Plate Motion / Convection Currents

[kingwinner]

1. Are ALL terranes on the continents coming from the ocean where they are scraped off in continental-oceanic subduction zones?

2. Is the "theory of microplate terranes" suggesting that ALL continents are a patchwork of terranes (from the ocean)?

3. I don't quite get the 3 underlined parts of this paragraph:
"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates (why is it sinking? so are we constantly getting closer to the core?) transport heat. Where the plates pull away from each other at the mid-ocean ridges, magma from the asthenosphere rises to the surface, where it cools to from new lithosphere. As the lithosphere moves away from the mid-ocean ridge, it cools and becomes more dense and begins to subduct into the asthenosphere. (how come the lithosphere suddenly subduct without any plate collision?) The downward pull of the subducting lithosphere is probably the main driving force for tectonic plate motion (is this whole paragraph describing the cause of plate motion as convection currents? I don't see how this describes the earth as a convecting system...it seems to me that this paragraph does not relate to convection currents in any way...)"

Can someone explain? I would appreciate!

[Southtown]

Utoh! We have a thinker...

http://www.creationscience.com/Trenches5.html#wp1111128

[Turtle]

Originally Posted by kingwinner

1. Are ALL terranes on the continents coming from the ocean where they are scraped off in continental-oceanic subduction zones?

2. Is the "theory of microplate terranes" suggesting that ALL continents are a patchwork of terranes (from the ocean)?

3. I don't quite get the 3 underlined parts of this paragraph:
"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates (why is it sinking? so are we constantly getting closer to the core?) transport heat. Where the plates pull away from each other at the mid-ocean ridges, magma from the asthenosphere rises to the surface, where it cools to from new lithosphere. As the lithosphere moves away from the mid-ocean ridge, it cools and becomes more dense and begins to subduct into the asthenosphere. (how come the lithosphere suddenly subduct without any plate collision?) The downward pull of the subducting lithosphere is probably the main driving force for tectonic plate motion (is this whole paragraph describing the cause of plate motion as convection currents? I don't see how this describes the earth as a convecting system...it seems to me that this paragraph does not relate to convection currents in any way...)"

Can someone explain? I would appreciate!

1. No; oceans came after a crust cooled on Earth.
2. I don't know that theory offhand.
3. Subduction is a plate boundry, but not all boundries subduct. The mechanics of the convection model remain speculative. If you need this particular version in memory for a test, then learn it verbatim. In any case, read widely.

[Tagred]

1. Though not ALL terrains are of the same age most continental rock was formed early on in earth's evolution.

2. Not too sure what its asking but, continental rocks are different in composition and density to oceanic rock again, micro terranes could be the last remnants of previous continental drift episodes

3. I would disagree that upwellings are driven by the sinking of lithosphere. Upwellings, hotspots, spreading centres etc are a result of either a wekaness or increased activity below the oceanic crust and when heated are less dense than cool rock, which is why at spreading centres you see ridges, as it spreads out it is eroded, cooled and increases in density.

Subduction of oceanic lithosphere is a driver of continental drift however, along with spreading centres. Oceanic rock will always subduct under continental rock because of density (without going into any other fault systems)

I think that passage doesnt describe enough about either convection systems of continental drift, it seems more like a half explanation for both, if u know what i mean.

[Turtle]

Originally Posted by Tagred

1. Though not ALL terrains are of the same age most continental rock was formed early on in earth's evolution.

2. Not too sure what its asking but, continental rocks are different in composition and density to oceanic rock again, micro terranes could be the last remnants of previous continental drift episodes

3. I would disagree that upwellings are driven by the sinking of lithosphere. Upwellings, hotspots, spreading centres etc are a result of either a wekaness or increased activity below the oceanic crust and when heated are less dense than cool rock, which is why at spreading centres you see ridges, as it spreads out it is eroded, cooled and increases in density.

Subduction of oceanic lithosphere is a driver of continental drift however, along with spreading centres. Oceanic rock will always subduct under continental rock because of density (without going into any other fault systems)

I think that passage doesnt describe enough about either convection systems of continental drift, it seems more like a half explanation for both, if u know what i mean.

1) Much of continental rock is sedimentary & metamorphic & is continuously formed/reformed.
2) The Columbia Plateau Basalts errupted from seams on dry land.
3) Hot spots & spreading centers occur on continents as well as undersea; Yellowstone Supervolcano is over a hotspot, & the Great Rift Valley in Africa is an active spreading center.
Plate boundries do not always subduct; the San Andreas Fault is a strike-slip plate boundry, the Himalyas continue to push up from a locked plate boundry.

[Turtle]

___Science is always amendable. New research in geology reveals evidence of how Earth's early geology developed.
http://uanews.org/cgi-bin/WebObjects...rticleID=11692

This quote from the linked article:
"Comets have played a relatively minor role in inner solar system impacts, Strom, Malhotra and Kring also conclude from their work. Contrary to popular belief, probably no more than 10 percent of Earth's water has come from comets, Strom said.

After the Late Heavy Bombardment, terrestrial surfaces were so completely altered that no surface older than 3.9 billion years can be dated using the cratering record. Older rocks and minerals are found on the moon and Earth, but they are fragments of older surfaces that were broken up by impacts, the researchers said.

Strom said that if Earth had oceans between 4.4 billion and 4 billion years ago, as other geological evidence suggests, those oceans must have been vaporized by the asteroid impacts during the cataclysm."

[Tagred]

Originally Posted by Turtle

1) Much of continental rock is sedimentary & metamorphic & is continuously formed/reformed.
2) The Columbia Plateau Basalts errupted from seams on dry land.
3) Hot spots & spreading centers occur on continents as well as undersea; Yellowstone Supervolcano is over a hotspot, & the Great Rift Valley in Africa is an active spreading center.
Plate boundries do not always subduct; the San Andreas Fault is a strike-slip plate boundry, the Himalyas continue to push up from a locked plate boundry.

1) In terms of the terraines they were formed early on in earth history (relatively). And terraines as most geologist would know them (at least the years of my study) were normally restricted to central land masses. Although I gather the goal posts can be moved, which is no bad thing.

2) Absolutely right, but in general terms density of continental rock is lighter than oceanic. trying to keep within the question, maybe im being too generalistic

3) I purposely added the caveat not including any other fault system. In subduction zones oceanic rock will always subduct under continental. I tried to reply in the context of the question rather than add perhaps more confusion.

I could have gone into minute details of the whole convection & tectonic cycle but though it best to keep it at a "general" level to answer the question. Perhaps I was wrong.

I need to read more posts on this forum, and gather more info as to what amounts of detail and qualification is needed to avoid confusing others and embarrassing myself.

Please excuse me if I'm not up to standard just yet, i need a little time . I hate being technical when theres no need and vice versa.

[HydrogenBond]

What confuses me is that originally there was only one singular super continent, Pangaea, earlier in the earth's evolution when there was more mantle heat. That remained until quite recently in geological time, after the earth had time to cooled, i.e., 200Million years ago. Why did a cooler earth cause such large scale continental drift when the hotter earth with its singular land mass, probably had more internal convection? It seems backwards to me.

[Turtle]

Originally Posted by Tagred

I could have gone into minute details of the whole convection & tectonic cycle but though it best to keep it at a "general" level to answer the question. Perhaps I was wrong.

I need to read more posts on this forum, and gather more info as to what amounts of detail and qualification is needed to avoid confusing others and embarrassing myself.

Please excuse me if I'm not up to standard just yet, i need a little time . I hate being technical when theres no need and vice versa.

___The post is fine Tagred; just the right mix to stimulate conversation.

Originally Posted by HydrogenBond

Why did a cooler earth cause such large scale continental drift when the hotter earth with its singular land mass, probably had more internal convection? It seems backwards to me.

___I think it may have to do not so much with the amount of heat driving the convection as the direction(s), amplitude, volume, & interference of the convection currents. As temperatures change, these interactions also change dynamically. Cooler stiffer rock no longer floats around at relative ease & begins locking & breaking with the uneven stresses.
___So much yet to learn about the Earth's core & it's magnetic dynamo I think as well in order to understand the surface effects.

[HydrogenBond]

Well that sort of makes sense except, shouldn't the early earth have more of all the internal dynamics. The oceans were already in place and life was on earth so the crust should have been relatively thick 250M years ago. Common sense would imply the energy from core increased 200M years ago. Maybe the earth's convection changed orientation due to the cooling of the poles increasing the thermal gradient north and south.