Logging Requirements
You will visit 3 gravestones all within 100 feet of each other. Note that each gravestone is listed as a Waypoint.
- Frederick George Andrew - Look at he feather shaped cluster of garnet on the bak of the gravestone, just left of the center top. There is a thin vein running lengthwise like the shaft of the "feather". Is this feature felsic or mafic? How do you know? Is it part of the melting or the original metamorphic part of the rock?
- Stevenson - Take a photo showing alternate light and dark banding either on the headstone or the ledger on the ground in front of the headstone.
- Maria Vu Thi Loan - Look at bottom of the back of the headstone. The right hand corner has a “Q-159” inscribed on the vertical part. Compare this to the left bottom. Which side is felsic melt (light colored) and which side is unmelted metamorphic mafic (dark-colored)?
- What is the main process that defines migmatite?
- Of the three rock types, sedimentary, igneous and metamorphic) migmatite is transitional between which two rock types?
Headington Cemetery
Posted coordinates will take you to the entrance of the Headington Cemetery on Dunstan Road in Oxford.
Opening Times
April to October: Monday to Friday: 7am to 8pm and Saturday and Sunday: 8am to 8pm
November to March: Monday to Friday: 7.30am to 6pm and Saturday and Sunday: 8am to 6pm
Parking: There is no designated parking but visitors can park on the driveway within the cemetery.
Description
A Rock Type in Transition
At some point in our academic journeys we probably all learned that there are three types of rock- sedimentary, igneous and metamorphic. Of course, things don’t always fit nicely into discreet categories. Here at Headington Cemetery we will visit three examples of a rock that is in transition between two of these rock types. In fact, all three rock types will come into play as our story unfolds.
The rock we will be observing is migmatite. Not only is it beautiful to look at but the story of its origin is quite fascinating. As we examine each gravestone we will be looking for clues that help us to piece together the story of how these rocks were formed.
Migmatite is a metamorphic rock. Or is it? Maybe it’s igneous? Or a little of both?!
Metamorphic Rocks
Figure 1 Metamorphic Rock Identification. Modified from Johnson, C., Affolter, M. Inkerbrandt, P, and Mosher, C. 2017.
In order to better understand the process that leads to migmatites, we need to understand metamorphism. Metamorphism literally means “change form.” A sedimentary or an igneous rock may change form and become a metamorphic rock.. A metamorphic rock may even change into a different metamorphic rock!
Perhaps you had learned that the sedimentary rock, shale, when subjected to heat and pressure may metamorphose or change to slate. That’s true…sometimes. There is a range of metamorphic conditions from low grade (little change due to relatively lower temperature and pressure) to high-grade (significant metamorphic change due to higher temperature and pressure). With increasing temperature and pressure our metamorphosed shale would progress as follows:
slate —> phyllite—> schist—> gneiss.
Migmatite
Figure 2. The typical transition in mineral content that resuts from the progressive metamorphism of shale. From Lutgens, F.K., Tarbuk, E.J.and Tasa, D. (2012)
So it looks like gneiss is the end of the line, right? Not exactly. If we were to add even more heat, some, but not all, of the minerals in the rock would begin to melt. Each mineral has its own melting point. So those with the lowest melting point would melt first. Quartz and feldspar would melt first. These are generally lighter colored minerals that are referred to as felsic (because they contain feldspar (FEL) and Silica (SI). The darker Magnesium and Iron bearing minerals (called mafic from the MA in magnesium and the FE from the chemical symbol for Iron) would require even higher temperatures to melt.
The word migmatite comes from the Greek, “migma” meaning mix. Indeed, as you will see in our gravestones, migmatites are a mix of lighter, melted and recrystallized igneous-like layers, and darker, high grade metamorphic laters. The key process in the formation of migmatite is partial melting.
Perhaps you noticed I did not mention pressure. Increasing pressure actually increases the temperature required to melt a substance. Reducing pressure increases melt!
WP1 Frederick George Andrew
One of the first things you notice here are the beautiful swirling patterns made by the intertwining of light and dark groups of crystals. These patterns represent where the partially melted, lighter felsic minerals e.g. quartz and feldspar, flowed into the darker mafic portions of metamorphic gneiss.
If you look more closely you will see pinkish-red crystals scattered through the gravestone. These are garnets. A very obvious cluster is seen just to the left of top center on the back of the stone. I‘ve seen this referred to as flame-shaped. I think of it as feather-shaped. Either way, it’s hard to miss. Combined with the dark and light crystals we noted before we can start to unpack what’s going on here.
We started out with a sedimentary rock, shale or mudstone. These parent rocks are made up primarily of clay minerals, quartz and some feldspar. The clay minerals contain a lot of aluminum. Under heat and pressure the clay minerals break down and recrystallize into, among other things, an aluminum rich garnet. Garnet forms at high temperatures, high enough to melt the quartz and feldspar. This partial melting results in the whitish swirls seem here. The dark swirls are mafic crystals i.e. those with a high magnesium and iron content such as the hornblende.
Had it been hot enough to melt all of the minerals the result would have been an igneous rock.
WP2 Charles Stevenson
Like the Andrew headstone, this rock is composed of clusters of light crystals e.g. quartz, feldspar, and layers of dark crystals e.g. hornblende, and pinkish red garnet. Closer inspection reveals a key difference. Unlike the more random swirls of dark and light on the Andrew stone, this one shows a more obvious banding and foliation or layering.
The sequence of events leading up to this migmatite were similar to the Andrew headstone. Sedimentary shale underwent high grade metamorphism. The high pressure, temperatures and deformation cause minerals to recrystallize and realign. Foliation develops as the light colored minerals, which melt and recrystallize at a lower temperature are separated from the darker minerals that melt and recrystallize at higher temperature. This results in the the formation of gneiss, more specifically in our current two examples, garnet gneiss. One of the distinctive characteristics of gneiss is the alternate light and dark banding.
It can be assumed that both the Andrew and the Stevenson rocks exhibited gneissic banding as conditions progressed from sedimentary shale through to high grade metamorphosed gneiss. The partial melting, which caused the felsic or light colored minerals to melt and intrude the mafic or dark would have resulted in layering and foliation. But if the melting continued even further the alignment may have been disrupted.
Migmatites sometimes show banding/foliation because they retain structures from earlier metamorphic stages or because the melt segregates in a structured way. They sometimes lack foliation when partial melting is intense enough to disrupt or override those structures.
It’s this hybrid nature—part metamorphic, part igneous—that makes migmatites so texturally variable.
WP3 Maria Vu Thi Loan
Our third example of migmatite tells a slightly different story. It too shows light quartz and feldspar swirling around darker streaks of hornblende. The difference is not what is here, but what isn’t. There is no garnet to be found on this gravestone. While not definitive, had it been derived from aluminum rich clays from sedimentary rock we would have expected to find garnet crystals. So it does suggest another pathway for migmatite to develop.
Figure 3. Magnified Crystals on Stevenson Gravestone (left) and Loan Gravestone (right)
Closer inspection of the crystals shows they are larger than the other gravestones (see Fig. 3 for a comparison). Crystals have a chance to grow larger when they cool more slowly deep below the surface.This combined with the lack of garnet, as well was crystal composition, suggest the parent rock was granite, an igneous intrusive (magma cools and solidifies deep within the Earth’s crust) rock.
This gravestone shows many examples of shearing, or portions sliding past each other, deforming the rock. When this happens near the surface, where rocks are colder and more brittle it can cause fractures or faults. Deeper in the crust, the more ductile or fluid rock deforms without cracking. We can see several large flow patterns of light quartz and feldspar, especially evident on the back. The white streaks of quartz and feldspar on the flat ledger stone indicate the direction of the shear the rock underwent, And one of the most interesting patterns are the zig-zag folds on the upper right of the front of the headstone, more evidence of shearing in a ductile zone.
One Rock, Two Pathways
So we see here in these three examples, two pathways to becoming migmatite. In the first two (Anderson and Stevenson) we started with a aluminum-rich clayey sedimentary rock, shale or mudstone, which under high grade metamorphism was changed to garnet gneiss and then heated even more to partially melt and become migmatite.
In our third example, Loan, we began with an igneous rock, granite, which also is going to be subjected to high metamorphism resulting in gneiss on its way to becoming migmatite through addition of even more heat and partial melting.
Summary
| |
Foliation |
Garnet? |
Parent Rock Type |
Parent Rock |
| Anderson |
Non-foliated |
Yes |
Sedimentary |
Shale/mudstone |
| Stevenson |
Foliated |
Yes |
Sedimentary |
Shale/mudstone |
| Loan |
Foliatted |
No |
Igneous |
Granite |
Heat and pressure changed rock (metamorphism) from sedimentary (shale/mudstone) or igneous (granite) to gneiss
Gneissic banding or foliation was at one time a common feature of all three.
Additional heat caused partial melting resulting in swirling patterns of light-colored patterns of crystals (felsic e.g. quartz, feldspar) and ark (magic e.g. hornblende).
The foliation the Anderson gravestone may have been obliterated by even higher temperature melts.
The combination of the melted igneous-like, then recrystallized lighter portions intermixed with the darker original metamorphic portions is what defines this as a migmatite.
Resources:
Brown, M. (2001). Orogeny, migmatites and the structure of the crust. Journal of Structural Geology, 23(9), 1457–1470.
Farndon, J. (2018). The Illustrated Guide to Rocks and Minerals. Anness Publishing Ltd.
Johnson, C., Affolter, M. Inkerbrandt, P, and Mosher, C. 2017. An Introduction to Geology. https://opengeology.org/textbook/, Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Lutgens, F.K., Tarbuk, E.J.and Tasa, D. (2012). Essentials of Geology (11th ed.) Pearson Education, Inc.
Morgan, N. Gravestone Geology. https://www.gravestonegeology.uk/information_stone_types.php accessed 2025 05 16
Morgan, N. And Powell, P. (2015). The Geology of Oxford Cemeteries.
Philpotts, A.R., & Ague, J.J. (2009). Principles of Igneous and Metamorphic Petrology (2nd ed.). Cambridge University Press.
Sawyer, E.W. (2008). Atlas of Migmatites. NRC Research Press.
Winter, J.D. (2010). Principles of Igneous and Metamorphic Petrology (2nd ed.). Pearson Education.
Yardley, B.W.D. (1989). An Introduction to Metamorphic Petrology. Longman Scientific & Technical.