PSN-L Email List Message

Subject: Re: Subduction Zone Question
From: Dave Nelson davenn@...............
Date: Tue, 01 Jan 2008 13:59:35 +1100

Hi Jerry,
                 happy New Year to you too

   Good questions you pose ...

    the angle of subduction does vary a bit in different locations
The SW of the South Island of New Zealand  is is almost vertical
under the North Island , NZ   it is much shallower ~ 30deg  give or take a 
few deg
(just a couple of examples)

An easy way to determine how far horizontally they go before melting is to 
look at
what is happening at the surface ....  look for ranges of volcanoes
ie.  the Cascades  in NW  USA,  the central North Island, NZ,  volcanoes
The volcanoes of the Andes  in Sth America,   the strings of  volcanoes along
  the islands of Sumatera and Java in Indonesia,  the volcanic island chain of
the Aleutians    etc etc   you get the picture  :)      <--- a quick North Is, 
NZ   cross-section

Yellowstone is a  Hot Spot  (correct term)   as is Hawaii   they are not 
related to
subduction they are a mantle plume that ongoing reguardless of the plate motion
going on above them.
Look at seabed maps of the Pacific Basin  and you can see many strings of
islands, atolls and seamounts  that indicate past and current Hot Spot 

A more interesting study is the depth to which the descending slab goes before
it melts.   and one of the deepest areas for this is in the Tonga - 
Kermadec Trench
in the South Pacific between Fiji and New Zealand.
The depth to which the slab descends is directly related to the rate of 
plate motion
in a given region.  and in the T - K Trench  you get quakes regularly to in 
excess of
600km, but horizontally maybe only a few 10's of km (20 - 100km)  away from 
trench.  Plate motion here is  ~ 7 - 8 cm/yr  and decreases as you head south
from the Fiji end of the zone towards the North Is of NZ.
    I could go into the why's of that   but thats another whole lesson in 
   The faster the plate is moving,  the faster the subduction, therefore 
the deeper the
  slab will descend before it melts.
Another interesting effect is in this region .... the type of quakes 
occurring at the
great depth.  Picture a subducting slab 10km thick, the norm for the seafloor.
and you can imagine as it starts to subduct  its cold (relativey speaking)
but as it subducts it starts to heat up from the outer layers towards its 
But because of the high speed of subduction (plate motion), the core of the 
stays cold for a very long time   ie.  there is very high temperature 
across the slab. This results in many tremors within the slab as it 
internally fractures rather than events just between the surface of the 
slab and the
surrounding rock its grinding past.

Here's a project for you ..... and you can do it for any subduction zone

Plot earthquakes (from the USGS/NEIC records)  on a graph showing depth Vs 
distance from the trench line.  (ie a cross-section across the subduction zone)
and it easily show you 1) .. the angle of subduction,  2) the distance from 
trench the subduction zone extends.

Now b4 everyone screams its already been done with Alan Jones's seismic
prog,  why repeat it ?  yes I know it has.

But to actually do the exercise yourself  and plot the data from the 
records for a given region.  It gives you a ( I believe)  a better insight 
and understanding into the processes going on   :)

cheers all

Dave Nelson

At 02:17 AM 01/01/2008, you wrote:
>Hi All,
>Happy New Year.
>I have a question about Subduction Zones and their angle of incidence down 
>to the mantle.  If I remember correctly, the Mariana Trench is quite steep 
>while others are not.
>I was wondering specifically about the Pacific and Juan de Fuca plates and 
>their subduction angle under the Cascade Mountains.  Specifically, I am 
>wondering how far the subduction angel goes inland before it melts into 
>the mantle?
>The 3.8 event at the Yellowstone Park area brought the question to 
>mind.  I know there is supposed to be a Hot Pocket under that area and 
>potentially explosive, but I was wondering if the subduction wedge 
>extended inland that far.  Truthfully, I don't know how far Yellowstone is 
>from the coast.
>Of course, the farther inland, the deeper the wedge would be.  The 
>Yellowstone event was shallow (6.8 km), and obviously not what I an asking 
>about.  Nevertheless, I was wondering about that specific area 
>anyway.  Anybody know?
>What the heck do you have to think about anyway, New Years parties?
>No virus found in this incoming message.
>Checked by AVG Free Edition.
>Version: 7.5.516 / Virus Database: 269.17.12/1203 - Release Date: 
>12/30/2007 11:27 AM


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