That Rock is What Color?

Hello Backyard Geology readers! This week’s featured rock likely does not exist in many of your backyards, but it is a very important rock that allows geologists a chance to better understand what happens when two plates collide and form what we call a subduction zone. Let’s take a deep dive into the red and green rocks, called eclogites.

Put simply, an eclogite is high pressure – moderate to high temperature rock that is mainly made up of the minerals garnet and clinopyroxene. The term ‘eclogite’ from the Greek ‘choice’, was first established by a famous french mineralogist, René-Just Haüy in 1822¹. He chose this name because of the unique pairing of minerals within the rock (it was uncommon at the time to see these two minerals found together). As geology evolved with technology the name eclogite became even more appropriate as geologists have learned so much about the complexities that occur at subduction zones because of these rocks. 

So What Makes Eclogites Special?

For starters, let’s break down some of the formation mechanisms of eclogites. I mentioned above that these rocks form at high pressures and medium-high temperatures. Let’s quantify that a bit further. Below is a diagram that geologists often use to help understand the range of pressures and temperatures that metamorphic rocks form at. Each of the colored sections with names in them represent what is called a metamorphic facies (a region that represents a set of minerals that form under those specific pressures and temperatures). If we look at the top of the diagram we see the large green shape that highlights eclogites. So from this observation we can say that the minerals that make up eclogites form at pressures between 11kbar-20kbar (159,541 psi – 290,075 psi; your tires would beyond explode with that much pressure) and 300-1,000 degrees Celsius (572 Fahrenheit – 1,832 Fahrenheit). Outside of these pressure and temperature constraints the minerals within our eclogites would not be able to form and different rocks would be created instead. 

We can achieve these pressure and temperature conditions when two tectonic plates collide at a subduction zone (or convergent boundary). The diagram below shows a very simple example of this process and where our eclogites form. In this diagram we have the oceanic crust made up of primarily an igneous rock, basalt, and sediments that accumulate on the ocean floor. The continental crust is made up of granites, other metamorphic rocks, and sedimentary rocks. Because the oceanic crust is denser (basalt is very dense), it will preferentially be forced under the continental crust. As this happens our oceanic crust travels into the lower lithosphere and upper mantle (the “asthenosphere”) where the basalt and other sediments will transform into new rocks under higher heat and pressure. Our eclogites form roughly where the yellow box is around 45km (28 miles) below the Earth’s surface! The minerals in our basalt are no longer happy, healthy minerals at these pressures and temperatures so they undergo chemical reactions and transform into our high pressure pyroxene (omphacite) and high pressure garnet (pyrope). This deep crust to upper mantle process at subduction zones is incredibly complex and studying eclogites allows geologists windows into these subduction zone dynamics.

You may be asking yourself at this point, well if they form so deep, how do they make it to the surface where we can see them? A valid question and a complicated one. One of the more common ways we can see these rocks at the surface is through what is called ‘exhumation’, where our rock is brought up to the surface from depth. In the case of eclogites, it is likely through earthquakes (a common feature at the boundary of two tectonic plates) that can shift kilometers of rock upwards to the surface. Unfortunately much of those materials are destroyed in the process, but at times they can be preserved. Because eclogites form so deep, the localities around the world where eclogites are well preserved and easily accessible is not common. A famous occurrence of incredibly well preserved eclogites is on a small island off the coast of France, called the Ile de Groix. This tiny island offers a view into the eclogites that formed around 400 million years ago³. The western coast of Norway is also home to many well preserved eclogite exposures. These eclogites formed around 450 million years ago with the closure of an ancient sea⁴.  Perhaps we have the locations for our next Backyard Geology vlog?

References:

¹Godard, G., 2001, Eclogites and their geodynamic interpretation: a history: Journal of Geodynamics, v. 32, p. 165-203.

²Arndt, N., 2013, Formation and evolution of the continental crust: Geochemical Perspectives, v. 2, p. 405-533.

³Blueschist, Ile de Groix, http://www.alexstrekeisen.it/english/meta/groix.php Accessed: 8/28/2021. 

⁴Torsvik, T.H., and Rehnström, E. F., 2003, The Tornquist Sea and Baltica-Avalona docking: Tectonophysics, v. 362, p. 67-82.

⁵Dudek, K., 1989, Deerite from Ile de Groix, Brittany, France, Mineralogical Magazine, v. 53, p. 603-612.

⁶Worthington, J.R., Hacker, B., and Zandt, G., 2013, Distinguishing eclogite from peridotite: EBSD-based calculations of seismic velocities: Geophysical Journal International, v. 193, p. 489-505.