(January 12, 2018 at 11:14 pm)Haipule Wrote: (January 12, 2018 at 11:02 pm)Anomalocaris Wrote: Impossible? Like the Bible? Why Do you think so?
You know how much I truly respect your intellect but, the earth has an inner core and an outer core. Super Continent is flat earth revisited. And as stupid as theology! Expanding Earth Theory makes perfect sense! Small earth + water = large earth and plate tectonics!
I know that that is pseudo science for now. But it makes sense of things observed.
Let’s talk first about expanding earth.
Expanding earth makes sense only for the first 5 - 10 million years of earth’s history, when the protoearth continuously collided with other planetesimals in a dense accretion disk around the sun and gained mass at a rate of maybe one lunar mass every few hundred thousand years. This rate of bombardment is 11 orders of magnitude higher than the rate of bombardment that came afterwards. There are multiple lines of evidence to show that this bombardment ended rather abruptly as the accretion disk is swept clean, and the size and the mass of the earth has remained effectively constant ever since, or for the last 4.5 billion years.
So, now stretch out your arms to either side. Let’s say your arms span is 6 feet. If that 6 feet were the 4.6 billion year history of the earth, with the beginning of the earth at the tip of your left index finger, and now being the tip of your right index finger, then the period in which earth was actually expanding is about the 1/10 inch at the tip of your left middle finger. Afterwards, the earth has effectively stay the same size and same mass for the rest of the 6 feet. By the way, on this scale a single gentlest stroke of the finest emery paper upon your right index finger tip would obliterate the entire history of the Christian god.
Do meteorites not continue to fall to earth? Yes. They do. But even the largest asteroid known to have impacted the earth in the last 4 billion years is so infinitesimal in mass and size compared to the earth that 4 billion year’s worth of bombardment makes no significant difference to the total mass and size of the earth.
To put it in perspective, the largest body to impact the earth in the last billion years is probably the chicxulub asteroid that hit Yucatán in Mexico 65 million years ago and did in most dinosaurs. It probably weighed a trillion tons. In 1 billion years there was just one such body that large known to have hit the earth. For there to be enough asteroid impacts to expand the earth’s total volume and mass in mass by just 1% over 4 billion years, there would need to be 1 such impact by an asteroid of Dino killing size, not every billion years, but every month, for the entire 4 billion years.
So no. The earth didn’t expand during the last 4.5 billion years.
Now, let’s talk about super continents.
What do you think a super continent is? I don’t know what is a supercontinent. Supercontinent is a nebulous popular science term to denote a really big continent, maybe one that is big enough to be split up into several continents. In professional geology there is no generally accepted definition for “supercontinent”. Some geologists have called the combined land mass of Europe, Asia and Africa as a single supercontinent. They have a point. Europe, Asia, and Africa are connected into one single contiguous natural land mass and continental geological structure, or contiguous at least until the British and the French hacked Africa away by building the Suez Canal. As a a single continuous continental mass, Europe, Asia and Africa contains well over half of the earth’s total continental mass. That seems pretty super to me. Since there is arguably a supercontinent on our round earth right now, it seems strange to say supercontinent is flat earth revisited.
Now let’s step back and see where the term supercontinent may have popped up in popular, rather than professional discussions. In Popular terms, supercontinent usually denotes Pangaea. Pangaea refers to a former contigous landmass that existed on earth between 335 million years ago and 150 million years ago. At its height, Pangaea contained the main parts of all the major continents that exists today. Pangaea probably contained 90% of all the continental mass that existed on earth at the time.
How came it to be there, does it require any special condition that does not exist on earth today? Such earth being flat? No. Let’s take a step back and get a bigger picture of what has been happening with continental masses on earth during much of the earth’s history.
To understand continents one has to understand that continents are not just high points of ocean floor that sticks up above the water. A proper continent is made of different stuff than the floor of the sea. The floor of the ocean are made from rocks heavy in iron and magnesium. The floor of the ocean are in other words made of stuff similar in density and composition to the mantle of the earth. The continents are made from silicate rocks that are light in iron and magnesium. The continents are made of stuff notably lighter than the mantle and of significantly different compositions.
An oceanic crust similar in composition to the mantle crystallizing out of the mantle is easy to envision. How the different substance of the continent crust came to be requires some more elaborate process. Early earth mantle has no way of directly making continental rock. So it is strongly believed that early earth has no real continents in the modern sense. The earth was probably almost entirely ocean, with lots of volcanic islands similar to Hawaii dotted all around the globe girdling ocean to vent the internal heat of the earth. It is not clear if there was plate tectonics as we know it back then. But some time during the first 2 billion years, some form of plate tectonics started. For plate tectonics to operate, for crust to move side ways relative to each other, new crust has to form where crust comes apart, but more importantly old crust has to disappear where they come together. Since oceanic crust is about the same density as the mantle, it is easy to see how as oceanic crust come together and pile up, the weight of the pile will drive some oceanic crust downwards back into the mantle and disappear.
Laboratory experiments show when Cold oceanic crust is heated in the mantle, it doesn’t melt all at once. Instead, lighter sicilicous parts of its composition melts first, oozes out of the rock, and rises up as magma with different composition to the surrounding mantle. This lighter material that first melts out of oceanic crust is remarkably like the continental crust in composition. So the first continents where small, and were made from partial melting of oceanic crust that were subducted into the mantle by plate tectonics.
Because continental rocks are lighter than mantle rocks, buoyancy makes it impossible for large blocks of continent to be subducted back into the mantle like heavier oceanic crust can. So where as oceanic crust continuously gets created and then destroyed by subduction back into the mantle, continental crust mostly are here to stay in one form or another once they’ve been created.
Because plate tectonics are on going, oceanic crust continues to be subducted and partially melted to rise up and become new continental rock in places like japan, the American West coast, and all along Indonesia, and old continental crust is mostly never totally destroyed, the total continental mass on the surface of the earth have been getting bigger since about 3 billion years ago and are still getting bigger.
Even as total continental mass slowly get bigger over 3 billion years, the continents are also getting ripped apart into piece and carried around the earth by plate tectonics. When continents were small, the chances of a continent so happen to fall on a rip in the plate caused by plate tectonics were small, also if a continent were ripped apart, the chances of two small ripped apart pieces randomly moving and then coming back together in collision were also small.
But by about 1-1.5 billion years ago, sufficient amount of continental mass have accumulated so that pieces are now quite large. When they move around they each form a big target and there is high chance of collision. When continents collide, they tend to stick together for some time, 100-200 million years. So the pieces that have collided form bigger targets for the other pieces still moving around. So this way the chances increase that as continents move around by plate tectonics, there will periodically be enormous pileups involving most of not all the continents.
This is the origin of Pangaea. Pangaea does not require special process. It is an periodic artifact of the routine on-going process of continental collision. Think of plate tectonics as being a slippery playground for bumper cars. The continents are the bumper cars. Bumper cars hit each other all the time. But from time to time, many bumper cars may all end up in a single big pile up. This is Pangaea.
