Tuesday, September 3, 2013

Hiatus: Some Geological Truths May Never Be Known...

A hiatus in geology is a significant interval of time where there is no rock record in an area. The boundary between the two beds is called an unconformity. Here a granite deposited 1,100 million years ago is directly touching a 500 million sandstone.  This unconformity represents a 600 million year hiatus.

Sometimes in geology we're very lucky and the right "recipe" leads to amazing insights into a period of time.

The Burgess Shale is a great example. Strange, soft animals living more than 500 million years ago were suddenly buried under a sheet of fine mud. The mud kept away scavengers and oxygen, locking the animals away like mummies in a tomb. Half a billion years of lucky chances lead these fossils to survive erosion and tectonic activity, leaving the exquisitely preserved remains to be discovered in Canada in 1909. It is an amazing snapshot of some of the earliest complex animals on Earth.

But usually we're not so lucky.

Sometimes rocks are destroyed or altered by the squeezing forces and heat that move continents, turning sandstone and shale into quartzite* and schist. Sometimes wind, ice and water scour away any trace of the rock. Sometimes the rock is never deposited at all.

We don't always know what causes a hiatus - a gap in the time represented in the rock record. Like a cut in video footage it can skew our interpretations of events.

But I have always loved the idea of a hiatus because it means that we not only don't know everything, we literally can't know everything. There will always be some mystery.

My favorite quote on the idea of hiatus comes from Darwin's "On the Origin of Species":

"For my part, following out Lyell's metaphor, I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated formations." - Charles Darwin, On the Origin of Species

*Pro tip: play "quartzite" on a triple word in Scrabble and your opponent will never know what hit him!

ABC Wednesday


  1. A very interesting use of the word Hiatus.
    Fascinating information.

  2. Wow! I hadn't thought about the creatures in the Burgess Shale in years, Even though I attended college in Southern California we learned about the Geology and geography of Canada due to the fact that our Prof. was from Toronto.
    We learned about the Canadian Shield first and only then moved on to the rest of North America.
    His method must have worked most of it all just came roaring back to me after a few decades!

  3. Great SCRABBLE advice! Though a 9-letter word means some 2 or more letter word had to be out there already.
    Re hiatus: when in doubt, blame space aliens.

    1. True. It can be played off of quartz, quart, art, and many more.

  4. Great to learn what hiatus is in geology. I only knew hiatus in blogging.

    Thanks for the pro tip. My mother is an excellent scrabble player. I will see how excellent when I make a surprise attack with that quartzite. Lol....

  5. That's very interesting - I didn't know that, and it is something to speculate about what was really happening during this hiatus.

  6. The past is even more interesting than the future especially when you have reached the age of eighty. We are the past, I think more and more of the things that happened in my life long ago.
    The ancient past is almost unthinkable. Thank you for this post!
    Wil, ABCW Team

  7. Fantastic post for H this week. Great shot.

  8. I studied geology in my first year at university - had no idea what I was signing up for but it was fascinating. Love the scrabble suggestion!

    abcw team

  9. I've always thought this area of geology was fascinating! I love looking at the different layers in rock formations. Great photo.