a time of extreme global warming, the Paleocene-Eocene Thermal Maximum (PETM), and some scientists have suggested that CO₂ emissions from the lava may have been the cause of this superlative episode of global warming. As with the end-Permian extinction, that is potentially really important: a matter of some importance to Rock-fluent humans who are living in their own time of global warming.

Bottom line: Igneous rocks are just as vital as sedimentary rocks in interpreting Earth’s history.

On the metamorphic front, consider this blueschist:

Blueschist is a metamorphic rock that is blue and flaky. But like all metamorphic rocks, it didn’t start that way. This blueschist started as basalt – the same sort of mafic volcanic rock we just saw at the Giant’s Causeway! The fact that it doesn’t look like that anymore is evidence of a profound journey it took – from Earth’s surface down deep into its interior, down 40 or maybe 50 kilometers into the mantle. Down there, temperatures were hotter, but more importantly, the pressure was insanely high. The minerals making up the basalt couldn’t take it: they reacted and fell apart, and their atoms recombined to make new minerals. The new minerals were chemically stable under the crushing pressure.

This distinctive blend of minerals is, therefore, evidence of a journey. Like an astronaut returning from the Moon, the blueschist has returned from a place none of us will ever get to go. Its minerals tell a story, like stamps in a passport. The best travel stories are written in Metamorphic.

Scars tell stories too, and Earth has picked up her share of scars through the years. Mostly, these “scars” are tectonic structures like folds, faults, and shear zones. Consider this photograph of the Andes Mountains in southern Argentina:

Folded turbidite strata in southern Argentina.
Photograph by Zoltán Sylvester; reproduced with permission.

Rock layers are laid down in layers that are pretty close to horizontal, perhaps as steep as 30° (the angle of repose). But when you see them as in this Patagonian mountainside, with some of the layers upside-down, you know that they must have been seriously contorted sometime after their original deposition. After all, you cannot deform a rock layer without first forming it.

Stories can be extracted not only from the sequence of sedimentary layers in this case but also from their contortion, orientation, their elevation, and their shape. The structure is the most passionate strain of Rock vernacular. If we ignore the structure of the mountainside, we potentially miss out on the most dramatic stories these rock layers have to tell.