From Sawtooths to Zircons

Picking away at the Sawtooth Metamorphic Complex

As we know, the Sawtooth Mountains are very old. The granite was formed over a hundred million years ago, longer than any human could ever lay claim to. When compared to other geological formations they are relatively recent though – for example, the Appalachians are about 480 million years old. Yet, within these walls of adolescent granite lies much more elderly rock.

Granite (technically granite diorite) of the area is known as igneous rock. This means it was formerly molten rock that cooled and hardened. Magma and volcanic action makes for great Dwayne ‘The Rock’ Johnson films, but they are not as exciting as metamorphic rocks. These are rocks that reached their current state through conditions of extreme heat and pressure, and lots of time. Usually they take on an extremely crunched and stretched appearance, such as gneiss or marble.

Sawtooth Range

The Sawtooths, despite being majority granite, have small samples of these metamorphic rocks present in them. There is one large section of the range that peculiarly has an abundance of these kinds of rocks, and they have some interesting traits – the Sawtooth Metamorphic Complex (SMC). This resides near some of our highest peaks, and has some extremely unique specimens. When observing the range the question that popped into geologists heads was, “how old is it?”

So I am diving in to, “Detrital-zircon geochronology of the Sawtooth metamorphic complex, Idaho: Evidence for metamorphosed lower Paleozoic shelf strata within the Idaho batholith,” a 2016 research project published in Geosphere if you want to read the source. Don’t be too scared, I’ll break things down a bit.

Chemical dating is done through measuring isotope ratios. Essentially, as certain elements decay over time and lose electrons, they turn into isotopes. Geologists can then measure the amount of isotopes present in a rock sample. There is a lot more to it, but an easy way to visualize it is to imagine a glass of ice. Ice naturally will melt in a room, and we can very easily measure at what rate the ice is melting at. When you first put the ice in the glass, the glass is 100% solid ice. After a while, there is about 50% ice and 50% water. Since you know how quickly ice melts, you can calculate how long that glass has been sitting there based on how much water and ice there is in it. Isotopes operate in a similar way, geologists can estimate how old a rock is based on how much of a certain isotope is present compared to the element that produced it.

The authors investigate this question through the method of analyzing detrital zircon crystals. No, this is not from Star Trek, these are instead extremely unique geological formations that are very helpful to geologists. Most rocks are not good at keeping isotopes stable. It would be like if someone added more ice to your cup, or someone else poured some water out without you knowing. Zircon crystals are the equivalent of putting your ice water in a locked safe. They are extremely hard, durable, and unlikely to react with other rocks.

For a metamorphic rock, their very nature is based on extreme change. High pressure, high heat, the whole gamut. In these instances, isotopes get lost, distorted, shifted around: your cup is being thrown at the wall. A great example of this glass chucking is the collision of the 150 million year old continent colliding with the Pacific Ocean plate. This caused the current mangled sharpened strip of rock splitting America (also known as the Rocky Mountains). Despite the endless slamming of ice water glasses into shredders, dendrite crystals can seemingly endure, capturing a snapshot of the rock before that destruction. Geologists have even found zircons as old as 4.4 billion years old – only 500 million years from the Earth’s origin.

Zircon crystals may seem simple (‘it’s just really hard rocks‘), but they can change how we view an area massively. We know that the Sawtooths are mostly 100 million years old granite, but the Sawtooth Metamorphic age isn’t as cut and dry. The granite hung out with t-rex, but who played cards with the gneiss?

Zircon crystals are vital here, because they’re the one friend counting cards. The authors of the study were able to extract zircon crystals from these metamorphic rocks, and fire lasers on them. They say this was for ‘mass spectroscopy’, I say it was for intimidation. Either way, the crystals revealed the secret of how old they are.

The highest number of analyses is linked to the most probable age (Ga = billions of years ago)

After reading that graph, you may be thrown from your seat, astounded by the age of these rocks. Collect yourself slowly, I know zircons are an exciting subject, but it’s true, when you look at the highest peaks in the Sawtooths you are admiring rocks that are billions of years old.

When these rocks formed it is almost impossible to think of what central Idaho would have looked like, or where it was located on the globe. Any mountains that would have existed likely didn’t resemble the Sawtooths at all. There could have been endless sand dunes, frozen ice fields, or inland seas. Living communities might have been comprised of spiny cacti, tropical ferns, or coral reefs.

So the next time you are in the Sawtooth National Recreation Area consider the abundance of history under your feet. These metamorphic rocks connect us back in time to former worlds that have existed here. Perceiving a mountain’s age as a singular, well defined number is the same giving a smartphone to a mule deer…it’s useless.

Our time in these mountains is like high tide, slowly beginning to pull back. As the water recedes it’ll leave behind the memories of all those who found solace and peace here. The next tide is creeping in underneath. In 2020, we could feel it shaking. Some day a child might dig up the pebbles that once comprised this iconic Idaho landscape.

Written by Bryce Johnston – 2020 Naturalist

The Idaho Geological Survey’s interactive map: