STRIKE the DIP!

Introduction to Geochronology

Have you ever wondered, “How do geologists know how old a rock is?” 

Geochronology is the science of determining how old rocks, sediments and fossils are. Whether they formed 4 billion years ago or within the historic record, geologists can employ geochronologic methods to determine either the relative or absolute ages of these materials. 

Radiometric dating–which measures the amount of decay of a radioactive isotope with a known half-life–is the most commonly utilized method in geochronology. There are a bevy of different isotope systems, each with different half-lives that dictate the time frame over which they can be applied.

Isotopes–which are elements with the same number of protons but a different number of neutrons–can be radioactive and decay over time. A half-life is the amount of time it takes for one half of theirs atoms to decay. As their atoms decay from what is the parent (P), they become daughter atoms (D).

Once a rock crystallizes, the clock for the half-life begins; all the atoms are still the original parent. After 1 half-life, half of the parent atoms have decayed to daughter atoms. Accordingly, after each consecutive half-life, half of the remaining parent atoms will continue to decay to daughter atoms until you are only left with the daughter.

By knowing the half-life of the isotope system you are working with, you can measure the daughter/parent ratio (D/P) to determine how much time has passed since the rock was formed. The table below shows a simple ratio schematic based on the decay from parent to daughter in the rocks above.

Different isotope systems can have half-lives ranging from seconds to billions of years; you need to choose an isotope system that has a half-life that will cover the time frame of the expected age of your rock (you need an appreciable amount of both parent and daughter atoms to measure the ratio for an age).

Over the next coming weeks, we’ll be giving a primer on some of the most common isotope systems and geochronologic methods to help understand how we can date the Earth and its materials.

Spectra

This colorful scene was photographed along the Kancamagus highway, a road that cuts east-west in Northern New Hampshire, which just passed its peak fall tree colors. The rocks of New Hampshire are a mix of igneous and metamorphic rocks, produced as a series of island arcs ran into and accreted onto the edge of the growing Laurentian continent prior to the collision with Africa and Europe.

-JBB

Image credit: Dennis Forgione https://flic.kr/p/zBdjF9

An angry mountain comic.

Stuck Slab

In basic geology classes we’ll tell you that at subduction zones, oceanic plates sink into the mantle where they’re recycled and fluids from the subducting plate produce volcanic arcs. While this picture is the case in some places in the world, in other places the story is much more complicated.

Caledonian folding

Back in the early Cambrian some 500 million years ago, the continental dance had an active phase that resulted in the assembling of many of the plate fragments that form the bedrock beneath the European continent. Chunks of rock that were joined then that have stayed together through the vagaries of supercontinental gathering (the formation of Pangaea, completed some 300 million years back) and dispersal. There were two main landmasses in those days, known as Gondwana and Laurentia (for a piece on the formation of Gondwana see http://on.fb.me/1D548m8).

From Mindat.Org Photo Of The Day; August 19, 2015:

Amethyst - Exposed to UV / Heat Amir Akhavan

A part of a deeply colored amethyst geode from Rio Grande Do Sul was cut into four pieces. Three of them were treated with heat or UV radiation. The four pieces were glued together again after treatment.

Top Left: Untreated, original color. Color seems to be evenly distributed in the tips, but is most intense 1-4 mm under the rhombohedral crystal faces.

Top Right: Exposed to UV light from two 18W low pressure mercury lamps (germicidal UV-C lamps) for 3 months. The crystals have lost almost all their color, but some slightly violet patches are still visible inside some crystals. Fading proceeds relatively quickly in the beginning, but slows down with time. The complete bleaching would probably have required another 2-3 months of exposure.

Bottom Left: Heating to about 380°C for 8 hours resulted in loss of most violet color and an overall patchy appearence. Some crystals are still colored violet in central parts while the outmost layers of the rhombohedral faces start to develop a yellow-brown color.

Bottom Right: Heating to about 450°C for 12 hours. The violet color is gone, and a orange-brown color appears that is most intense close to the surface. Note that the most intense violet color of the untreated crystals is not directly at the surface, but a few millimeters inside the crystals.

I guess I can finally share my pieces from the Iceland Residency Exhibition at Light Grey Art Lab!  Here’s my little blurb from the show, so I don’t have to write it out all over again:

“I love weird landscapes. I love inscrutable waterfalls, I love columnar basalt, I love incomprehensibly massive glacial tongues and unexpected volcanic activity.  I love all of this stuff, and so it frustrates me to no end that it’s so rarely reflected in my own work.  I spend most of my time (both professionally and personally) designing characters and costuming, so this show seemed as good a chance as any to step out of my comfort zone and focus on honing my landscape painting skills.  Hard to go wrong with Iceland in the landscape department. I did a few plein air studies while on-site in Iceland, but most of my time during the residency was spent just taking photos and trying to absorb the scenery.  I ended up with ten finished paintings, and tried to focus on some of the imagery during the trip that really stuck with me – the acidic green mosses, the almost eternally overcast sky, the force of water, the semi-abandoned structures.  As much as I enjoyed the scenery, it’s not hard to admit that I enjoyed the people even more; this residency wouldn’t have been half as worthwhile if it weren’t for the amazing and talented people I got to spend it with.”

Fieldwork: Not just a geology holiday

As a geologist I often come across people who think my fieldwork is just an excuse to go relax in the sun while looking at the occasional rock. I can understand the confusion, why fly out to a different continent to study rocks when the department is already full of samples?

A couple of students checking out a syncline and anticline in the Kolob area of Zion National Park. We were hiking to Double Arch Alcove. March 2015.