#outer core

Nothing is ever as simple as we learned in school, and the composition of the Earth is no different. The mantle is not a uniform, spherical layer; it actually has enormous "blobs" that sit at the bottom of the mantle above the outer core. Like the mantle, these blobs appear to be made from rock, but may be hotter and heavier than their surrounding material. As of the this 2019 article, scientists have not decided what to call these blobs. The most common name is "large low shear velocity provinces," or LLSVP.

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A new study from Arizona State University (ASU) suggests that these blobs are remnants of the proto-planet named Theia that collided with Earth. Part of Theia's mantle got stuck in the Earth's mantle, and other pieces of Theia were flung into space to create the moon. Initially, the Theian mantle bits were scattered through Earth's mantle. They did not "mix" because they are made of different materials - Theia was more iron-rich. After billions of years, the Theian fragments merged to create the LLSVPs. This origin hypothesis is not new, but the ASU team argues that their work is the most comprehensive to date.

A Tale of Two Poles

The first rule of navigation is that a compass needle points towards the North. But where exactly is ‘North’? The Earth has two North poles, a geographic North pole and a magnetic North pole. The geographic North pole is what we most commonly think of as the North pole. This is the fixed point where the surface of the Earth intersects its axis of rotation and where the meridians (lines of longitude) converge in the northern hemisphere.

Our planet also has a magnetic North pole. This is the location where the Earth’s magnetic field lines converge at the surface. Put simply, the Earth can be though of as a giant bar magnet with ends at the magnetic North and South poles. The Earth’s magnetic field is the result of the motion of molten metals in the outer core; a self-sustaining process known as the geodynamo.

A compass needle will align with the Earths Magnetic North pole. While the geographic pole is a fixed point on the surface, the magnetic pole migrates across the Earth’s surface over time. This ‘polar wander’ is due to changes of the motions in the inner core. Currently, the magnetic pole is located to the East of the geographic pole, and is moving up to 10’s of kilometres per year towards Siberia. Scientists have been tracking the movement of the magnetic North pole for almost 200 years, as shown in the image below.

The angular difference between the geographic and magnetic North poles is called magnetic declination. Declination varies depending on where you are located on the globe, generally increasing closer to the poles and changing at you move from East to West. Take two cities at similar longitudes but different latitudes, Toronto (~ 43 degrees N) and Miami (~ 25 degrees N). Toronto has a magnetic declination of ~ 14 degrees to the West while Miami has a declination of ~6 degrees to the West. If you are interested in finding out what the magnetic inclination is where you live, check out http://bit.ly/1ZWPful.

Setting magnetic declination on your compass is a critical first step toward successful navigation, or you might just end up somewhere you don’t expect.

CD

Sources The Solid Earth: An Introduction to Global Geophysics 2nd ed. Fowler, C.M.R. 2010. Cambridge University Press. http://bit.ly/2r6TZIG http://bit.ly/2EG7vF1 https://go.nasa.gov/2wquiAa http://bit.ly/2arsQbc

S Wave Shadow Zone

From and early age, we are all taught that the interior of the Earth can be divided into four distinct layers; a solid crust, a plastic mantle, liquid outer core, and solid inner core. But have you ever wondered how we know these layers exist? Obviously, nobody has every managed to drill down to the center of the Earth to check. The deepest hole drilled so far, the Kola Superdeep Borehole in Russia, is just over 12 km deep, far shy of the 6, 371 km that would be required to reach the center of the Earth. Most of what we know about the interior structure of the Earth comes from studying how seismic waves generated by large earthquakes move through the planet. By using networks of sophisticated seismometers installed in bedrock around the world, geophysicists can record even the smallest vibrations. Seismic waves that travel through the Earth are generally divided into two types; compressional (P waves) and shear (S waves). P waves compress and expand material in the direction the wave is propagating. S waves shear the material perpendicular to the direction the wave is propagating. P waves travel much faster than S waves, and can be differentiated on a seismograph on the basis of their arrival times. A great way to visualize how these waves move is with a slinky, here’s an example: http://bit.ly/2xXRXgq

Liquids cannot sustain shear forces, meaning S waves can only propagate through solid material. When an earthquake occurs, seismic waves are generated at the hypocenter and propagate in all directions through the Earth. S waves are able to pass through the solid crust and mantle and can detected by seismometers on the other side. When an S wave crosses the boundary between the mantle and the outer core, some of the energy contained in the wave gets reflected back toward the surface, and some is transmitted through the core as a P wave. In the 1930’s, Beno Gutenberg found a lack of certain previously predicted S wave recordings at seismometers located more than 103 degrees of latitude from where the earthquake took place. This is known as the ‘S wave shadow zone’ for a given earthquake, and is directly caused by the inability of S waves to pass through the outer core.

CD

http://bit.ly/2xySNPt http://bit.ly/1m0UbMt http://bit.ly/2wMJT2c Fowler, C.M.R. 2005. The Solid Earth: An Introduction to Global Geophysics (2nd Edition). Cambridge University Press, Print.

The motion of Earth’s inner core

To first order, the Earth is made up of 3 layers; a solid mantle (and crust), a liquid outer core, and a solid inner core.

This setup allows for a very interesting possibility. We all ride on the outer solid layer and we know pretty well how fast it rotates; the solid outer layers of the Earth rotate once every 24 hours. However, the Earth’s outer core is a liquid so it can flow in different directions from the Earth’s mantle. There can be currents and flows in the outer core just like there are currents in the ocean, and those currents don’t have to follow the motion of the mantle.

On top of that, there is one more solid layer to the Earth; the inner core. The solid inner core is separated from the mantle by an enormous layer of molten iron that is very hot and fluid. There is nothing solid to connect the 2 solid layers, so there is no reason they need to move together; any movement of the inner core can be distinct from movement of the Earth’s solid outer layers.

A few months ago we covered a new study that identified actual motions in the Earth’s inner core based on how seismic waves move through it (https://www.facebook.com/TheEarthStory/posts/513792705348445). Building on that work, a new study has just come out in the Proceedings of the National Academy of Sciences putting together a detailed model for how this rotation works.

The Earth’s magnetic field is generated in the outer core by currents flowing in the incredibly-hot iron. That magnetic field is expressed both at the Earth’s surface and inside the planet and can be measured to actually give information about the outer core.

As long ago as 1692, Edmund Halley, better known for a comet of course, discovered that there were features in the magnetic field at the surface which were drifting to the west with time, moving in the opposite direction of rotation of the Earth’s surface. This westward drift is slow, but relative to the Earth’s surface it can be readily measured and is believed to reflect the direction currents are moving in the outer core.

Based on the results of the seismic surveys, these authors conclude the Earth’s inner core is actually drifting East relative to Earth’s surface. Have to do a bit of relativity here to say this correctly so bear with me; from the perspective of an observer in space, the Earth’s surface turns to the East, so from that perspective the Earth’s inner core turns to the East the fastest, the Earth’s outer core turns the slowest, and the Crust/Mantle are in-between.

From the perspective of the Earth’s surface, the outer core is turning to the west relative to me right now and the inner core is moving to the East.

The authors explain these behaviors through the magnetic field. That property is illustrated in this figure from their paper; the magnetic field is generated in the outer core, causing a force which slows down the rotation of the liquid outer core (represented by the vertical orange cylinder), but that same magnetic field applies a force to the inner core which speeds it up (represented by the blue contour around the core).

On top of that, the magnetic field varies through time. It changes in intensity and direction, even flipping every few hundred thousand years. As a consequence, the force on the inner core changes as well, leading to the wobbling back and forth that the other authors measured.

The mechanisms are a little hard to follow and I hope everything makes sense. If you want a take home message about how the Earth is moving; the liquid outer core beneath you is convecting with iron rich fluids moving at a velocity of perhaps a millimeter per second or so. That fluid is generally moving to the West relative to you at about a kilometer per year and taking the magnetic field with it. The Inner core beneath it is going the other way; it’s turning to the East relative to me right now, moving faster than the Earth and being pushed by the magnetic field.

-JBB

Image credit and original manuscript: http://www.pnas.org/content/early/2013/09/13/1307825110.full.pdf+html

Press report: http://phys.org/news/2013-09-insights-year-old-problem-dynamics-earth.html

Plate tectonics is one of the fundamental concepts in Earth science, explaining how the Earth’s surface is constantly changing. The theory of plate tectonics revolutionized our understanding of geology, earthquakes, volcanoes, and mountain formation. This article explores the mechanisms behind plate movement, the types of tectonic plates, and the effects on the planet’s surface, providing a comprehensive guide for students, researchers, and enthusiasts alike.

Introduction to Plate Tectonics

The Earth’s lithosphere is divided into several large and small plates that float on the semi-fluid asthenosphere beneath. These plates are not static—they move slowly over geological time, causing earthquakes, volcanic activity, and continental drift.

Understanding plate tectonics helps explain phenomena like:

The distribution of earthquakes and volcanoes.

The formation of mountains and ocean basins.

Continental drift over millions of years.

The concept was first widely accepted in the 1960s after Alfred Wegener’s theory of continental drift gained supporting evidence through seafloor spreading and paleomagnetic studies.

The Structure of the Earth

To understand plate tectonics, it’s essential to grasp the Earth’s internal structure:

Crust – The outermost layer of Earth, which includes continental and oceanic crust.

Mantle – A thick layer beneath the crust, where convection currents drive plate movement.

Outer Core – A liquid layer composed mainly of iron and nickel.

Inner Core – A solid, dense sphere of iron and nickel at the center of the Earth.

The lithosphere (crust + upper mantle) floats on the asthenosphere, which behaves like a viscous fluid over long periods. This movement of the lithosphere over the asthenosphere is the primary driver of plate tectonics.

Types of Plate Boundaries

The interactions between tectonic plates occur at boundaries, which are classified into three main types:

1. Divergent Boundaries

At divergent boundaries, plates move away from each other. Magma rises from below the crust to fill the gap, forming new crust. This process is common at mid-ocean ridges.

Examples include the Mid-Atlantic Ridge and East African Rift.

It can result in volcanic activity and shallow earthquakes.

2. Convergent Boundaries

Convergent boundaries occur when plates collide. Depending on the types of plates involved, several outcomes are possible:

Oceanic-Continental Collision: Oceanic plate subducts beneath continental plate → volcanic mountain chains (e.g., Andes).

Oceanic-Oceanic Collision: One oceanic plate subducts → island arcs (e.g., Japan).

Continental-Continental Collision: Both plates push up → high mountain ranges (e.g., Himalayas).

3. Transform Boundaries

Transform boundaries occur when plates slide past each other horizontally. These boundaries are marked by frequent earthquakes.

Example: San Andreas Fault in California.

Mechanisms Driving Plate Movement

Several forces drive plate tectonics, including:

Mantle Convection: Hot mantle material rises and cooler material sinks, moving plates along the surface.

Ridge Push: Elevated mid-ocean ridges push plates apart due to gravity.

Slab Pull: Sinking plates at subduction zones pull the rest of the plate along.

These forces work together to keep the Earth’s crust in constant motion, reshaping continents and ocean basins over millions of years.

Effects of Plate Tectonics

The movement of tectonic plates has significant effects on Earth’s surface, environment, and human activity:

Earthquakes: Sudden plate movement releases energy, causing seismic waves.

Volcanic Activity: Subduction and divergent boundaries allow magma to reach the surface.

Mountain Building: Continental collisions create mountain ranges.

Ocean Formation: Divergent boundaries and rifting lead to new ocean basins.

Bullet Points: Additional Impacts

Tsunamis triggered by underwater earthquakes.

Formation of natural resources like minerals and fossil fuels.

Climate and habitat changes due to continental drift.

Plate Tectonics and Earth History

Over geological time, plate tectonics has dramatically reshaped Earth’s surface. Supercontinents like Pangaea formed and broke apart due to plate movement. Fossil distribution and rock formations across continents support this dynamic history.

Pangaea: Formed approximately 335 million years ago and broke apart around 175 million years ago.

Modern Continents: Continents continue to drift, changing ocean currents and climate patterns.

Modern Methods of Studying Plate Tectonics

Scientific advancements have allowed detailed study of plate tectonics:

GPS Technology: Measures the precise movement of plates in millimeters per year.

Seismic Studies: Map the distribution and intensity of earthquakes.

Ocean Floor Mapping: Reveals mid-ocean ridges, trenches, and rift valleys.

Paleomagnetism: Tracks the historical movement of plates using magnetic orientation in rocks.

Read This Blog in Detail Please Visit This Blog Plate Tectonics

For more in-depth resources and interactive explanations, you can visit studylab24.com.

Common Misconceptions About Plate Tectonics

Misconception 1: Plates move rapidly. → Truth: Most plates move only a few centimeters per year.

Misconception 2: Earthquakes happen randomly. → Truth: Earthquakes mainly occur at plate boundaries.

Misconception 3: Volcanoes only appear at hot spots. → Truth: Most volcanoes are along convergent and divergent plate boundaries, though hot spots like Hawaii exist.

Why Plate Tectonics Matters

Plate tectonics is not just a geological concept—it affects our daily lives:

Predicting natural disasters.

Understanding the distribution of resources.

Explaining Earth’s past climate and life evolution.

By studying plate tectonics, scientists can develop better strategies for disaster preparedness, resource management, and environmental protection.

Conclusion

Plate tectonics is the framework that explains the dynamic nature of our planet. From the drifting continents to the formation of mountains, earthquakes, and volcanoes, it shapes both the Earth’s physical landscape and human society. As technology advances, our understanding of plate movements continues to deepen, highlighting the ongoing, fascinating story of Earth’s crust in motion.