On finding out my ‘profession’, there’s one fact that people proudly announce to me on a regular basis: “I know the three rock types: sedimentary, igneous and metamorphic!”. What usually emerges from deeper probing is that most people are comfortable with the concept of sediments and magma, but metamorphism is a bit of an enigma. Like the mysterious stranger, lurking in a dark forgotten corner, it is true to say that for most people, this holds even throughout undergraduate geology courses.
Metamorphism means “to change shape”, and metamorphic rocks are those whose have been converted from their original form into new rock types. All it takes really, is pressure (P), and time (with apologies to the Shawkshank Redemption), and two very important additional components: elevated temperatures (T) (above ~ 150oC) and fluids. When exposed to P-T conditions beyond their natural stability, minerals in the original rock (the protolith) are said to be metastable, and if kinetics are favourable, can react to form new minerals and structures. In this way, if subject to intense compressive forces during an episode of mountain building a volcanic ash can become a slate, or a limestone be transformed into marble by the presence of an intruding body of magma.
A sample of hornfels from Sinen Gill, Lake District. This rock started off life as a humble mudstone, and was then thermally metamorphosed by a nearby intrusion of Skiddaw Granite. The white needles are crystals of chiastolite (a type of andalusite) – closer to the granite contact, the hornfels also contains cordierite. Credit: Charly Stamper
If you’re lucky enough to have studied something you truly love, then I’m sure you’ll have experienced one of those life-affirming moments when you think “Wow, THIS is why I am studying X”. Despite now dealing solely in solidified magma, it was metamorphic rocks that provided me with said defining moment during my undergraduate mapping dissertation in Syros, Greece.
The island of Syros is part of the Cycladic Blueschist unit (CBU), a pile of granite, sediments and ophiolite that was buried to ~40km depth during subduction in the Eocene (about 50Ma) and exhumed to the surface some 15 Myr later. The rocks were subject to both prograde blueschist and retrograde greenschist metamorphism, and these processes are most stunningly manifest in the mangled ophilolite sequence. Originally coarse grained gabbros and ultramafic cumulates now comprise massive boulders of blue glaucophane, bottle green omphacite, bright red garnet, gleaming white phengite and impressive rhomb-shaped lawsonite pseudomorphs.
Metagabbro from Syros, Greece. Although the original igneous texture has been retained, the protolith igneous minerals have been replaced by glaucophane (dark blue), omphacite (green), garnet (red) and zoisite (white). Credit: Charly Stamper
It’s not only the mineral-magpie in me that finds these rocks so interesting. The sheer wealth of information that can be gathered from one exposure is staggering. The mineral assemblage tells you about the protolith; mineralogy reveals the P-T conditions of different stages of metamorphism; and relative timing of events are evinced by textures relative to deformation. A complete history of the outcrop can be deduced with only a hand lens and a compass-clino, and with a little background knowledge of the region, tied into global tectonic events.
Lawsonite pseudomorph in glaucophane epidote schist from Syros, Greece. The original high pressure mineral lawsonite has been replaced by phengite and epidote, but has retained its original shape. Credit: Charly Stamper
My Cycladic introduction to the unique complexity of metamorphic rocks encouraged me to do my MSci project on a different type of metamorphic rock: Archean gneisses from Greenland. These ancient rocks were originally clay-rich mixtures of volcanic ash and clay, but now exhibit spectacular reaction textures between cordierite, orthoamphibole and garnet; snapshots of metamorphic evolution frozen in time. A combination of compositional data (electron microprobe analysis) and thermodynamic modelling and allows us to place accurate constraints on the P-T conditions that caused these reactions. Add in U-Pb zircon dating from cross-cutting intrusions, and you have a full geological post-mortem from a single thin section.
Photomicrograph (plane-polarised light) of cordierite-bearing gniess from south-west Greenland. The texture is preserved from the metamorphism at approximately 2.8 billion years ago. High pressure kyanite is replaced by its high temperature polymorph sillimanite. At lower P-T conditions, the two aluminosilicates then react with staurolite, quartz (and either gedrite or garnet, not pictured) to form cordierite. Credit: Charly Stamper
I could wax lyrical about metamorphic rocks for hours, but I might not find much of an audience. So why do they have such an enigmatic and downtrodden reputation? Firstly, metamorphic rocks tend to contain loads of minerals, and weird ones at that. We’re not talking your standard olivine-clinopyroxene-plagioclase assemblage, think sapphirine, kornerupine, gedrite, gahnite, vesuvianite…a nearby copy of DHZ is a must. Furthermore, the minerals in metamorphic rocks are commonly not in textural, chemical or isotopic equilibrium, meaning classical thermobarometry and phase equilibria are often not applicable. In fact, it’s pretty tricky to find out much about these types of rocks. The relatively low temperatures (compared to igneous processes) severely limit the extent to which experimental petrology can help. Thermodynamic modelling of metamorphic reactions suffers from the same limitations as any other type of numerical modelling. And the long timescales involved means we can’t measure the processes as they occur in nature.
I think that the main problem is that unlike volcanic eruptions or wave-formed ripples, no human will ever be able to directly observe the formation of metamorphic phenomena. To some, the inherently intangible nature of metamorphic rocks makes them less appealing than lava or limestone. Though the study of such confusing rocks may seem a little esoteric, they are in fact the subject of intense commercial interest. Many of the world’s most valuable gemstones (e.g., sapphires, rubies, diamonds) are associated with metamorphic lithologies. In my opinion, metamorphic rocks also hold a bigger prize; the key to some of the biggest unanswered questions in geology, such as how plate tectonics have changed over the planet’s history.
Co-existing quartz and coesite inclusion in garnet. Coesite is a high pressure polymorph of quartz and its presence indicates that the rock underwent ultrahigh pressure metamorphism. This type of assemblage is absent from the Earth’s oldest rocks, indicating tectonic plates have thickened (through cooling) over time. Credit: Christian Nicollet.
I can’t help but think that in the geology version of rock-paper-scissors*, metamorphic beats sedimentary and igneous every time.
*Colloquially known as ‘rock-rock-rock’
