16 March 2012

Heat Flow

How are the other planets like - and unlike - our Earth?  To answer that very fundamental question, at the dawn of the Space Age my friend and fellow USGS scientist Gene Shoemaker founded the Branch of Astrogeology in Flagstaff, Arizona. He and his many fellow scientists there have figured a LOT out about the other planets by using data and imagery provided by NASA.


I am told that the core of the earth is as big as the moon and as hot as the surface of the sun, and that the mantle is pretty darn hot too...

Why doesn't all of this heat transfer, move through, conduct up through the crust so that the surface of the earth would at least be very warm to the touch?

peter h

The Earth's core is not quite as large as the Moon - it's about 70% of the Moon's radius using evidence accumulated from the seismic tomography studies over the past half century or so. Think: earthquakes send sound waves downwards, and sophisticated calculations convert the refracted waves and their arrival times into an image of the Mantle and the Outer Core and the Inner Core. No one has ever measured the Core's temperature directly, of course, but laboratory high-pressure experiments, along with theoretical calculations, suggest that the temperature may be in the 5,400C/9,800F range - pretty close to the surface temperature of the Sun.

Actually the Core's heat DOES transfer outwards, and in some pretty spectacular ways: parts of the Earth's crust (Kamchatka, for instance) are moving as much as 8 cm/3 inches per year under the convective force of that heat trying to escape.  Think: this crustal movement is analogous to the skin moving on the surface of a pot of cooking Cream of Wheat. This is the reason we are seeing those monstrous earthquakes off the coast of Chile, Japan, and in Haiti.  The fact that heat escapes from the core is also manifested in the hundreds of volcanoes we see, for instance, all around the Pacific Ring of Fire.  The continental crust rides up and over the down-going (denser) oceanic crust, which melts as it goes deeper and gets hotter, and the lighter water-and-gas-saturated components work their way upward through that continental crust to give us things like Mount St Helens.  

Heat flow is actually a venerable (old and respected) field of geoscience. There are specialists who study heat flow all their professional lives - you have to put sensors deep in wells and block the fluids from convecting in order to get accurate numbers. There are places like Battle Mountain, Nevada, where the heat flow - the amount of heat escaping through a square meter of the surface - is several times higher than it is, on average, elsewhere on the Earth's crust. Another manifestation of that heat flow is the fact that no matter where you are on the Earth's surface, you can go down in a mine a few tens of meters/yards, and the temperature will almost always be about 55 degrees F (12 degrees C). It could be 122F/50C on the surface, and it will still be that cool at depth. It could be 'way below freezing on the surface in the Arctic, and it will STILL be 55F/12C at a drill-able depth. As you go much deeper, perhaps 4,000m/12,000 feet deep in some of the South African gold mines, the temperature gets hotter and hotter the deeper you go. A friend told me that in one South African gold mine, the temperature at the rock face at those depths can be 140F/60C.  The deeper you go, the hotter it gets.  

Why isn't the crust hot to the touch?  For the same reason that a cinder-block wall is good insulation against the heat of the day-time sun. Rock is just not thermally conductive like a metal is - it's usually a pretty good insulator, in fact. For this reason, when the heat can't easily get out by conduction, it gets out by convection, but at a much slower rate.  Think of that pot of Cream of Wheat again - that's convective heat transfer going on. On the global scale of the Earth's crust, this is the same thing as continental drift... which gives us huge subduction earthquakes and volcanoes.  

14 March 2012

Sea Level Rise

One of the more startling side effects of Climate Change is sea level rise. (Yes both are very real, and the only partially unanswered question is how much of it you and I as human beings have caused ourselves.) There is a loud scientific cat-fight going on about sea level rise - and another equally loud one as to whether climate change is the reason for the increasingly violent weather extremes we've been seeing. These recent extremes include multi-year droughts in Texas and the Southwest, EF-4 tornadoes in the Midwest in March, monster hurricanes in the East Coast and similar typhoons in the western Pacific, etc.  Oh: One of the more obvious pieces of data is that nine of the hottest ten years on record historically have been in the past decade: in this century. 

For some people there is a growing realization that sea level rise can have some rather direct personal consequences. As in property-loss. As in losing your entire country.


Sorry for the rather twisted syntax here, but my kids are interested to know whether about 250,000 years ago Virginia's Atlantic coast was generally farther east or west of today's coastline. In other words, was more of what is now Virginia covered by the Atlantic ocean then than today? Thanks for any help or direction you can provide.
Mike M.


Good question - you have smart kids to even think about this - and a firm number here is hard to pin down precisely. The following link will give you an image of seawater high-and-low stands over the past 900,000 years: http://en.wikipedia.org/wiki/File:Sea_level_temp_140ky.gif

The short answer is that 250,000 years ago there was substantially more of Virginia than there is today: the coastline went much farther east than it does now.

I know relatively little of Virginia's coast and offshore topography except for the (probably exposed then) Smith Shoals, where I dragged an electrical geophysical streamer behind a ship not so many years ago looking for titanium-bearing placer sands. All I remember was days of 5-meter (15-foot) seas and a long-term case of seasickness. Only 4 people of a ship's compliment of 19 even bothered to go for lunch or dinner. The data were good, though.

To put things in perspective, just 17,000 years ago the northern Florida coastline was about 50 kilometers farther east than where it is today. In a Gulf Coast estuary I've seen side-scan records that show a beach berm 6 meters below modern sea-level - and when it was tested with a vibracore, there were burned seashells in the sample.

Translation: They saw and drilled a paleo-indian campsite that is now 20 feet below modern sea-level.

What is interesting to me in that chart is not so much the cyclic nature of sea-level stands, but the fact that the most recent sea-level stand is the highest. THAT tells me two things:
1. The Virginia coast today has the least land area in the past million or so years, and...
2. At current generation of CO2 (historically this was around 280 ppm in our atmosphere; it is now closing in on 400 ppm), it means that Virginia is about to lose even more land to the sea as Antarctica and the Greenland ice cap melt - which appears to be accelerating vis-a-vis their state even 50 years ago.

So... don't look for long-term investment on beach-front property.


Was it because the atmosphere was colder and there was more sea ice than now?

Seawater stands were lower then because monstrously huge glaciers covering most of the northern hemisphere were greatly expanded and tied up so much surface water. In Norway the glaciers were 3 kilometers (two miles) thick. And yes, that means colder (as an average) by many degrees C everywhere.

Were ALL the seas lower?

Yes. There are tectonic movements - for instance, Scandinavia has been and still is rebounding many meters (about 1 cm/year) since the "ice monster" glacial cover melted - and these sea level changes are 'relatively' persistent. Ports have had to be repeatedly relocated in Scandinavia because of this. In most of the rest of the world, however, sea level is creeping up. Also, seawater seeks its level worldwide - like if a flood inundates a city, the water gets everywhere quickly. Tides and large storms (typhoons, hurricanes) will temporarily raise local sea level, but it will always re-equilibrate to provide a "Mean Low Water". This is a legal term used to define edges of seafront property - I was once threatened as part of a USGS survey team in Alaska because of a drunk property owner not understanding this. MLW is what is now starting to change as the glaciers melt ever more rapidly, and huge chunks of the Ross Ice Shelf break off and float away from Antarctica. I suspect sea level rise will soon (if not already) match or overmatch the isostatic rebound in Scandinavia. Low-lying places like Tuvalo and the Seychelles are already actually witnessing the frightening loss of their entire countries! Because of tides and storms, it takes a lot of recording over many years to be certain of the actual sea level rise.

What about on the west coast?


Same - all sealevels rise at the same time. The latest world-wide data says the water is moving up about 3.1 mm per year (EOS, 6 December 2011). That's an inch every 8 years, 16 inches a century.  Another recent paper (EOS, 8 November 2011) points out that the huge groundwater depletion going on all around the world is increasing river flows... and adding to the sea-level rise. The Washington Post (22 March 2012) reports that a new long-range military assessment is predicting water wars this century - nations going to war over water rights!