Mineral Monday: AMCG

That’s “anorthosite-mangerite-charnockite-garnet***,” but don’t go to sleep just yet.

This unplanned post is really about the Adirondack Mountains of Upstate New York and, for me, a forty-year-old life circle that has just closed.

Minerals aren’t usually something to take personally, but when they form themselves into mile-high massifs and strew garnet-bearing boulders all over the neighborhood, minerals are hard to ignore.

Rockhounds, of course, do not ignore them. There are garnet “mines” open to the public, but this is what you can see in stream bed boulders, too. In these AMCG rocks, the light-colored stuff is anorthosite (plagioclase feldspar); dark colors are the “MCG” part.

I’m talking about Mount Marcy and the other High Adirondacks. They’re made of solid AMCG.

I was up in that neck of the woods (literally woods: it’s a huge state park) to earn a two-year college degree (all that the college offered then), ten years after graduation from high school and soon after getting my feet under me for the first time after leaving the very troubled family I grew up in.

We almost-Boomers were lucky to still have such flexibility and social leeway back then.

I was at a crossroads personally but was neither ready for marriage/family nor driven toward some particular form of employment — being at loose ends was not a good position for a single woman in 1980.

This is new, I think. We had Longtin Cafeteria.
(Image: mwanner via Wikimedia, CC BY-SA 3.0.)

A high-school aptitude test had highlighted an interest in outdoors work, so I enrolled in forestry at Paul Smith’s College, then a two-year school a little over ten miles west of the Adirondack town of Saranac Lake.

My formal academic career there was not stellar. Somehow I graduated, and even won a writing scholarship, but the scenery outside the classroom windows and the intellectual frontiers that I was beginning to perceive meant more to me than did cumulative grade points.

This chunk of anorthosite is from the Moon! (Image: alkivar via Wikimedia, public domain)

This college isn’t in the High Peaks region, but there were still plenty of opportunities to see rocks studded with big blocky crystals of creamy white plagioclase (teachers told us what it was, though no one needed to tell us it was beautiful).

That’s anorthosite.

The light-colored stones and boulders sometimes showed narrow layers of dark material and smoky-looking blotches — the mangerite and charnockite, respectively — as well as garnets.

It’s always fun to find a gemstone “in the wild,” though these garnets were usually tiny and not high in quality.

My forestry studies and encouragement from professors who were also rockhounds showed me how the underlying geology influenced the plants and, indirectly, the people that could inhabit the Adirondacks.

These might have included First Peoples:

And they definitely included more recent folks:

I was at Paul Smiths during the “Sagan era” of popular science and spent more time reading about earth science and astronomy than I did on school studies (so graduation was indeed a miracle).

The weather is challenging. I almost got hypothermic to the point of irrationality while finishing up an overdue surveying assignment on this point one misty June morning! (Image: mwanner via Wikimedia, CC BY-SA 3.0)

That foundation built of wonder, laid down in the early 1980s, has stayed with me and is now blossoming after retirement.

One thing I never understood was the High Adirondacks, which are very different from other surface geology in the area.

The chunky white rock appealed to me. Too, while reading I picked up the factoid that the Moon’s highlands are made of anorthosite. (I had no idea of the “MCG” part yet.)

Anorthosite isn’t very visually exciting under the microscope. (Image: Kevin Walsh via Wikimedia, CC BY-SA 2.0)

The High Peaks were over a billion years old — the most ancient dating I had ever heard of then.

The Moon was old, too (I hadn’t a clue yet just how old — about 4.5 billion years — though I would soon see my first geological thin section at Alfred University and that would be from a lunar rock collected by Apollo astronauts).

Could there be a link between Adirondack anorthosite and that on the Moon?

This fascination with anorthosite, plus the Adirondacks’ beauty, made me promise to return to them someday as I drove off after graduation in the summer of 1982.

I never have, physically. But this past week I did, deep inside.

It was a two-step process.

  1. Yesterday’s post on Mount St. Helens’ 1980 eruption. I was oblivious to it at the time, other than that a volcano had erupted somewhere and people had died.

    Sunday’s reminder of 1980 naturally turned my thoughts back to those wonder-filled days when I first saw a little of the interconnections between Earth systems.

  2. Reading up on the next supercontinent post here, which covers Rodinia as well as plate tectonics. For reasons that will become apparent in that post, I focus on the core of North America (Laurentia), which includes a lot more than just the Canadian Shield.

    Basically, it’s a collection of about eight cratons, “cemented” (Santosh et al.) together by one of the world’s largest known mountain-building episodes: the Grenville Orogeny that occurred about a billion years ago.

    These mountains are vast repositories of anorthosite, stored as the AMCG group. (Mukherjee and Das)

See where I’m heading here?

Hey, Geotime Ginger! Welcome back! How was Germany? (Images: Ginger by Shutterstock/Grenville orogeny timeline by G. Mills via Wikimedia, public domain.

Yes, the Adirondacks are a Grenville range. Most of the exposed Grenville formations are in Canada, with this one little outlier in upstate New York. (Much of the orogeny now sits underneath younger formations, and as we’ll see later, the system actually covers a vast area.)

The reason no one could answer my 1980s questions about the High Peaks and where they came from was simply that the Grenville Orogeny is incredibly difficult to study. Scientists were just starting to check it all out with modern tools. (Rivers, 2015)

That work is ongoing, but with much more data available now, the boffins have come up with some breathtaking speculation.

Adirondack High Peaks backstory

As I understand it, anorthosite is igneous rock, but it has never erupted. (Mukherjee and Das)

The current working hypothesis, I think, is that anorthosite forms slowly in Earth’s upper mantle, under high pressure and in extremely hot conditions, when a high-aluminum type of basalt called tholeiite ponds at the base of the planet’s outer crust and veeery sloowly changes its chemistry.

In the lower crust next to this intense magma-body heat source, some minerals morph into the “MCG” group.

Meanwhile, in the magma body itself, “A” (anorthosite) crystallizes out of the melt as almost pure plagioclase feldspar.

Anorthosite, being fairly buoyant, will rise toward the surface if and when a pathway opens for it. It will gather up some of that lower crust MCG mineral banding along the way.

But it doesn’t reach the surface and erupt. As I understand it, Earth’s crust in such slow-collision settings tends to be extremely thick.

This works less well. (Image: Dalmalmation via Wikimedia, CC BY-SA 4.0)

Instead, the rising diapir of anorthosite stalls out (reaches neutral buoyancy), cools, and hardens into a pluton underground (think Pluto, the underworld’s ruler in Ancient Greek mythology).

You know where something like this is happening today?

The Andes, per current thinking. (Rivers, 2009, 2015)

The crust is very thick in such regions, making it possible to store huge amounts of magma, such as today’s Altiplano/Puna batholith.

How did the AMCG magma get up into the crust without erupting?

I can’t find a helpful video showing the process, at least insofar as I’ve been able to follow the boffins thinking on that, but this GIF of a salt dome forming gives you the general idea:

Salt rises because it is much more buoyant than rock. The anorthosite that became the Adirondacks rose because it was a little more buoyant than surrounding country rock. (Image: yuweihgeo via Wikimedia, CC BY-SA 4.0)

Per Rivers, 2015, extension — inverted back-arc spreading — during continental collisions as supercontinent Rodinia came together fractured Earth’s crust, opening up pathways for that MCG-banded anorthosite diaper 🙂 diapir to rise.

I think he means this (jargon alert).

There must have been some tremendous earthquakes back then. But the rising AMCG melt never reached the surface. Instead, it went all plutonic and solidified underground as geologic time passed.


With this new (to me) knowledge I now have returned to the Adirondacks in the sense that I’ve learned where anorthosite and those awesome Adirondack Mountains might have come from.

“Proterozoic: It’s mellow yellow, baby! We’re talking about 1 Ga (billion years ago) here.” — Geotime Ginger(Images: Shutterstock)

While reading up on Rodinia, the mind has also been boggled by how geologic processes shaped those particular AMCG Proterozoic plutons.

Imagine Andes-style mountains rumbling away on the edge of some landmass, with back-arc spreading and the molten diapir that will someday be the Adirondacks just sitting there in the crust, a little ways inland, accumulating more and more volume.

Only there is another landmass offshore, not the wide Pacific that fronts the Andes.

Let’s let the geologists decide on a name for it.

Forces involved in the Grenville Orogeny are bringing it closer and closer to our own landmass — and then the two masses collide slowly, majestically, and over a very long time, smashing the volcanoes and squeezing the vast AMCG pluton behind them upwards much as the India-Asia continental collision is raising the Tibetan Plateau today.

The collision eventually ends. Over the next billion years, weathering and various geologic processes then uncover the uplifted AMCG pluton, while water, wind, ice, and the biosphere sculpt it into a massif-style mountain range.

Now put yourself back in the Proterozoic and imagine such a collision occurring over some 3,100 miles of coast — lands that now stretch from the US Southwest through, yes, Upstate New York and Canada, but also Greenland and Scandinavia to Eastern Europe and the Ukraine Shield.

That’s all Grenville rock. And it holds together a supercontinent: Rodinia.

We don’t know for sure what Rodinia or the supercontinent before it — Columbia/Nuna — looked like.

But Laurentia, which today is one of the world’s oldest and largest cratons (Hoffman), probably sat at the core of both these supersized jigsaw puzzles.

Columbia/Nuna we’ve already met; I’m still working on Rodinia.

Thank you so much for your interest and encouragement.

Back in the 80s, I was more interested in experiencing life than in recording it on film. I never rafted the turbulent rivers, but I did get out in a canoe now and then, like this, only with two-person canoes.

*** May 26, 2022 update: More objective reading of the same papers tells me that this is actually “anorthosite-mangerite-charnockite-granite (not garnet, although these rocks carry large quantities of low-grade garnet). Since this post is really a personal reminiscence, and because it’s unlikely that lay readers of the blog will really care about the difference, I have simply removed the word “technically” and left in the shiny.

Featured image: Mount Marcy summit in 1999, by petersent via Wikimedia, public domain.


Hoffman, P. F. 1988. United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia. Annual Review of Earth and Planetary Sciences, 16(1): 543-603.

Moore, E. S., and Dickin, A. P. 2011. Evaluation of Nd isotope data for the Grenville Province of the Laurentian shield using a geographic information system. Geosphere, 7(2): 415-428.

Mukherjee, A., and Das, S. 2002. Anorthosites, granulites and the supercontinent cycle. Gondwana Research, 5(1): 147-156.

Rivers, T. 1997. Lithotectonic elements of the Grenville Province: review and tectonic implications. Precambrian Research, 86(3-4): 117-154.

_____. 2015. Tectonic setting and evolution of the Grenville Orogen: An assessment of progress over the last 40 years. Geoscience Canada: Journal of the Geological Association of Canada/Geoscience Canada: journal de l’Association Géologique du Canada, 42(1): 77-124.

Santosh, M.; Maruyama, S.; and Yamamoto, S. 2009. The making and breaking of supercontinents: some speculations based on superplumes, super downwelling and the role of tectosphere. Gondwana Research, 15(3-4): 324-341.

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