Plate Techtonics

In this episode of the I Can’t Sleep Podcast, fall asleep learning about Plate Tectonics. Shifting plates, growing mountains, slow change. These are great topics to bore you to sleep tonight. Happy listening!

Transcript

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Bostor.

Today's episode is from a Wikipedia article titled Plate Tectonics.

Plate Tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a large number of smaller plates of the Earth's lithosphere since tectonic processes began on Earth between 3.

3 and 3.

5 billion years ago.

The model builds on the concept of continental drift,

An idea developed during the first decades of the 20th century.

The geoscientific community accepted plate tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

The lithosphere,

Which is the rigid outermost shell of a planet,

The crust and upper mantle,

Is broken into tectonic plates.

The Earth's lithosphere is composed of seven or eight major plates,

Depending on how they are defined,

And many minor plates.

Where the plates meet,

Their relative motion determines the type of boundary,

Convergent,

Divergent,

Or transform.

Earthquakes,

Volcanic activity,

Mountain building,

And oceanic trench formation occur along these plate boundaries or faults.

The relative movement of the plates typically ranges from 0 to 100 millimeters annually.

Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere,

Each topped by its own kind of crust.

Along convergent boundaries,

Subduction,

Or one plate moving under another,

Carries the lower one down into the mantle.

The material lost is roughly balanced by the formation of new oceanic crust along divergent margins by seafloor spreading.

In this way,

The total surface of the lithosphere remains the same.

This prediction of plate tectonics is also referred to as the conveyor belt principle.

Earlier theories since disproven proposed gradual shrinking,

Contraction,

Or gradual expansion of the globe.

Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere.

Lateral density variations in the mantle result in convection,

That is the slow creeping motion of Earth's solid mantle.

Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges.

Due to variations in topography,

The ridge is a topographic high,

And density changes in the crust.

Density increases as newly formed crust cools and moves away from the ridge.

At subduction zones,

A relatively cold,

Dense oceanic crust is pulled or sinks down into the mantle over the downward convecting limb of a mantle cell.

Another explanation lies in the different forces generated by tidal forces of the sun and moon.

The relative importance of each of these factors and their relationship to each other is unclear,

And still the subject of much debate.

Key Principles The outer layers of the Earth are divided into the lithosphere and asthenosphere.

The division is based on differences in mechanical properties and in the method for the transfer of heat.

The lithosphere is cooler and more rigid,

While the asthenosphere is hotter and flows more easily.

In terms of heat transfer,

The lithosphere loses heat by conduction,

Whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient.

This division should not be confused with the chemical subdivision of these same layers into the mantle,

Comprising both the asthenosphere and the mantle portion of the lithosphere and the crust.

A given piece of mantle may be part of the lithosphere or the asthenosphere at different times,

Depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates,

Which ride on fluid-like viscoelastic solid asthenosphere.

Plate motions range up to a typical 10-40 mm per year,

Mid-Atlantic ridge,

About as fast as fingernails grow.

To about 160 mm per year,

Nazca plate,

About as fast as hair grows.

The driving mechanism behind this movement is described below.

Oceanic lithosphere plates consist of lithospheric mantle,

Overlaying by one or two types of crustal material.

Oceanic crust in older texts called sema from silicon and magnesium,

And continental crust,

Seal from silicon and aluminum.

Average oceanic lithosphere is typically 100 km thick.

Its thickness is a function of its age.

As time passes,

It conductively cools and subjacent cooling mantle is added to its base.

Because it is formed at mid-ocean ridges and spreads outwards,

Its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed.

For a typical distance,

That oceanic lithosphere must travel before being subducted.

The thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones.

For shorter or longer distances,

The subduction zone,

And therefore also the mean thickness,

Becomes smaller or larger respectively.

Continental lithosphere is typically about 200 km thick,

Though this varies considerably between basins,

Mountain ranges,

And stable cratonic interiors of continents.

The location where two plates meet is called a plate boundary.

Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features,

Such as mountains,

Volcanoes,

Mid-ocean ridges,

And oceanic trenches.

The majority of the world's active volcanoes occur along plate boundaries,

With the Pacific Plate's Ring of Fire being the most active and widely known today.

These boundaries are discussed in further detail below.

Some volcanoes occur in the interiors of plates,

And these have been variously attributed to internal plate deformation and to mantle plumes.

As explained above,

Tectonic plates may include continental crust or oceanic crust,

And most plates contain both.

For example,

The African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans.

The distinction between oceanic crust and continental crust is based on their modes of formation.

Oceanic crust is formed at seafloor spreading centers,

And continental crust is formed through arch-volcanism and accretion of terrains through tectonic processes.

Though some of these terrains may contain ophiolite sequences,

Which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation,

And spreading centers and subduction beneath continents.

Oceanic crust is also denser than continental crust owing to their different compositions.

Oceanic crust is denser because it has less silicon and more heavier elements,

Mafic,

And then continental crust,

Felsic.

As a result of this density stratification,

Oceanic crust generally lies below sea level,

For example most of the Pacific Plate,

While continental crust buoyantly projects above sea level.

Types of Plate Boundaries Three types of plate boundaries exist,

With the fourth,

Mixed type,

Characterized by the way the plates move relative to each other.

They are associated with different types of surface phenomena.

The different types of plate boundaries are 1.

Divergent Boundaries Constructive Occur where two plates slide apart from each other.

At zones of ocean-to-ocean rifting,

Divergent boundaries form by seafloor spreading,

Allowing for the formation of new ocean basins.

As the ocean plate splits,

The ridge forms at the spreading center,

The ocean basin expands,

And finally the plate area increases causing many small volcanoes and or shallow earthquakes.

At zones of continent-to-continent rifting,

Divergent boundaries may cause new ocean basin to form as the continent splits spreads,

The central rift collapses,

And ocean fills the basin.

Active zones of mid-ocean ridges,

E.

G.

The mid-Atlantic ridge and East Pacific rise,

And continent-to-continent rifting such as Africa's East African Rift and Valley and the Red Sea are examples of divergent boundaries.

2.

Convergent Boundaries Distructive or Active Margins Occur when two plates slide toward each other to form either a subduction zone,

One plate moving underneath the other,

Or a continental collision.

At zones of ocean-to-continent subduction,

E.

G.

The Andes Mountain Range in South America and the Cascade Mountains in western United States,

The dense oceanic lithosphere plunges beneath the less-dense continent.

Earthquakes trace the path of the downward-moving plate as it descends into a thinosphere,

A trench forms,

And as a subducted plate is heated it releases volatiles,

Mostly water from hydrous minerals,

Into the surrounding mantle.

The addition of water lowers the melting point of the mantle material above the subduction slab causing it to melt.

The magma that results typically leads to volcanism.

At zones of ocean-to-ocean subduction,

E.

G.

Aleutian Islands,

Mariana Islands,

And the Japanese Island Arc,

Older,

Cooler,

Denser crust slips beneath less-dense crust.

This motion causes earthquakes in a deep trench to form in an arc shape.

The upper mantle of the subduction plate then heats and magma rises to form curving chains of volcanic islands.

Deep marine trenches are typically associated with subduction zones,

And the basins that develop along the active boundary are often called foreland basins.

Closure of ocean basins can occur at continent-to-continent boundaries,

E.

G.

Himalayas and Alps.

Collision between masses of granitic continental lithosphere,

Neither mass is subducted,

Plate edges are compressed,

Folded,

Uplifted.

Transform boundaries,

Conservative,

Occur where two lithospheric plates slide,

Or perhaps more accurately grind past each other along transform faults,

Or plates are neither created nor destroyed.

The relative motion of the two plates is either sinistral,

Left side towards the observer,

Or dextral,

Right side toward the observer.

Transform faults occur across a spreading center.

Strong earthquakes can occur along a fault.

The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.

Plate boundary zones occur where the effects of the interactions are unclear,

And the boundaries,

Usually occurring along a broad belt,

Are not well defined and may show various types of movements in different episodes.

Driving Forces of Plate Motion It has generally been accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere.

Motion of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming.

The current view,

Though still a matter of some debate,

Asserts that as a consequence,

A powerful source of plate motion is generated due to the excess density of the oceanic lithosphere sinking in subduction zones.

When the new crust forms at mid-ocean ridges,

This oceanic lithosphere is initially less dense than the underlying asthenosphere,

But it becomes denser with age as it conductively cools and thickens.

The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones,

Providing most of the driving force for plate movement.

The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

Although subduction is thought to be the strongest force driving plate motions,

It cannot be the only force since there are plates such as the North American plate,

Which are moving yet are nowhere being subducted.

The same is true for the enormous Eurasian plate.

The sources of plate motion are a matter of intense research and discussion among scientists.

One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movement,

As some patterns may be explained by more than one mechanism.

In short,

The driving forces advocated at the moment can be divided into three categories based on the relationship to the movement,

Mantle dynamics related,

Gravity related,

Main driving force accepted nowadays,

And earth rotation related.

Driving Forces Related to Mantle Dynamics For much of the last quarter century,

The leading theory of the driving force behind tectonic plate motion envisaged large-scale convection currents in the upper mantle,

Which can be transmitted through the asthenosphere.

This theory was launched by Arthur Holmes and some forerunners in the 1930s,

And was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the century.

However,

Despite its acceptance,

It was long debated in the scientific community because the leading theory still envisaged a static earth without moving continents up until the major breakthroughs of the early 60s.

Two- and Three-dimensional Imaging of Earth's Interior Seismic Tomography shows a varying lateral density distribution throughout the mantle.

Such density variations can be material from rock chemistry,

Mineral from variations in mineral structures,

Or thermal through thermal expansion and contraction from heat energy.

The manifestation of this varying lateral density is mantle convection from buoyancy forces.

How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics.

Somehow,

This energy must be transferred to the lithosphere for tectonic plates to move.

There are essentially two main types of forces that are thought to influence plate motion,

Friction and gravity.

Basal Drag Friction Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere.

Slab Suction Gravity Plate motion driven by local convection currents that exert a downward pull on plates in subduction zones at ocean trenches.

Slab suction may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle,

Although perhaps to a greater extent acting on both the under and upper side of the slab.

Lately,

The convection theory has been much debated as modern techniques based on 3D seismic tomography still fail to recognize these predicted large-scale convection cells.

Alternative views have been proposed.

Plume Tectonics In the theory of plume tectonics followed by numerous researchers during the 1990s,

A modified concept of mantle convection currents is used.

It asserts that superplumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells.

These ideas find their roots in the early 1930s in the works of Bilosev and Fenbemelin,

Which were initially opposed to plate tectonics and placed the mechanism in a fixistic frame of verticalistic movements.

Fenbemelin later on modulated on the concept in his undulation models and used it as the driving force for horizontal movements,

Invoking gravitational forces away from the regional crustal doming.

The theories find resonance in the modern theories,

Which envisage hot spots or mantle plumes,

Which remain fixed and are overridden by oceanic and continental lithosphere plates over time and lead their traces in the geological record,

Though these phenomena are not invoked as real driving mechanisms,

But rather as modulators.

The mechanism is nowadays still advocated for,

For example,

To explain the breakup of supercontinents during specific geological epochs.

It has also still numerous followers,

Also amongst the scientists involved in the theory of earth expansion.

Surge tectonics Another theory is that the mantle flows neither in cells nor large plumes,

But rather as a series of channels just below the earth's crust,

Which then provide basal friction in the lithosphere.

This theory,

Called surge tectonics,

Became quite popular in the geophysics and geodynamics during the 1980s and 1990s.

Recent research based on three-dimensional computer modeling suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere.

Driving forces related to gravity Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism,

Such as the various forms of mantle dynamics described above.

In modern views,

Gravity is invoked as the major driving force through slab pole along subduction zones.

Gravitational sliding away from a spreading ridge According to many authors,

Plate motion is driven by the higher elevation of plates at ocean ridges.

As oceanic lithosphere is formed at spreading ridges from hot mantle material,

It gradually cools and thickens with age,

And thus adds distance from the ridge.

Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived,

And so with increasing thickness it gradually subsides into the mantle to compensate the greater load.

The result is a slight lateral incline with increased distance from the ridge axis.

This force is regarded as a secondary force and is often referred to as ridge push.

This is a misnomer as nothing is pushing horizontally and tension features are dominant along ridges.

It is more accurate to refer to this mechanism as gravitational sliding,

As variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the first prominent feature.

Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate,

Which produces a clear topographical feature that can offset,

Or at least affect,

The influence of topographical ocean ridges and mantle plumes in hot spots,

Which are postulated to impinge on the underside of tectonic plates.

Slab pull Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere.

Slab pull is therefore most widely thought to be the greatest force acting on the plates.

In this current understanding,

Plate motion is mostly driven by the weight of cold,

Dense plates sinking into the mantle at trenches.

Recent models indicate that trench suction plays an important role as well.

However,

The fact that the North American plate is nowhere being subjected,

Although it is in motion,

Presents a problem.

Same holds for the African,

Eurasian,

And Antarctic plates.

Gravitational sliding away from the mantle doming According to older theories,

One of the driving mechanisms of the plates is the existence of large-scale asthenosphere mantle domes,

Which cause the gravitational sliding of lithosphere plates away from them.

This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism.

It finds its roots in the undation model of Van Bimelen.

This can act on various scales,

From the small scale of one island arc up to the larger scale of an entire ocean basin.

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I definitely need more science content. I learn a lot before I fall asleep :) I love it!!

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Worked well to send me off to sleepsville

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Great! Thank you!

Fabulously boring. I love these recordings of 'someone reading random Wikipedia articles' and I'm happy they are on Insight Timer now!

How does he not put himself to sleep? So boring, and spot on, I was almost out in 30 seconds flat but I stayed up and motivated myself to get the boring nightly routine done first - flossing, brushing teeth, scrubbing face, moisturizing, brushing hair, ugh. I used to fall asleep on my boring Honors History book and barely passed with a C, so as an adult, I am laughing along w this concept, and hell, pushing myself to maybe even try to learn something. But the lithosphere....YAWN.

It ended just went it started making sense. Five stars for the description alone.

Helped me fall asleep and study for my geography test!

Works like a charm. 😊 Quiets my ever thinking mind.

I really enjoyed the information.
Thank you so much!

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This definitely did the trick! Thank you!