Photo of the Week #2

Ok, it has been more than a week since I last put up a photo of the week post so I apologize for the lateness of this next installment.

Anyways, here it is:

The Burgess Shale

The topic of this weeks post is the nightmare world pictured above. Believe it or not, this is an artists representation of what life 530 million years ago would have looked like in a shallow equatorial sea located in Alberta, Canada at a location now known as the Burgess Shale in Yoho National Park. The Burgess shale represents a rare window into what life was like in the Cambrian period. Part of what makes the Burgess shale so special is the degree of preservation of soft tissue found in the fossil organisms. Most fossils that we find today only preserve the "hard parts" such as shells, bones and exoskeletons because all of the soft body tissues have decayed. However, at the Burgess shale everything is preserved including soft tissues that normally decay quite quickly.  This allows paleontologists to examine organisms that would never have been preserved otherwise and to see details of organisms that would not have been preserved under normal circumstances. Fossil deposits such as the Burgess shale that preserve unusual features or large number of fossils have a special geologic term: lagerstatten, which is the German word for "mother lode".

So what are these abnormal circumstances that have allowed the Burgess Shale to preserve the soft tissues? To answer this question we must examine how the Burgess Shale was formed in the first place. The creatures of the Burgess shale lived on or near a large carbonate reef platform, kind of like a large underwater tower of algae and large sponges that were capable of building reefs similar to the coral reefs of today.

Schematic of the Burgess Shale

However, one unlucky day, a large mudslide occurred transporting the creatures into deep water and burying them under millions of tonnes of mud. Not a pleasant way to die! The wonderful thing about this mudslide is that once the creatures were buried there was not enough oxygen to cause decay. In order for decay to occur oxygen must be present, and in these anoxic conditions the soft tissues were preserved. As you can see below every detail of these creatures is visible.

Here are some pictures of the Burgess shale fossils:

Hallucigenia

Hallucigenia

Marella (this poor guy got squished during the slide) The black mark on the rock behind it are its insides.

Marella

Opabinia

Opabinia

The importance of the Burgess shale is more than just well preserved and attractive fossil specimens. The Burgess shale has preserved over 140 different species of organism, many of which belong to previously unknown phyla and have given us insight into the very beginnings of life in a much more detailed and complete way than was previously known. Furthermore, many of the Burgess shale fauna have descendants that still exist today, which elucidates a family tree extending back to the beginning of complex life.  Alternatively, some of the Burgess shale fauna represent failed experiments in evolution and are dead-ends in the tree of life, which, without such well preserved fossils we would never have dreamt existed. 

For more information on the Burgess shale visit these websites:

The Burgess Shale Geoscience Foundation: http://www.burgess-shale.bc.ca/

The Smithsonian Museum of Natural History: http://paleobiology.si.edu/burgess/

Matt

How old is the Earth??

Well, since this is a blog about all aspects of geology I figure it is best to start at the beginning and ask a question that has been hotly debated by humans throughout our history: How old is the Earth? The age of the Earth that is globally accepted by geologists is 4.6 billion years.

4.6 billion is a ridiculously large number, so before, we delve into how we came up with it in the first place lets get some perspective on how much time 4.6 billion years actually is. Most of us can't envision much more than 1000 years accurately so trying to understand the incomprehensibly large amount of time 4.6 billion years represents is just that: incomprehensible. There are many ways of putting it into perspective but one that works for me is to imagine the age of the Earth is represented by distance. Imagine a path 100 metres long. If we were to let the length of this pathway represent the entire age of the Earth each year would be:

100m / 4,600,000,000 years

= 2.17 x 10^-8 m/year

= 21.7 nanometres/year

Each year is 21 nanometres which is about the width of 10 atoms lying side by side, or the size of a virus or 2000x less than the width of a human hair...so really, really small! Therefore, when we think about this in terms of our 100m pathway we can start to understand how much time 4.6 billion years actually represents.

Of course, this scenario assumes that the age of the Earth really is 4.6 billion years old and begs the question how did we come up with this number in the first place? Time for a bit of history.

One of the first attempts to date the Earth, which was largely accepted at the time, was performed in 1650 by Archbishop James Ussher. Ussher calculated the age of the Earth by adding up the ages of people in the Bible and correlating the dates of events in the Bible with historical records from other cultures. In this way he found that the  Earth was created on the evening of October 22nd, 4004 BC.

Ussher's age for the Earth was accepted until Lord Kelvin (the namesake of the temperature unit "Kelvin") proposed one of the earliest scientific methods for dating the Earth in 1846. Lord Kelvin, a physicist, suggested that the rate of cooling of the Earth could be used to predict when the Earth formed. He assumed the Earth was molten at the time of formation and by simply calculating the time it would take for the Earth to reach its present temperature given a steady rate of cooling he was able to calculate an age. Kelvin's calculations resulted in an age for the Earth between 20-30 million years ago. Many geologists disputed Kelvin's results, but were unable to offer a satisfactory rebuttal until the assumption that the Earth had begun in a molten state was challenged and because, in my opinion, they were afraid to challenge a man with such an epic beard. Ironically, the major flaw in Kelvin's calculations was that he had not accounted for heat produced by radioactive decay, which would later become the very tool used to date the Earth.

Lord Kelvin and his beard

The next step in dating the Earth did not come about until the concept of radioactive decay and radioactive dating was created. Some elements have very long half lives which enables geologists to use them to date rocks. For example, by calculating the amount of a certain daughter product present and the initial amount of the parent element we can calculate the time it took for the parent to decay into the daughter giving us an age for a rock. Radioactive dating relies on dating isotopes trapped in rocks to find a date. Some of the oldest rocks that have been found thus far come from Northern Canada in a geological unit called the Acasta gneiss. The rocks from the Acasta gneiss have been dated at 3.96 billion years using uranium-lead dating. Grains that are even older of the mineral zircon have been discovered in metamorphosed sandstones in Australia that date at around 4 billion years old. There is even an argument currently raging in the geologic community about the new oldest rocks that have been found in the Northern Canada. I personally have seen these rocks and I can tell you they don't look like anything special. However, there is still a problem: none of these rocks actually tell us how old the Earth is as the crust did not form immediately and may have taken many millions of years to form. So then how did we come up with this crazy 4.6 billion number? Primordial lead ratios.

The Acasta gneiss

Yes, I realize primordial lead ratios sounds like gibberish a geologist might spit out after a few too many drinks in the field, however, they are the key to our 4.6 billion year old date for the Earth. About a third of the lead on today's Earth is a product of the radioactive decay of uranium and two thirds is from the original formation of the planet. But, some meteorites contain no lead produced by radioactive decay so they have lead isotope ratios that have not changed since they were formed. These are the primordial lead ratios. The reason this matters is because it allows us to extrapolate from our current lead ratios, which have been changed by the addition of radiogenic lead, back to the original lead ratios at the time of the formation of the Earth. Since we know the rate that radiogenic lead is produced by the decay of uranium we can calculate how long it has taken to get from our primordial lead ratio to our current lead ratio. The answer is 4.6 billion years!!! Phew.

Meteorite

So there you have it. The progression of the age of the Earth up to our currently accepted value of 4.6 billion years and how we got there. Thanks for reading.

Matt

Reference: Prothero and Dott, Evolution of the Earth , 7th Edition, 2004.

Photo of the Week #1

Ok, here is the first actual geology related post of the blog.

I took this photo in the Yukon Territory near Red Creek while I was doing field work last spring. Any guesses as to what it is?

I'll give you a hand on this one. Red Creek is very deserving of its name. It is highly saturated with iron from the local shale bedrock and in certain conditions can precipitate as iron oxide. This photo is iron oxide, likely Fe2O3. The weird shapes that cross though it are made by ice crystals that have since melted, but left their impression on the soft iron oxide powder. The iron is at the bottom of a shallow puddle sitting on thicker ice from the winter. Pretty cool eh??

Here is a photo of Red Creek.

Matt

Welcome

Hi,

As of the time I write this post I realize no one has looked at this blog yet. Therefore, I guess I am just writing for the sake of writing...and procrastination. To any future readers of this post I welcome you to this blog.

I am a geologist and current PhD. candidate in geology and one of the things I have noticed is the lack of geological knowledge and education the general public has and is exposed to. This blog is my attempt to fill that void and bring a few interesting geological tidbits into the public view. With this in mind I will be posting as I find the time about different aspects in geology from rock, mineral and fossil collecting, current geological research and new scientific developments to interesting photos. I feel that gaining an understanding of the world around us the processes that have shaped it over the last 4.6 billion years has given me a great appreciation of the natural world and I would hope that it can do the same for you.

I hope you enjoy it.

Matt