Rockaholic Adventures

Hi,

if you search for some years old articles in SciAm you will find a super article on these events: any 300 +-? years the tsunami is triggered by a sudden uprise (half a meter) of the North America Plate.

This is deducted from the discontinuities in peat deposits and river sediments.

There are other examples of these geologic impulse generators that may cause severe damage. The plates do not glide continuously, but are driven with nearly constant velocity, so compression and stress is storing considerable elastic energy.

The tsunami is triggered by the uprise of the water above the North America Plate and the simultaneous down movement of the Yuan de Fuca Plate.

So two waves will be triggered: one from coast to Pacific by the water above the uprising area and the other from the Pacific towards the coast by the trough formed by sudden down-step. Both will have devastating energies.

RHABE

The Juan de Fuca Plat is rather thin in comparison to most oceanic crust, due to the close proximity of the subduction complex to the spreading ridge. This explains the abundance of fracture zones and significant transform faults that developed and fragmented the plate about 7MYA.

Since the plate is shrinking in size, slab pull might be the predominant force acting on the body with its thin rather buoyant figure resisting subduction and causing a significant amount of stress to build up before there is a faulting. I would be interested to see measurements of the buldge of the plate before it enters the subduction complex. I would suggest it is abnormally large in size and might help explain why this area might be prone to causing catastrophic tsunamis.

Hi Shawn,

the Tsunami was explained in the SciAm article only as consequence of the vertical displacement of the plates. Step height times area displaces a big volume (up or down) and the resulting water flow triggers the tsunami.

Why should a thin plate be more buoyant than a thick one? The only consequence of thickness I can imagine is lower heat conductance thus hotter below and thus lower in density. But oceanic plate consists of different material of better heat conductivity so I don' know. This would decrease the difference in top level of the two plates(?)

Bulging action as crack generation sounds interesting, but what force is pulling?

I thought about lower boundary layer friction force of slowly moving magma as the force generating mechnism, and this would be pushing towards subduction.

RHABE

The forces driving subduction are slab pull and ridge push. The thickest part of the plate, bearing more gravitational forces, is the subducted slab. The abundance of fracture zones dissecting the plate shows its fragility.

Buoyancy in most oceanic plates is due to crustal contamination, which is not the case here. Since there is a lack of a deep trench and less elevation difference between the spreading ridge and the subducted slab than has been observed in other areas, the plate material near the subduction complex may not be any more buoyant, but exists at a higher elevation than normal. This phenomenon may resist subduction and cause accretion wedges to form.

The theories you explained that generate tsunamis are correct. I'm attempting to explain why the oceanic crust in this area isn't so readily subducted and furthermore why this geographical location has become associated with the generation of catastrophic events.

Resource:

http://academic.emporia.edu/aberjame/tectonic/cascade/cascade.htm#setting

Hi Shawn,

"forces driving subduction are slab pull and ridge push"

I estimate that these forces are only a minor part of total forces.

Pull will readily break and extend the plate as no plastic flow of the plate material is possible, so explain some peculiarities of this plate.

Push will not only accumulate a bulge (existing) but this will be composed of multiply broken parts (I don't know if existing).

But all these plates are much too large to be driven mostly by push or pull or both.

A length to thickness ratio of 1:8 will be the limit which may be stable if pushed, pull will be worse.

As plates are typically 5km thick any plate longer than 40km will brake , and being pulled apart if pulled or crumbled if pushed. This is not existing, so:

Primary cause is upwelling hot magma (not really what we know as a liquid but very very viscous.

Upwelling causes the ridge as the upwelling magma is hotter than the surrounding magma and has to be reoriented in the direction of flow to horizontal.

And upwelling is causing new material to cool and form new oceanic crust.

The horizontal flow of the magma (minimum horizontal travel is a few hundred kilometers) beneath the cooled and rigid plate of older seafloor is the main driving force. It generates a viscous force that acts in the horizontal shear zone beneath the plate. This will drive plates of any size.

If the plate hits a continental crust there may develop a subduction zone - regardless if a oceanic plate hits a continental plate or two continental plates collide - I don't know an example ? where two oceanic plates are colliding.

If there is no clear decision (by buoyancy) which part is going down there may be in the Juan de Fuca situation a head on collision with breaking and mountain building by piling up the broken parts.???

"Buoyancy in most oceanic plates is due to crustal contamination", ok if existing but to be modified by the temperatures of both the plates that are colliding and the temperatures and densities of the mantle material beneath.

"I'm attempting to explain why the oceanic crust in this area isn't so readily subducted"

If there is no clear distinction who will win? What will happen if you move two concrete slabs, mud below, head on against each other?

May be this exists may be the start of subduction has not yet happened - how long will it take to develop a subduction zone - 600km deep until the material is re-molten - at a velocity of 1 to 10cm/year?

" and furthermore why this geographical location has become associated with the generation of catastrophic events."

Going back to the model of the plate that is driven by viscous forces below the total area of the plate and pointing towards the direction of movement.

If the motion is stopped by hitting the continent, the plate will be compressed elastically at a rate according to the plate velocity before the hit.

In this compression a large amount of energy is stored as elastic energy (elastic energy is the square of the stress multiplied by the volume and divided by two times the elastic modulus).

The question is no more how such a big energy can be stored in the plate but when and how this energy will be released.

Where is the point of the plate that will break or slip first and initiate the event?

This cannot be answered as nobody knows the rupture stress of these materials in the condition existing there and nobody knows the exact shape - may be not smooth, may be interlocking, may be many other conditions.

Breaking at compression (elastic buckling if thin, brittle breaking if thick) is a complicated field of mechanics.

So this geologic impulse generator is firing in 300 years intervals: this is an action of continuous (?) stress buildup and reaching a level of failure (also not constant).

Certainly there are other periodic geologic events that have a big impact but most may have a lower frequency or less energy. The knowledge can be generated only by careful observation of geologic deposits. In the Sahara there is a fault that jumps any 2000 years, but this has no big consequences as no water is displaced. I do not know other regular events of similar nature. Do you?

RHABE

I understand it may be hard to believe that compression and tension cause the plate to move and for tectonic process to exist, but after several debates with my professor I came to believe this theory held the most water.

You state, "no plastic flow of the plate material is possible", which couldnt be any further from the truth. As you enter the Moho, you are dealing with a non-unform Visco-elastic half melt. Material accumulates to the plate making them thicker as you drift away from the hot zone, "the spreading ridge".

The real force causing these compression and tension forces are density gradients. As you have partial melting the subducted slab meamorphs into an extremely dense rock. This viscoelastic material enters a convection cycle and plumits towards the outer core.

If friction along the Moho caused the plates to drift you might seem more fracturing within the interior of the plate.

Hi Shawn,

"that compression and tension cause the plate to move"

Look at the plate: for example 5km thick and 500km long (in the direction of viscous flow beneath the plate).

From these "typical" dimensions (maybe the size has to be adjusted considerably but this would not change the arguments) of seafloor plates it is absolutely clear that viscous shear is the dominant force in plate tectonics.

(Go and ask the mechanics experts in the ME department of your university for a proof if you doubt).

You are absolutely right with visco-elastic half melt below Moho-discontinuity, above gradually changing to solid behaviour, so if temperature is high there is the possibility of plastic flow, if temperature is low there is plastic flow existing only if very long time and compression is acting, else there will be brittle behaviour.

You can experimentally model this with tar or asphalt, also with honey if cooled.

Generate spotwise or linewise upwelling by spotwise or linewise heating at the bottom of a pot that is filled with one of theses materials.

Let swim some plates made from molten sugar on the asphalt melt. Dimensions may be 100mm x 100mm plate area x 1mm thickness. On honey I would take sheets of glass frit that was prepared from hollow glass or ceramic spheres to adjust density.

I completely agree to your statement about density gradients and convection cycle.

You will see this internal fracturing by viscous forces between convection flow and plates, who made the statement that the JdF plate is severely fractioned?

There is an argument for suppressed fractioning in driven plates: the stress if driven by friction from below is likely to be compression, if this is valid, then the additional bending stresses are responsible for cracking or not.

As these plates have to plow through some existing material or are subducted or stopped by whatever means - being driven from beneath or behind is giving compression stress (in the direction of viscous flow movement).

RHABE

some references:

http://www.uoguelph.ca/geology/geol2250/glossary/HTML%20files/slabpull.html

http://en.wikipedia.org/wiki/Plate_tectonics

If Basal drag or convection cycles dictated plate behavior, why do hot spots exist with little to no interruption of continental drift? Slab pull also explains why plates encompassing the Pacific ocean move at faster rates. Continental drift seems to be the result of inertia and subduction roll-back.

ps A figure that fights for your theory

http://www.windows.ucar.edu/tour/link=/earth/interior/how_plates_move.html

These nicely drawn convection cycles are highly unlikely. Where is the hot spot that causes the Hawaiian Islands to form?

Hi,

the first link is definitely wrong, the second (Wiki) is ok. but the naming of density sinking as pull is not at all correct.

Density driven sinking of one part in a viscous fluid may have some parts of the plate subject to tension but if so there will be early rupture and no longer a "plate" but a lot of chunks that are enlarging the distance between each other.

This is definitely not existing.

"why do hot spots exist with little to no interruption of continental drift?"

Because one spot has much less driving force than a ridge.

(How much force can be exerted by a wire compared to a foil?)

The hot spot can puncture the continental or maritime plate but cannot rupture it along long fault lines.

"Slab pull also explains why plates encompassing the Pacific ocean move at faster rates"

There are many other possible reasons. Slab pull will easily rupture the plate, as there is no new upwelling apart from the ridge there is no rupture thus no pull.

You can calculate the maximum pull from any data about tensile strength.

No inertia at all in these effects, too slow.

Continental drift is mostly convection currents viscous (shear) forces.

Some effect of continuity because of different densities and sinking is accepted.

I did not look at the third link if you think it unlikely it is not worth to discuss lengthy.

Did you ever see the Marangoni effect of upwelling?

Take a pot with water (adding some viscosity modifier may be a good idea), fill in 1 cm of water, heat cautiously from below, observe the pattern on the surface with reflected light. There will be a regular honeycomb pattern if heating is very slowly and a more irregular pattern if heating is more intense.

What about mixing milk with coffee (no good example as turbulence has a major part)

RHABE

There are two accepted unproven theories that explain how tectonics work and what the driving forces are behind them. I chose to believe that visco-elastic flows are not uniform and that bouyant hot magmas only cause anomolies.

I suggest that the predominant force driving plate tectonics is the downward pull of relatively cool super-dense half-melt oceanic plates that show the last signs of brittle fracturing at about 150km's deep into the crust.

In some manor I see this process analogous to deep water convection cycles in our oceans where downwelling is observed in the nothern atlantic ocean and upwelling in the weatern pacific.

I could lead you on with other questions, but why bother, when you make a good arguement. I think you underestimate the varying thickness of oceanic plates, the flow of material that accumulates to plates over time and the predominant mass that exists near the subduction zone.

Hi,

please try to think about a possibility to bring the heat that is generated in the earth to the surface.

There exists an estimate how much it is.

The few hot spots are not at all sufficient.

The mid-ocean ridges transport most of the heat up.

I agree that cooling plates tend to subduct inside warmer (?) surrounding material.

Can you provide a number for the viscosity at the Moho?

And a value for tenmsile strength of oceanic plate material?

If so I can calculate the length where the plate will start to crack and thus defining the length a pulling plate can have.

RHABE

I live in Port Angeles, WA which is directly across the Straight of Juan De Fuca from Victoria, BC. The straight comes in from the Pacific and makes a turn to the South and runs down to Hood Canal and Puget Sound. Port Angeles is right in the area where the straight makes the turn South.

It is an interesting area geologically. We literally have water out our back door and the Olympic Mountains out our front door. Not far from our house is the Lower Elwha river part of the Olympic National Park. A 1 1/2 hour or so hike in (and up) takes you to several natural hot springs. We have hiked for a soak in the hot springs several times.

I can't see how an earthquake in the straight would create a tital wave in the Pacific ocean. It would seem that if there was an earthquake under the water in the straight and if it had vertical displacment then it would cause a tital wave of such in the straight but I would think it would slosh back and forth from one side of the straight to the other and be pretty much contained in the straight, Puget Sound and Hood Canal.

Travis

You live well within the North American Plate boundary. Subduction would occur some hundred miles off shore and give you about 30 minutes warning before your town was devastated. The USGS has looked at the probability of transformation from a subduction complex to a strike slip fault as you see in LA. This process takes millions of years and the faulting observed in association with the Juan de Fuca plate occurs at regular intervals of 300+/- years throughout observed history.

reference:
http://walrus.wr.usgs.gov/tsunami/GIFanimation.html