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1.4: Magma, metamorphism, and mountains

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    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. Click on it and spin it around to see its distinctive shape.

    Figure \(\PageIndex{1}\): Igneous is a dialect of Rock with elegant structure and fiery intonation. (3D model by Marissa Dudek & Callan Bentley)

    The pattern you observe in the 3D model 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 it must have been constructed by a conscious mind. 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 own 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. Collectively, all of these sites are 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 prior to the various landmasses separating 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 urgency 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 history.

    On the metamorphic front, consider this blueschist:

    Blueschist is a metamorphic rock which is blue and flaky. But like all metamorphic rocks, it didn’t start off 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 any more 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 coming back from the Moon, the blueschist has come back 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 tells stories too, and Earth has picked up her share of scars through the years. For the most part, these “scars” are tectonic structures like folds, faults, and shear zones. Consider this photograph of the Andes Mountains in southern Argentina:

    A sunlit mountain, with trees at its base and a bit of snow dusting the top. The area of the mountainside in the middle shows zigzag outcrops of folded and faulted sedimentary layers. The folds have an apparent interlimb angles of 35 to 45 degrees: they are tightly folded!
    Figure \(\PageIndex{2}\): 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\(^{\circ}\) (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 having formed it.

    Stories can be extracted not only from the sequence of sedimentary layers in this case, but also from their contortion, their orientation, their elevation, and their shape. 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.


    This page titled 1.4: Magma, metamorphism, and mountains is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts (VIVA, the Virginia Library Consortium) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.