Plate Tectonics; Continental Drift; Radio-isotope Dating of Oceanic Crust

Geophysicist John Tuzo-Wilson (1963):  "It will be difficult for most of us to accept that large amounts of what we have written and taught has been erroneous".

In my seminar last week at Glasgow University I had described, from a physicist's perspective,  how Radiometric Dating (RD) is used to determine the age of the Earth and the Solar System and in that process provides an absolute time scale for geological events.  RD has played an equally important role in establishing the Continental Drift theory or Plate Tectonics on a firm footing.  What was amazing to realize is that the world’s oceans are continuously created and destroyed and that the oldest ocean crust is merely 200 million years (My) old.  In contrast, continental crusts were formed as the earth cooled 4550 million years ago and are not subject to recycling like the oceanic crusts are.  The following two slides show the measured ages of the oceanic crusts:  

The first slide is for the Atlantic Ocean while the second slide shows similar data for the oceans of the world. Youngest rocks in the crust lie along the mid-oceanic ridge where new crust is being created by extruding magma from the mantle of the Earth.  As new magma is introduced, it collects over the older magma (now solidified) and slides down by gravity as shown in slide 3, on both sides of the ridge and creates the oceanic crust.  The oldest oceanic crust is merely 180 million years old.

The question then is what causes the mid-oceanic ridge to extrude magma.   The continental crust is transported, albeit very slowly, by the convection currents in the mantle and as the two continental crusts move apart, space is created for magma to leak through and create what becomes the mid-oceanic ridge.  As magma solidifies, it forms igneous rock known as basalt which has a higher density (3 gm/cc).   The continental crust, largely made of silicates has a density of 2.7 gm/cc.  The oceanic crust, being heavier, subsides below the continental crust and creates a subduction zone, mixes with the mantle and melts.  Some of the molten crust seeps through the cracks in the stressed continental crust and leak out as volcanoes.  Such volcanoes form in subduction zones - 'Pacific Ring of Fire' is an excellent example of some of the regions where volcanoes occur (see slide). 

Active volcanoes (red dots) define the Pacific Ring of Fire where they form along tectonic plate boundaries. Only volcanoes on land are shown in this figure. Figure modified from USGS (Topinka, 1997).

The overall topology of an ocean floor might look as in the next slide.  Sometimes the ridge may be high enough to stick out of the water level and form an island.  Iceland is an example and it sits right on top of the mid-Atlantic ridge. 

The region of the subduction zone, where the oceanic crust goes under the continental crust, there is a deep trench created and these are the places where the ocean has the greatest depths.  In the Abyssal plain, sediments from the continent deposit and form a nice flat surface.  

WHY CONTINENTS DRIFT - what causes them to move?
People had suspected for a long time that continents drift. The matching shapes of the continental boundaries were the first indications and were commented on by many scientists and others.  

Alfred Wegener 1912-15 first proposed his theory of continental drift but could not provide a credible mechanism for causing this drift.

Wegener's theory essentially was that about 200 million years ago, all continents were joined together as a super-continent called Pangaea meaning 'all land'.  Pangaea broke up into individual continents which drifted apart and now occupy their current positions.

There was a lot of evidence in the form of similar fossils, rock formations and climate found in different continents that could not be explained by any other way except by assuming that all these continents were joined together at some distant past.  Wegener's theory had firm empirical backing but the main question as to how the continents could move - what cases the movement - just could not be explained.  
Naturally a great debate started in geology with many geologists in favour and equally large numbers against the idea of drifting continents.  Arthur Holmes liked the theory and proposed that convection cells in the mantle of the Earth can provide such a mechanism.  Earth's interior was not well understood in the 1930s and the idea met with great opposition and was to some extent ridiculed. 
Holmes persevered with his theory but nothing really happened until the 1950s when firm evidence of sea-floor spreading became available and the reality of drifting continents could not be ignored.  The evidence was in the form of magnetic zebra stripes that were mapped and found to be symmetrically placed on either side of the mid-oceanic ridge.

The formation of magnetic zebra stripes is interesting.  Earth’s magnetic field has reversed many times over the planet’s history—with the magnetic north sometimes facing south, or vice versa, as it is today. 

When seafloor lava solidifies at the seafloor, its magnetic crystals are quenched in alignment with Earth’s magnetic field, and the rocks’ magnetic “polarity” is preserved. 

Radio-isotope dating of oceanic crusts tells us accurately the time that a particular part of the crust was formed.  It is then possible to calculate the rate at which oceanic floors have been increasing and continue to spread.  This is shown in the next two slides:

These days, satellite observation of fixed points on the Earth allow us to measure the spreading rates very accurately and the data in the slides reflect these measurements.

We still have to address the mechanism of continental drift - or as it is now known - plate tectonics. There is a lovely set of slides, 147 in total, that really cover the subject in great detail.  I shall briefly summarize it here.  

Our earth started life as a molten sphere with chemical composition of the Solar Nebula.  As the earth cooled, heavier metals (Iron and Nickel) sank to the bottom ( to the centre of the sphere), the lighter materials floated to form a layered structure (rich in silicon, oxygen, aluminium, calcium, potassium etc.).  This is shown in the following two slides 

 

Building on Arthur Holmes theory of convection currents in the mantle, Harry Hess developed the idea of sea-floor spreading in the 1960s. The following two slides summarize the situation

 

In 1965, John Tuzo Wilson put forward the theory of plate tectonics by suggesting that the Earth's crust is made of a number of rigid plates.  Plate Tectonics is an interesting subject and it is easy to get carried away with writing pages. I shall not do it here and refer you to the web site that has a detailed very readable description of plate tectonics.

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Future of Intelligence – How do we define Intelligence anyway?

In December 2015, at the Nobel Prize gatherings in Gothenburg, Sweden, the theme was the future of intelligence.  Ray Kurzweil was the keynote speaker – Ray is somebody I admire for his many wonderful qualities and his ability to project the technological progress in a way that sounds credible but frightens many people for what it portends.  In his projections, Ray focuses on different technologies that are changing our ability to see and understand large sets of information and create computer systems that might reach and then quickly surpass human level thinking.

Personally, I have some difficulty in understanding what intelligence is – what is the definition?  It is always good to know what one is talking about.  I did some legwork.  The results are as follows:  Intelligence is…

Collins Dictionary:  The capacity for understanding;
                               Ability to perceive and comprehend meaning

Oxford Dictionary:  The faculty of reasoning, knowing and thinking as distinct from feeling

From the Web:       A person’s cognitive abilities to learn.
                               Is an estimate of the quality that we attribute to the decision- making and abstract thinking of people around us.

                               Refers to one’s cognitive abilities which include memory, comprehension, understanding, reasoning, abstract thought.

Wiki says:  Human intelligence is the intellectual capacity of humans, which is characterized by perception, consciousness, self-awareness, and volition. Through their intelligence, humans possess the cognitive abilities to learn, form concepts, understand, apply logic, and reason, including the capacities to recognize patterns, comprehend ideas, plan, problem solve, make decisions, retain information, and use language to communicate. Intelligence enables humans to experience and think.

It became clear to me that intelligence is not something that can be measured by the IQ tests. The well-known Flynn effect (paradox) about the drastic increase of IQ score in the 20^th century has its own rationalisations and make interesting reading (http://www.personalityresearch.org/papers/cherry.html)

Without getting too bogged down in defining intelligence, may be we should come back to the Nobel Week Dialog and what Ray had to say about the accelerating increase in technological progress.  I also notice that AI used to mean artificial intelligence but it is frequently used to mean accelerating intelligence – I call it the Kurzweil effect!

Ray’s keynote speech may be found at https://www.youtube.com/watch?v=9Sy3Vp7DNpE

and is very enjoyable to listen to. Particular attention should be given to the difference between linear thinking and exponential thinking.  Humans are used to linear extrapolations and that is what our common sense dictates in most circumstances.  I have some beautiful examples of exponential change in my talk on population growth

http://st21ravi.blogspot.co.uk/2015/06/sustainability-population-dynamics-what.html

It is worth having a look at those.

Ray Kurzweil was interested in the accelerating growth of technology and gave examples of transistor price, transistor cycle time, DNA sequencing costs, growth in supercomputer power, transistor per chip and many more.  It is amazing that the smart phone in your pocket is a billion times more powerful than the computer I had in my nuclear physics lab in Canada in 1966. 

All this points to the way big data can be handled and made sense of.  This is what the human brain is so capable of doing.  With accelerating growth – I like the phrase exponential growth more – computers are getting more versatile and capabilities like pattern recognition which were impossible a few decades ago are well within the reach of super-computers.  We are not at a stage that computers can match human intelligence fully or even partially but there are signs that progress is in the correct direction.  The main bottleneck is that we still do not understand properly how the brain processes information and what gives rise to abstract thinking – where does intelligence lie in the brain?

Artificial Intelligence (AI) purports to match human intelligence sometime in the near future and then surpass it very soon after that.  Actually, it is better to think AI at three different level  http://waitbutwhy.com/2015/01/artificial-intelligence-revolution-1.html  and I quote

“1) Artificial Narrow Intelligence (ANI): Sometimes referred to as Weak AI, Artificial Narrow Intelligence is AI that specializes in one area. There’s AI that can beat the world chess champion in chess, but that’s the only thing it does. Ask it to figure out a better way to store data on a hard drive, and it’ll look at you blankly.

2) Artificial General Intelligence (AGI): Sometimes referred to as Strong AI, or Human-Level AI, Artificial General Intelligence refers to a computer that is as smart as a human across the board—a machine that can perform any intellectual task that a human being can. Creating AGI is a much harder task than creating ANI, and we’re yet to do it. Professor Linda Gottfredson describes intelligence as “a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience.” AGI would be able to do all of those things as easily as you can

3) Artificial Superintelligence (ASI): Oxford philosopher and leading AI thinker Nick Bostrom defines superintelligence as “an intellect that is much smarter than the best human brains in practically every field, including scientific creativity, general wisdom and social skills.” Artificial Superintelligence ranges from a computer that’s just a little smarter than a human to one that’s trillions of times smarter—across the board. ASI is the reason the topic of AI is such a spicy meatball.”

There is still a long way to go to reach ASI but with accelerating growth in technology – who knows?  There are many apprehensions, worries and expectations generated by the ever quickly developing technologies.  Ray Kurzweil neatly summarized the task at hand and again I reproduce his list of topics for discussion:

·         When will artificial intelligence exceed human intelligence?

·         Are fears of super-intelligent systems justified?

·         Does our developing relationship with technology change our brains?

·         How well do we understand the basis of human intelligence?

·         What are the economic consequences of increasingly intelligent systems?

·         What role will creativity have in the future?

·         Who will benefit and who will lose out?

·         What is the link between technology, education and inequality?

·         What will humans do when robots take over even more of our roles?

·         How can society best prepare for the changes ahead?

·         What should we learn in the future?

·         How will learning change in the decades ahead?

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Age of the Earth - Radiometric Equations - Isochrons Explained

The radiometric equations given here provide theoretical details of the construction of isochrons. Please refer to the main article for background information.  This part may be skipped without loss of continuity in the main article. 

In an actual radiometric analysis to determine the age of a sample, it is necessary to measure the abundance of the parent and daughter isotopes and also we need to know how much of these isotopes were present at the time of formation of the sample. How these abundances are related to the elapsed time (age of the sample) is determined by radiometric equations.

Now back to the main article

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Radio-Isotope Dating: Age of the Earth - An Outreach Seminar

In the introduction to radio-isotope dating, I had indicated that recently tremendous advancements have been made in geology because now we have an absolute time clock for measuring geologically significant events - after all Geology is defined as the study of the Earth's origin, history, structure and composition.  Without a reliable way of measuring absolute time - only became available since about 1950 - Geology was in limbo - subject to much speculations and full of mysterious observations.  Geology is a science that relies very much on visual observations and measurements and there is a vast amount of data on physical features of rocks, fossils they contain, land forms, oceans and much more.  

The fundamental question in Geology has been - what is the age of the Earth? A significant step in this direction was the realization by James Hutton followed by Charles Lyell and others in the 18th and 19th centuries that rocks recycle over time - they form, erode, sediments collect and compress to form new rocks - the process repeating again and again. The sediments include in them remains of plants and animals that inhabit the Earth at that time - fossils - and the layers of such sedimentary rocks are essentially a catalogue of geological events in a time ordered fashion - just the absolute time scale is missing.  Grand Canyon is an excellent example.

The following few slides summarize the position:
(please click on a slide to view its bigger image)

The way I think of the relative geological time scale is like a stack of books arranged in the correct chronological order of the historic events as they happened but with no dates provided - as I show in the next slide

It is not that people did not try to determine a value for the age of the Earth.  It is just that the understanding of the physical processes was not good enough and all attempts gave results which were wide off the mark.  Even the church got involved and adopted the ridiculously small value of 6000 years as dogma and caused a lot of grief to lot of people.  The next few slides explain some of the attempts made






I must say that physicists and astronomers also did not help in the process of finding the correct age of the Earth.  Lord Kelvin did not know about convection currents in the molten magma in the interior of the Earth nor did he know about radioactive elements which were a source of heat energy.  But Kelvin was an eminent scientist and everybody listened carefully to what he said - in 1900 he said that the Earth is 20 million years old and that was something you do not challenge.  Evolutionary biologists did object to such a young Earth as this did not allow enough time for species to evolve etc. - a great debate - often ill tempered - ensued but there was no way out of the dilemma as to how to find a true reliable value of the age of the Earth.
In the 1920s and 30s, radio-isotope dating was getting established and was measuring ages of some rocks as 2 or more billion years.  Astronomers were discovering that the Universe is expanding and theorizing that the Universe might have started from a small compact volume less than 1.5 billion years ago. The Earth has to be younger than the Universe and this gave geologists a great deal of ammunition to aim at scientists like Arthur Holmes who believed that radio-isotope dating was a method that is capable of providing an absolute age of the earth. It should be stated that the astronomers have got their act together and in 2015 measured the Hubble constant and hence the age of the Universe as 13.8 Billion years with a tiny uncertainty.

Radioactivity was discovered in 1896 and soon it was proving useful in medical applications.  Rutherford was the first scientist to apply it for determination of age of geological samples.  Systemic errors were serious and the science of radioactivity was not understood well at that time.  Even isotopes were not known until about 1915.

This certainly gave the established geologists a big excuse to criticize the new science and it was really the solo fight by Arthur Holmes that eventually brought radio-isotope dating technique to universal acceptance by about 1950.

Let us first look at the radioactive isotopes of interest for geological dating.  It is believed that the Solar System was formed from a distribution of matter known as the Solar Nebula.  As the nebula contracted, most of the matter collapsed to form the Sun and a tiny percentage of the nebula formed the planets, asteroids, meteorites and the comets.  All of this happened over a very short period of a few hundred million years at most and it would be correct to say that our Earth formed at the same time as the Sun and the meteorites.  In fact, we can plot the abundance of elements found in the Sun and compare this to elemental abundance of early formed meteorites and find an exact correlation in these abundances. This is shown in the slide

It is now accepted that meteorites are the best objects for dating the solar system.  They were formed early on in the process for the nebula contracting and the chondrules in the meteorites have preserved their purity and initial composition at the time of their formation.  Claire Patterson have actually shown that the chondrules have similar age as some of the ocean sediment rocks confirming that they were formed at roughly the same time.

The next question is which radio-isotopes are useful for determining the age of the earth.  Unstable isotopes decay with a characteristic rate - called the half-life.  Half life is the time that an isotope will require for half of the original amount to decay to another isotope (called the daughter).  The daughter isotope may or may not be stable and can decay in turn but eventually produces a stable isotope that maintains its quantity over time.  The best isotopes are those whose half lives are similar to the age of the earth - a few billion years.  

The most frequently used isotopes for dating meteorites and earth's crustal rocks are those of uranium-238 and rubidium-87.
For dating oceanic crust, K-40 is a good choice.  These are shown in the next slide

 Carbon -14 has a short half life of 5730 years and is only useful for dating samples that are less than about 50000 years old. 

 Modern mass spectrometers are very sensitive and provide sufficient sensitivity to measure the tiny amount of radioactive isotope or the daughter product that is present in the sample.

Meteorites are good approximation to a closed system - particularly the small spherical chondrules, that meteorites contain, are very stable and have undergone little or no change since they were formed 4.65 billion years ago.

 

Zircon is an ideal mineral for dating terrestrial rocks.

In an actual analysis to determine the age of a sample, it is necessary to measure the abundance of the parent, daughter isotopes and also we need to know how much of these isotopes were present at the time of formation. How these abundances are related to the elapsed time (age of the sample) is determined by radiometric equations which are described in this article.  Essentially, an isochron is constructed - isochron is a straight line graph whose slope gives the age of the sample.

The first comprehensive measurement of the age of the earth was done by Claire Patterson in 1956 and we present the results obtained by Patterson which still stand.  By the way, Patterson was responsible for banning of leaded petrol - this he achieved through a thirty year struggle with the government bodies and the petrochemical industry!

In conclusion, it seems reasonable to say that the Earth came in existence at the same time as the Solar System which is about 4.568 billions of years old. 

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