Title: Plate Tectonics: An Insider’s History Of The Modern Theory Of The Earth
Author: Naomi Oreskes
Date/Year: January 20, 2003 (Westview Press)
Format: Paperback and Hardcove
We rely on coordinates, often using them as an absolute fixed location.
But we often don’t add time nor recognize the fact that the ground is not fixed while the coordinate is.
I live in Sonoma County, California.
The part of the North American Plate upon which Sonoma County rides moves on average about 2 centimeters a year north at a velocity a little slower than the rotating edge of the Pacific Plate to my west (west of the San Andreas Fault) which is moving north around 10 centimeters a year.
The rest of the North American Plate (roughly the part west of Nevada) moves about 2 centimeters a year to the west-southwest
That means that the coordinates for my desk chair are about four feet north of that location’s coordinates when I was born (in that time we’ve also invented and accepted the concept of Plate Tectonics).
Also a noticeable shift in my life time in the location of the parcel that the chair sits inside…more than my outstretched arms, more than the height of the fence that surrounds most parcels in this neighborhood.
A thousand years ago it was ½ a block south.
A million years ago the land below this chair I’m typing from was 16 miles south.
When the Chicxulub crater was formed by the asteroid associated with wiping out the dinosaurs and things had settled, it was roughly about where Bakersfield, California is now.
When the land all moves together, that is pretty handy –everything is off a bit including property boundaries, but off roughly the same in relation to everything else.
A parcel boundary is based on the distance and angles from a starting point as one walks (traverses) around the property, most often in a clockwise direction.
In my area, the coordinates for the starting point of the parcel boundary are based on the distance and angle from the top of Mt. Diablo.
Most of the time, Mt. Diablo is moving in the same direction and speed as most of Sonoma County.
Most of the time.
The rest of the time, the land we call Sonoma County erodes into the sea, jerks, lurches, lifts up, sinks down, folds, buckles, splits, bulges, twists, shifts, slumps, or moves in a different direction more quickly.
The land we call Sonoma County is like a pair of sweatpants that move and stretch with and over us throughout the day.
At the end of day, the knee is still roughly the same distance away from the cuff whether we sat in a chair or ran a mile.
Sometimes our sweats get caught on something, a nail or a twig, and they are stretched and pulled into an uncomfortable shape.
You adjust to release that tension of a wedgie or binding around a limb, like an earthquake releasing.
What do we do about the location of Mt. Diablo (our reference point) and my property move when the surface of the earth moves in different directions one morning as much as 6 feet?
Hopefully it will be relatively easy to solve, because all my neighbors’ properties will have moved in sync with mine, and we’ll all add a few feet and subtract a few degrees from our distance to Mt. Diablo.
Likely though, if I am looking at everyone’s property in the county I will see distortions and uneven shifts that add up to big differences to the fabric of parcels from one side to the other and foul up accounting in the deed recorder’s office.
The warping of the land and its offset relative to a set of coordinates is called deformation.
Last year’s 7.8 Gorkha earthquake in Nepal (April 25, 2015) created regular and significant deformation, with some displacements over 2 meters around the area of Kathmandu.
The deformation was so great across the whole country that surveyors there are considering a National Deformation Model which makes a great improvement to the process of adjusting survey data acquired before and after the recent large earthquakes. (read a more technical brief in Towards a modernized geodetic datum for Nepal)
The invention of GPS means we have different points of reference of equivalent accuracy to compare to each other.
A location reference point is called a datum.
Many older or “classic” datums were established before we understood plate tectonics.
By comparing coordinates from different datums we can measure our location relative to stars, relative to Mt. Diablo and relative to a constellation of satellites.
This means we can calculate the discrepancies between them to account for error, distortions of the atmosphere, observational bias, and movement of the earth’s surface with the precision of millimeters.
The rate of deformation obliges Nepal to modernize their datum to have a more elastic understanding of coordinates and correction of error.
This means that with a modernized datum, Nepal would not only synchronize different coordinates by a reference location (datum), but also by time (the epoch).
This is so that movement and change of the land accounts for the fact that some reference points move.
By modernizing the way it uses the datum and epoch, Nepal will be able to more easily adjust their land surveys for punctuated change (earthquakes) and the slow creep of deformation caused by being on the collision boundary of the Indian and Eurasian plates.
I wonder about this because I’m not sure how surveying in California adjusts for tectonic plate movement and the coordinates of parcel boundaries.
If my property boundary fence stands after the next big earthquake, I’ll let you know how it all shakes out.
This is a pedestrian example of the bureaucracy associated with land ownership as it is affected by the movement of the earth’s tectonic plates.
Tectonic plates are a description of the earth we accept, like that it is round, goes around the sun and has gravity.
About 40 years ago this wasn’t the case and the idea that continents drifted was dismissed as quackery.
For a more engaging view of the big picture, read about the history of the contributions to our understanding of how the crust of the earth is assembled and moves by the people who made those discoveries.
Get yourself to a library, haunt your used bookstore or download it to your phone.
” Why are reviewing an older book about Plate Tectonics on a blog about maps?” you might ask. And my answer would be “Maps of the earth would be as formless as your skin without the bones underneath if it wasn’t for tectonic plates.”
However you do it, read a copy of Plate Tectonics: An Insider’s History of the Modern Theory of the Earth by Naomi Oreskes.
Until it arrives in your hands read this summary with a great collection of animated visuals describing these forces from the perspective of the Pacific Northwest of the North American Continent: Plate Tectonics by the Pacific Northwest Seismic Network