The mantle is solid, so how did the oceanic crust subduct into it?

There is a question on Zhihu: "The mantle is solid, so how did the oceanic crust subduct into it?"

In my opinion, this is due to differences in density and viscosity , and two very important factors, temperature and time . Let me explain.

Short version

According to current understanding, the mantle is indeed solid, but at higher temperatures and longer geological time scales, the mantle can undergo relative flow.

Due to the long-term evolution of the oceanic lithosphere, the gradual cooling and thickening and rock phase changes lead to an increase in density, allowing it to sink into the mantle at a certain depth where the density and viscosity are relatively small.

The early geological history may have been vertical tectonic, and then the transition from initial plate tectonic to modern plate tectonic evolution stage | Gerya, 2019

Part 1

The power of time

I would like to talk about the power of time first.

Geological evolution often spans tens of millions of years, so the time scale perspective is very important.

On a larger time scale , it is just as the ancient Greek philosopher Heraclitus said: Everything flows .

For example, a cat appears to be solid, but can flow... nevermind, let's change the example.

For example, a piece of dough, although it is solid, we know that it can flow.

Because, after being left standing for a period of time, the dough will flow around and gradually flatten under the action of its own gravity and atmospheric pressure.

We can verify this process with our own eyes without spending too much time.

I couldn't find a suitable diagram, so I used a cheese rheology diagram (the flow is not so obvious because of the added boundary conditions) | Selverstone, 2005

But if we just stare at the dough for a few seconds, it's hard to notice any changes.

Unless you are Funes in Borges's novel "Funes the Learned".

Funes has an extraordinary memory and can remember every detail of things. It takes him a whole day to recall what happened on any given day, because he can remember every detail of that day, just like watching a video replay. He can see the quiet process of decay. In his world, the evolution of everything is like a high-resolution video at double speed.

Funes constantly saw the silent march of decay, tooth decay, and fatigue. He noted the progression of death and dampness:

We can see three wine glasses on a table at a glance; Funes can see all the branches of a vine, the bunches and individual grapes. He remembers the shape of the dawn in the south on April 30, 1882, and compares it in his memory with the texture of a leather-bound book he had seen only once, with the ripples of the Rio Negro caused by the oars on the eve of the Cabrazo riot. Those are not simple memories; each visual image is linked to the feeling of muscles, cold and heat, and so on. He can recreate all dreams. Two or three times he recreated a whole day; never vague, but each time it took a whole day... The images we can fully perceive are a circle on a blackboard, a right triangle, a rhombus; Ireneo can perceive the flying mane of a horse, the rearing of an animal on the mountain, the ever-changing flames and countless ashes, and the various faces of the dead during the long wake. I don't know how many stars he saw in the sky.

If we had Funes' ability, the dough would always be flowing in our eyes.

Not only water, oil, dough, etc. can flow, but ice, glass, and rocks can also flow.

The fluid properties of oil and dough can be observed in a few minutes;

The flow of glaciers and salt rivers takes weeks, months, or even years to be observed;

Flowing glacier

Over centuries, you can even watch glass and rock flow.

For example, the glass of some medieval cathedrals in Europe has been deformed; the marble benches in many ancient parks have sunk and bent under the long-term effects of their own weight and the weight of tourists.

The time of geological evolution is usually measured in millions of years (Ma), and this slow deformation is called "creep". The rock flow understood in geology is the gradual and continuous accumulation of deformation, which is the plastic deformation process.

For example, in the picture below, the rock is subjected to compression, but it does not break, but instead wrinkles.

We can still see this kind of fold structure in the wild today, but the squeezing force of that time has long since disappeared and has become a thing of the past, recorded in the deformed rocks.

This kind of deformation caused by force, which can be maintained after the force is removed, is usually called plastic deformation.

This plastic deformation is actually the "flow" of rock under the action of force.

Both the mantle and lithosphere are solid, but over geological timescales they flow, and this flow is recorded in deformed rocks.

In a short time, you can easily stuff a piece of chocolate into the dough;

Similarly, over long periods of time, oceanic lithosphere can also enter the mantle through subduction.

So, a lot of materials are solids, but they're actually slowly deforming, just over a lot more time. At certain time scales, a lot of materials behave like fluids.

The term " viscosity " is usually used to describe the fluidity of a substance.

Part 2

A Moveable Feast—Viscosity

Viscosity refers to the fluidity of a substance. Any fluid has viscosity.

Just search for a common substance viscosity table on the Internet:

At room temperature, the viscosity of water is 1 cP;

At 49°C, the viscosity of chocolate is 17000cP.

We can use this to feel the differences in fluidity of common substances.

As mentioned above, rocks can also flow on the geological time scale, so rocks also have viscosity, but this viscosity is very high.

How big is it?

The viscosity of water at room temperature is 1cP=10^-3Pa·S.

The viscosity of upper crustal rocks is about 10^22Pa·S, which is about 25 orders of magnitude different from the viscosity of water at room temperature.

The viscosity of rock is about 20 orders of magnitude different from the viscosity of chocolate at 49°C.

The viscosity of the mantle is approximately 10^21Pa·S.

As you can imagine, without the perspective of geological time scale, it is simply unimaginable.

Temperature is also an important factor.

Temperature has a significant impact on rock viscosity.

Increased temperatures reduce the viscosity of rock, making it more fluid.

For example, glass at room temperature has a high viscosity and is difficult to flow. If an external force is applied, it would rather break than bend.

However, as the temperature of the glass increases, it gradually softens, the viscosity decreases significantly, and it becomes easier to bend.

As shown in the figure below, for the rocks in the earth's crust, the temperature gradually increases on the geological time scale (the crust thickens and the radioactive elements warm up). After 80Myr (80 million years later), the viscosity of granite rocks at a depth of 30km underground can drop by 5 orders of magnitude.

Viscosity changes of a double-layered crust with time-temperature increase | Gerya, 2002

So what force drives this flow?

The horizontal tectonic force of plate movement is like an invisible hand.

However, research suggests that the thrust of the mid-ocean ridge is an order of magnitude smaller than the drag force of the subducting plate itself.

This downward drag is gravity, which is essentially caused by density differences.

Part 3

Clear things float while turbid things condense - density difference

In the Romance of the Three Kingdoms, Qin Bi asked Zhang Wen, who was sent by the Eastern Wu:

"Sir, you are a famous scholar from Eastern Wu. Since you are asking me about the affairs of the sky, you must be able to understand the principles of heaven. ... The light and clear things float up to become the sky, and the heavy and turbid things condense down to become the earth ... Since the sky is light and clear and floats up, why does it tilt to the northwest? I also don't know what else is there besides light and clear. I hope you can teach me." Zhang Wen could not answer.

It is a pity that Wu envoy Zhang Wen had not studied geology, otherwise he could have explained to Qin Mi why "the sky tilted to the northwest and the earth sank to the southeast."

It says that "the light and clear float up to form the sky, and the heavy and turbid condense down to form the earth", which is the basic principle of the Taoist view of heaven and earth, from chaos to the distinction between clear and turbid. In fact, it is Archimedes' buoyancy principle, which is determined by density differences.

There is a cup with oil at the bottom. Pour water on the oil. After a while, the oil will float up and remain on the water.

Conversely, it can also be understood that water sinks below oil, which is a density inversion - also known as Rayleigh-Taylor instability , a very simple and easy to understand principle. Rayleigh later won the 1904 Nobel Prize in Physics (for his research on gas density and the discovery of Ar).

Rayleigh and Taylor 1883, only a few pages are excerpted

For example, many phenomena in geology can be explained by this principle, such as diapirism .

Magma can diapirize, salt can diapirize, deep rock mixtures can diapirize, and ultra-high pressure solid rock at a depth of 100km can diapirize to the surface.

These are all caused by density differences, which produce buoyancy.

Left-middle image source: Fossen, 2016; right image source: Marschall, 2012

Conversely, heavy objects with high relative density sink into materials with low relative density, which is called " negative buoyancy " in geology.

To put it bluntly, it means sinking .

It can be seen that academic work often involves conceptualizing common sense principles.

The collapse of the slab and its sinking into the mantle is usually due to negative buoyancy.

Suppose in a relatively closed system, some pebbles are sprinkled on the mud. Under the action of gravity, the pebbles will gradually sink into the mud.

Likewise, if the oceanic lithosphere is denser than a portion of the mantle beneath it, the oceanic crust can sink into the underlying mantle like a stone sinking into mud.

So the question is, how does the density of the oceanic lithosphere compare to that of the mantle?

In fact, the oceanic lithosphere, especially the ancient oceanic lithosphere, has a density slightly greater than the mantle beneath it (at least greater than the mantle 100 km deep beneath it).

The age of the oceanic lithosphere is related to its thickness. The older and thicker it is, the lower the overall temperature and the higher the average density. This should not be difficult to understand.

This will have two effects on the ocean subduction process:

(1) The relatively high density of the oceanic lithosphere produces gravitational instability in the ocean-continent transition zone, initiating oceanic subduction;

(2) After subduction starts, the higher the density of the plate entering the subduction zone, the greater the gravity drag on the plate, and the easier it is to subduct deep into the mantle.

Digital simulation studies have found that, with other factors remaining unchanged, as the age of the oceanic crust increases, at the same depth, the older the initial oceanic lithosphere, the greater the overall density of the subducting plate, which in turn generates greater plate tension and a greater subduction angle of the subducting plate.

Huangfu Pengpeng, 2016 (1Ma=1 million years)

In addition, in order for the oceanic lithosphere to sink into the lower mantle, another condition must be met, that is, the viscosity of the underlying mantle must be low.

Just like although the density of stone is greater than that of plastic, it is difficult for stone to sink into plastic.

If you heat the plastic and soften it, that is, reduce its viscosity, the stone will sink into the plastic.

Similarly, the deeper, hotter mantle has a lower viscosity, so the oceanic crust can sink into the mantle at a certain depth.

In addition to the above-mentioned situation where the oceanic lithosphere becomes older and the lithosphere density increases,

Another very important change is rock phase change, which will also lead to the appearance of rock density.

Part 4

Metamorphosis - Rock Phase Changes

Kafka's representative work: The Metamorphosis, English translation: The metamorphosis.

In geology, the word for metamorphism is metamorphic.

Gregor, the protagonist of the novel, woke up one morning and found himself turned into a giant beetle.

Transforming from a human into a beetle is obviously not as simple as metamorphosis (change of shape), even the species has changed. This is "metamorphosis", a phase transition.

When rocks are subducted to a certain depth, under higher temperatures and pressures, they will flow not only due to the accumulation of deformation.

There is also a phase change of minerals, i.e. metamorphism, such as the transformation from basalt to eclogite.

This process, a solid-to-solid phase transition, will result in a significant increase in rock density.

This chemical phase change process of material conversion is also the driving mechanism of physical mechanical movement, and the driving mechanism of eclogite phase change is called the eclogite engine.

Density and shear wave velocity stratification of the crust and mantle (a) and the driving force mechanism of the eclogite delamination cycle (b) (modified from Anderson, 2007 after Li Sanzhong, 2019)

Not all eclogites will continue to sink after entering the mantle. In most cases,

At a depth of 410 km, the density of eclogite is less than that of the surrounding mantle, so it accumulates and gathers at a depth of 410 km.

Of course there are exceptions. Due to factors such as thermal changes and mineral phase changes, some subducted materials can continue to sink to a depth of 660km.

The mantle between 410-660km depth, where many subducted materials gather, is also called the mantle transition zone.

Hybrid mantle convection pattern diagram | Li Jianghai, 2019 modified from Chen Jiuhua, 2016; Tackley, 2008

Some of the subducted material can even reach the boundary between the mantle and the core.

Such, then, is the fate of subducted material.

Geometric features of the subducting slab in the transition zone revealed by seismic tomography | Li Jianghai, 2019 Modified from Goes, 2017

In addition to phase change increasing rock density, there is another effect, which is the extraction effect of partial melting, which leads to an increase in the density of the residual body.

When rocks are heated, some minerals with low melting points melt into liquid first. Under the action of buoyancy or seepage generated by tectonic stress, these liquids that melt first migrate.

Seepage can be understood as squeezing a water-rich sponge with your hands, and the water that flows out is caused by seepage. The remaining minerals with high melting points are still solid, which we call residual bodies. These residual bodies have a higher density, which will cause the density of the remaining rocks to increase.

Partial melting of rock, with the melt (yellow) being gradually extracted, leaving a remnant (grey) | Olivier Vanderhaeghe, 2009

Density of partially molten rock as the melt composition increases | Olivier Vanderhaeghe, 2009

Similar to the total density of a mixture of rice and sand, which is less than the density of pure sand.

A few more words, in addition to the oceanic lithosphere sinking into the mantle, the eclogite phase undergoing rock phase transition, or the lower part of the continental lithosphere remaining after melt extraction, will also detach and sink into the mantle. This effect is called "delamination."

For example, one mainstream view on the destruction of the North China Craton is the delamination of the bottom of the lithosphere.

Chen Ling, 2020; Hu, 2018

In short, rock phase changes and partial melting residues will lead to a further increase in the density of the oceanic lithosphere, which is the "deformation record" of the oceanic lithosphere.

In this way, the oceanic lithosphere with a relatively high density can sink into the mantle at a relatively low viscosity and density at a relatively deep depth under the action of "negative buoyancy" on a larger time scale.

References to this article are listed in the figure captions

The above are just my personal understanding and learning experience, please forgive me for not doing anything ridiculous.

This article comes from the answer by Zhihu answerer @铜马弓手 to "The mantle is solid, so how did the oceanic crust subduct into it?"

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