What is the Mid-Atlantic Ridge?

If you took geology in high school, then chances are you remember learning something about how the Earth’s crust – the outermost layer of Earth – is arranged into a series of tectonic plates. These plates float on top of the Earth’s mantle, the semi-viscous layer that surrounds the core, and are in constant motion because of convection in the mantle. Where two plates meet, you have what it is known as a boundary.

These can be “divergent” or “convergent”, depending on whether the plates are moving apart or coming together. Where they diverge, hot magma can rise from below, creating features like long ridges or mountain chains. Interestingly enough, this is how one of the world’s largest geological features was formed. It called the Mid-Atlantic Ridge, which run from north to south along the ocean floor in the Atlantic.

Description:

The Mid-Atlantic Ridge (MAR) is known as a mid-ocean ridge, an underwater mountain system formed by plate tectonics. It is the result of a convergent plate boundary that runs from 87° N – about 333 km (207 mi) south of the North Pole – to 54 °S, just north of the coast of Antarctica.

Transform Plate Boundary
The different types of Tectonic Plate Boundaries, ranging from convergent and transform to divergent. Credit: USGS/Jose F. Vigil

Like other ocean ridge systems, the MAR developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates. In the North Atlantic, it separates the Eurasian and North American Plates; whereas in the South Atlantic, it separates the African and South American Plates.

The MAR is approximately 16,000 km (10,000 mi) long and between 1,000 and is 1,500 km (620 and 932 mi) wide. The peaks of the ridge stand about 3 km (1.86 mi) in height above the ocean floor, and sometimes reach above sea level, forming islands and island groups. The MAR is also part of the longest mountain chain in the world, extending continuously across the oceans floors for a total distance of 40,389 km (25,097 mi).

The MAR also has a deep rift valley at is crest which marks the location where the two plates are moving apart. This rift valley runs along the axis of the ridge for nearly its entire length, measuring some 80 to 120 km (50 to 75 miles) wide. The rift marks the actual boundary between adjacent tectonic plates, and is where magma from the mantle reaches the seafloor.

Where this magma is able to reach the surface, the result is basaltic volcanoes and islands. Where it is still submerged, it produces “pillow lava”. As the plates move further apart, new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process, known as “sea floor spreading”, is happening at an average rate of about 2.5 cm per year (1 inch).

The Earth’s Tectonic Plates, with convergent and divergent boundaries indicated with red arrows. Credit: msnucleus.org

In other words, North America and Europe are moving away from each other at a very slow rate. This process also means that the basaltic rock that makes up the ridge is younger than the surrounding crust.

Notable Features:

As noted, the ridge (while mainly underwater) does have islands and island groups that were created by volcanic activity. In the Northern Hemisphere, these include Jan Mayen Island and Iceland (Norway), and the Azores (Portugal). In the Southern Hemisphere, MAR features include Ascension Island, St. Helena, Tristan da Cunha, Gough Island (all UK territories) and Bouvet Island (Norway).

Near the equator, the Romanche Trench divides the North Atlantic Ridge from the South Atlantic Ridge. This narrow submarine trench has a maximum depth of 7,758 m (25,453 ft), one of the deepest locations of the Atlantic Ocean. This trench, however, is not regarded an official boundary between any of the tectonic plates.

History of Exploration:

The ridge was initially discovered in 1872 during the expedition of the HMS Challenger. In the course of investigating the Atlantic for the sake of laying the transatlantic telegraph cable, the crew discovered a large rise in the middle of the ocean floor. By 1925, its existence was confirmed thanks to the invention of sonar.

The super-continent Pangaea during the Permian period (300 – 250 million years ago). Credit: NAU Geology/Ron Blakey

By the 1960s, scientists were able to map the Earth’s ocean floors, which revealed a seismically-active central valley, as well as a network of valleys and ridges. They also discovered that the ridge was part of a continuous system of mid-ocean ridges that extended across the entire ocean floor, connecting all the divergent boundaries around the planet.

This discovery also led to new theories in terms of geology and planetary evolution. For instance, the theory of “seafloor spreading” was attributed to the discovery of the MAR, as was the acceptance of continental drift and plate tectonics. In addition, it also led to the theory that all the continents were once part of subcontinent known as “Pangaea”, which broke apart roughly 180 million years ago.

Much like the “Pacific Ring of Fire“, the discovery of the Mid-Atlantic Ridge has helped inform our modern understanding of the world. Much like convergent boundaries, subduction zones and other geological forces, the process that created it is also responsible for the world as we know it today.

Basically, it is responsible for the fact that the Americas have been drifting away from Africa and Eurasia for millions of years, the formation of Australia, and the collision between the India Subcontinent and Asia. Someday – millions of years from now – the process of seafloor spreading will cause the Americas and Asia to collide, thus forming a new super continent – “Amasia”.

We have written many interesting articles about Earth here at Universe Today. Here’s 10 Interesting Facts About Earth, What are Plate Boundaries?, What are Divergent Boundaries?, Mountains: How are they Formed?, What is a Subduction Zone?, What is an Earthquake?, What is the Pacific Ring of Fire?, and How Many Continents are There?

For more information, check out the Geological Society’s page on the Mid-Atlantic Ridge.

Astronomy Cast also has episodes that are relevant to the subject. Here’s Episode 51: Earth and Episode 293: Earthquakes.

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10 Interesting Facts About Earth

Planet Earth. That shiny blue marble that has fascinated humanity since they first began to walk across its surface. And why shouldn’t it fascinate us? In addition to being our home and the place where life as we know it originated, it remains the only planet we know of where life thrives. And over the course of the past few centuries, we have learned much about Earth, which has only deepened our fascination with it.

But how much does the average person really know about the planet Earth? You’ve lived on Planet Earth all of your life, but how much do you really know about the ground underneath your feet? You probably have lots of interesting facts rattling around in your brain, but here are 10 more interesting facts about Earth that you may, or may not know.

1. Plate Tectonics Keep the Planet Comfortable:

Earth is the only planet in the Solar System with plate tectonics. Basically, the outer crust of the Earth is broken up into regions known as tectonic plates. These are floating on top of the magma interior of the Earth and can move against one another. When two plates collide, one plate will subduct (go underneath another), and where they pull apart, they will allow fresh crust to form.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

This process is very important, and for a number of reasons. Not only does it lead to tectonic resurfacing and geological activity (i.e. earthquakes, volcanic eruptions, mountain-building, and oceanic trench formation), it is also intrinsic to the carbon cycle. When microscopic plants in the ocean die, they fall to the bottom of the ocean.

Over long periods of time, the remnants of this life, rich in carbon, are carried back into the interior of the Earth and recycled. This pulls carbon out of the atmosphere, which makes sure we don’t suffer a runaway greenhouse effect, which is what happened on Venus. Without the action of plate tectonics, there would be no way to recycle this carbon, and the Earth would become an overheated, hellish place.

2. Earth is Almost a Sphere:

Many people tend to think that the Earth is a sphere. In fact, between the 6th cenury BCE and the modern era, this remained the scientific consensus. But thanks to modern astronomy and space travel, scientists have since come to understand that the Earth is actually shaped like a flattened sphere (aka. an oblate spheroid).

This shape is similar to a sphere, but where the poles are flattened and the equator bulges. In the case of the Earth, this bulge is due to our planet’s rotation. This means that the measurement from pole to pole is about 43 km less than the diameter of Earth across the equator. Even though the tallest mountain on Earth is Mount Everest, the feature that’s furthest from the center of the Earth is actually Mount Chimborazo in Ecuador.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

3. Earth is Mostly Iron, Oxygen and Silicon:

If you could separate the Earth out into piles of material, you’d get 32.1 % iron, 30.1% oxygen, 15.1% silicon, and 13.9% magnesium. Of course, most of this iron is actually located at the core of the Earth. If you could actually get down and sample the core, it would be 88% iron. And if you sampled the Earth’s crust, you’d find that 47% of it is oxygen.

4. 70% of the Earth’s Surface is Covered in Water:

When astronauts first went into the space, they looked back at the Earth with human eyes for the first time. Based on their observations, the Earth acquired the nickname the “Blue Planet:. And it’s no surprise, seeing as how 70% of our planet is covered with oceans. The remaining 30% is the solid crust that is located above sea level, hence why it is called the “continental crust”.

5. The Earth’s Atmosphere Extends to a Distance of 10,000 km:

Earth’s atmosphere is thickest within the first 50 km from the surface or so, but it actually reaches out to about 10,000 km into space. It is made up of five main layers – the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface.

Winter Solstice
Earth, as viewed from the cabin of the Apollo 11 spacecraft. Credit: NASA

The bulk of the Earth’s atmosphere is down near the Earth itself. In fact, 75% of the Earth’s atmosphere is contained within the first 11 km above the planet’s surface. However, the outermost layer (the Exosphere) is the largest, extending from the exobase – located at the top of the thermosphere at an altitude of about 700 km above sea level – to about 10,000 km (6,200 mi). The exosphere merges with the emptiness of outer space, where there is no atmosphere.

The exosphere is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules – including nitrogen, oxygen and carbon dioxide. The atoms and molecules are so far apart that the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or with the solar wind.

Want more planet Earth facts? We’re halfway through. Here come 5 more!

6. The Earth’s Molten Iron Core Creates a Magnetic Field:

The Earth is like a great big magnet, with poles at the top and bottom near to the actual geographic poles. The magnetic field it creates extends thousands of kilometers out from the surface of the Earth – forming a region called the “magnetosphere“. Scientists think that this magnetic field is generated by the molten outer core of the Earth, where heat creates convection motions of conducting materials to generate electric currents.

The magnetic field and electric currents in and around Earth generate complex forces that have immeasurable impact on every day life. The field can be thought of as a huge bubble, protecting us from cosmic radiation and charged particles that bombard Earth in solar winds. It's shaped by winds of particles blowing from the sun called the solar wind, the reason it's flattened on the "sun-side" and swept out into a long tail on the opposite side of the Earth. Credit: ESA/ATG medialab
Artist’s impression of the Earth’s protective magnetic field and the dynamo effect in its core that gives rise to it. Credit: ESA/ATG medialab

Be grateful for the magnetosphere. Without it, particles from the Sun’s solar wind would hit the Earth directly, exposing the surface of the planet to significant amounts of radiation. Instead, the magnetosphere channels the solar wind around the Earth, protecting us from harm. Scientists have also theorized that Mars’ thin atmosphere is due to it having a weak magnetosphere compared to Earth’s, which allowed solar wind to slowly strip it away.

7. Earth Doesn’t Take 24 Hours to Rotate on its Axis:

It actually takes 23 hours, 56 minutes and 4 seconds for the Earth to rotate once completely on its axis, which astronomers refer to as a Sidereal Day. Now wait a second, doesn’t that mean that a day is 4 minutes shorter than we think it is? You’d think that this time would add up, day by day, and within a few months, day would be night, and night would be day.

But remember that the Earth orbits around the Sun. Every day, the Sun moves compared to the background stars by about 1° – about the size of the Moon in the sky. And so, if you add up that little motion from the Sun that we see because the Earth is orbiting around it, as well as the rotation on its axis, you get a total of 24 hours.

This is what is known as a Solar Day, which – contrary to a Sidereal Day – is the amount of time it takes the Sun to return to the same place in the sky. Knowing the difference between the two is to know the difference between how long it takes the stars to show up in the same spot in the sky, and the it takes for the sun to rise and set once.

8. A year on Earth isn’t 365 days:

It’s actually 365.2564 days. It’s this extra .2564 days that creates the need for a Leap Year once ever four years. That’s why we tack on an extra day in February every four years – 2004, 2008, 2012, etc. The exceptions to this rule is if the year in question is divisible by 100 (1900, 2100, etc), unless it divisible by 400 (1600, 2000, etc).

9. Earth has 1 Moon and 2 Co-Orbital Satellites:

As you’re probably aware, Earth has 1 moon (aka. The Moon). Plenty is known about this body and we have written many articles about it, so we won’t go into much detail there. But did you know there are 2 additional asteroids locked into a co-orbital orbits with Earth? They’re called 3753 Cruithne and 2002 AA29, which are part of a larger population of asteroids known as Near-Earth Objects (NEOs).

The asteroid known as 3753 Cruithne measures 5 km across, and is sometimes called “Earth’s second moon”. It doesn’t actually orbit the Earth, but has a synchronized orbit with our home planet. It also has an orbit that makes it look like it’s following the Earth in orbit, but it’s actually following its own, distinct path around the Sun.

Meanwhile, 2002 AA29 is only 60 meters across and makes a horseshoe orbit around the Earth that brings it close to the planet every 95 years. In about 600 years, it will appear to circle Earth in a quasi-satellite orbit. Scientists have suggested that it might make a good target for a space exploration mission.

10. Earth is the Only Planet Known to Have Life:

We’ve discovered past evidence of water and organic molecules on Mars, and the building blocks of life on Saturn’s moon Titan. We can see amino acids in nebulae in deep space. And scientists have speculated about the possible existence of life beneath the icy crust of Jupiter’s moon Europa and Saturn’s moon Titan. But Earth is the only place life has actually been discovered.

But if there is life on other planets, scientists are building the experiments that will help find it. For instance, NASA just announced the creation of the Nexus for Exoplanet System Science (NExSS), which will spend the coming years going through the data sent back by the Kepler space telescope (and other missions that have yet to be launched) for signs of life on extra-solar planets.

Europa's cracked, icy surface imaged by NASA's Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute.
Europa’s cracked, icy surface imaged by NASA’s Galileo spacecraft in 1998. Credit: NASA/JPL-Caltech/SETI Institute.

Giant radio dishes are currently scan distant stars, listening for the characteristic signals of intelligent life reaching out across interstellar space. And newer space telescopes, such as NASA’s James Webb Telescope, the Transiting Exoplanet Survey Satellite (TESS), and the European Space Agency’s Darwin mission might just be powerful enough to sense the presence of life on other worlds.

But for now, Earth remains the only place we know of where there’s life. Now that is an interesting fact!

We have written many interesting articles about planet Earth here on Universe Today. Here’s What is the Highest Place on Earth?, What is the Diameter of the Earth?, What is the Closest Planet to Earth?, What is the Surface Temperature of Earth? and The Rotation of the Earth?

Other articles include how fast the Earth rotates, and here’s an article about the closest star to Earth. If you’d like more info on Earth, check out NASA’s Solar System Exploration Guide on Earth. And here’s a link to NASA’s Earth Observatory.

And there’s even an Astronomy Cast episode on the subject of planet Earth.

How Do Volcanoes Erupt?

Cleveland Volcano Eruption

Volcanoes come in many shapes and sizes, ranging from common cinder cone volcanoes that build up from repeated eruptions and lava domes that pile up over volcanic vents to broad shield volcanoes and composite volcanoes. Though they differ in terms of structure and appearance, they all share two things. On the one hand, they are all awesome forces of nature that both terrify and inspire.

On the other, all volcanic activity comes down to the same basic principle. In essence, all eruptions are the result of magma from beneath the Earth being pushed up to the surface where it erupts as lava, ash and rock. But what mechanisms drive this process? What is it exactly that makes molten rock rise from the Earth’s interior and explode onto the landscape?

To understand how volcanoes erupt, one first needs to consider the structure of the Earth. At the very top is the lithosphere, the outermost layers of the Earth that consists of the upper mantle and crust. The crust makes up a tiny volume of the Earth, ranging from 10 km in thickness on the ocean floor to a maximum of 100 km in mountainous regions. It is cold and rigid, and composed primarily of silicate rock.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

Beneath the crust, the Earth’s mantle is divided into sections of varying thickness based on their seismology. These consist of the upper mantle, which extends from a depth of 7 – 35 km (4.3 to 21.7 mi)) to 410 km (250 mi); the transition zone, which ranges from 410–660 km (250–410 mi); the lower mantle, which ranges from 660–2,891 km (410–1,796 mi); and the core–mantle boundary, which is ~200 km (120 mi) thick on average.

In the mantle region, conditions change drastically from the crust. Pressures increase considerably and temperatures can reach up to 1000 °C, which makes the rock viscous enough that it behaves like a liquid. In short, it experiences elastically on time scales of thousands of years or greater. This viscous, molten rock collects into vast chambers beneath the Earth’s crust.

Since this magma is less dense than the surrounding rock, it ” floats” up to the surface, seeking out cracks and weaknesses in the mantle. When it finally reaches the surface, it explodes from the summit of a volcano. When it’s beneath the surface, the molten rock is called magma. When it reaches the surface, it erupts as lava, ash and volcanic rocks.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

With each eruption, rocks, lava and ash build up around the volcanic vent. The nature of the eruption depends on the viscosity of the magma. When the lava flows easily, it can travel far and create wide shield volcanoes. When the lava is very thick, it creates a more familiar cone volcano shape (aka. a cinder cone volcano). When the lava is extremely thick, it can build up in the volcano and explode (lava domes).

Another mechanism that drives volcanism is the motion the crust undergoes. To break it down, the lithosphere is divided into several plates, which are constantly in motion atop the mantle. Sometimes the plates collide, pull apart, or slide alongside each other; resulting in convergent boundaries, divergent boundaries, and transform boundaries. This activity is what drives geological activity, which includes earthquakes and volcanoes.

In the case of the former, subduction zones are often the result, where the heavier plate slips under the lighter plate – forming a deep trench. This subduction changes the dense mantle into buoyant magma, which rises through the crust to the Earth’s surface. Over millions of years, this rising magma creates a series of active volcanoes known as a volcanic arc.

Cross-section of a volcano. Credit: 3dgeography.co.uk/#!
Cross-section of a volcano. Credit: 3dgeography.co.uk

In short, volcanoes are driven by pressure and heat in the mantle, as well as tectonic activity that leads to volcanic eruptions and geological renewal. The prevalence of volcanic eruptions in certain regions of the world – such as the Pacific Ring of Fire – also has a profound impact on the local climate and geography. For example, such regions are generally mountainous, have rich soil, and periodically experience the formation of new landmasses.

We have written many articles about volcanoes here at Universe Today. Here’s What are the Different Types of Volcanoes?, What are the Different Parts of a Volcano?, 10 Interesting Facts About Volcanoes?, What is the Pacific Ring of Fire?, Olympus Mons: The Largest Volcano in the Solar System.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

What is the Difference Between Lava and Magma?

Few forces in nature are are impressive or frightening as a volcanic eruption. In an instant, from within the rumbling depths of the Earth, hot lava, steam, and even chunks of hot rock are spewed into the air, covering vast distances with fire and ash. And thanks to the efforts of geologists and Earth scientists over the course of many centuries, we have to come to understand a great deal about them.

However, when it comes to the nomenclature of volcanoes, a point of confusion often arises. Again and again, one of the most common questions about volcanoes is, what is the difference between lava and magma? They are both molten rock, and are both associated with volcanism. So why the separate names? As it turns out, it all comes down to location.

Earth’s Composition:

As anyone with a basic knowledge of geology will tell you, the insides of the Earth are very hot. As a terrestrial planet, its interior is differentiated between a molten, metal core, and a mantle and crust composed primarily of silicate rock. Life as we know it, consisting of all vegetation and land animals, live on the cool crust, whereas sea life inhabits the oceans that cover a large extent of this same crust.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

However, the deeper one goes into the planet, both pressures and temperatures increase considerably. All told, Earth’s mantle extends to a depth of about 2,890 km, and is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause pockets of molten rock to form.

This silicate material is less dense than the surrounding rock, and is therefore sufficiently ductile that it can flow on very long timescales. Over time, it will also reach the surface as geological forces push it upwards. This happens as a result of tectonic activity.

Basically, the cool, rigid crust is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries. These are known as convergent boundaries, at which two plates come together; divergent boundaries, at which two plates are pulled apart; and transform boundaries, in which two plates slide past one another laterally.

Interactions between these plates are what is what is volcanic activity (best exemplified by the “Pacific Ring of Fire“) as well as mountain-building. As the tectonic plates migrate across the planet, the ocean floor is subducted – the leading edge of one plate pushing under another. At the same time, mantle material will push up at divergent boundaries, forcing molten rock to the surface.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

Magma:

As already noted, both lava and magma are what results from rock superheated to the point where it becomes viscous and molten. But again, the location is the key. When this molten rock is still located within the Earth, it is known as magma. The name is derived from Greek, which translate to “thick unguent” (a word used to describe a viscous substance used for ointments or lubrication).

It is composed of molten or semi-molten rock, volatiles, solids (and sometimes crystals) that are found beneath the surface of the Earth. This vicious rock usually collects in a magma chamber beneath a volcano, or solidify underground to form an intrusion. Where it forms beneath a volcano, it can then be injected into cracks in rocks or issue out of volcanoes in eruptions. The temperature of magma ranges between 600 °C and 1600 °C.

Magma is also known to exist on other terrestrial planets in the Solar System (i.e. Mercury, Venus and Mars) as well as certain moons (Earth’s Moon and Jupiter’s moon Io). In addition to stable lava tubes being observed on Mercury, the Moon and Mars, powerful volcanoes have been observed on Io that are capable of sending lava jets 500 km (300 miles) into space.

Igneous rock (aka. "fire rock") is formed from cooled and solidified magma. Credit: geologyclass.org
Igneous rock (aka. “fire rock”) is formed from cooled and solidified lava. Credit: geologyclass.org

Lava:

When magma reaches the surface and erupts from a volcano, it officially becomes lava. There are actually different kinds of lava depending on its thickness or viscosity. Whereas the thinnest lava can flow downhill for many kilometers (thus creating a gentle slope), thicker lavas will pile up around a  volcanic vent and hardly flow at all. The thickest lava doesn’t even flow, and just plugs up the throat of a volcano, which in some cases cause violent explosions.

The term lava is usually used instead of lava flow. This describes a moving outpouring of lava, which occurs when a non-explosive effusive eruption takes place. Once a flow has stopped moving, the lava solidifies to form igneous rock. Although lava can be up to 100,000 times more viscous than water, lava can flow over great distances before cooling and solidifying.

The word “lava” comes from Italian, and is probably derived from the Latin word labes which means “a fall” or “slide”. The first use in connection with a volcanic event was apparently in a short written account by Franscesco Serao, who observed the eruption of Mount Vesuvius between May 14th and June 4th, 1737. Serao described “a flow of fiery lava” as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain.

Such is the difference between magma and lava. It seems that in geology, as in real estate, its all about location!

We have written many articles about volcanoes here at Universe Today. Here’s What is Lava?, What is the Temperature of Lava?, Igneous Rocks: How Are They Formed?, What Are The Different Parts Of A Volcano? and Planet Earth.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

What is the Pacific “Ring of Fire”?

What if someone were to tell you that there’s a region in the world where roughly 90% of the world’s earthquakes occur. What if they were to tell you that this region is also home to over 75% of the world’s active and dormant volcanoes, and all but 3 of the world’s 25 largest eruptions in the last 11,700 years took place here.

Chances are, you’d think twice about buying real-estate there. But strangely enough, hundreds of millions of people live in this area, and some of the most densely-packed cities in the world have been built atop its shaky faults. We are talking about the Pacific Ring of Fire, a geologically and volcanically active region that stretches from one side of the Pacific to the other.

Definition:

Also known as the circum-Pacific belt, the “Ring of Fire” is a 40,000 km (25,000 mile) horseshoe-shaped basin that is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. This ring accounts for 452 volcanoes (active and dormant), stretching from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand – with several active and dormant volcanoes in Antarctica closing the ring.

Tectonic Activity:

The Ring of Fire is the direct result of plate tectonics and the movement and collisions of lithospheric plates. These plates, which constitute the outer layer of the planet, are constantly in motion atop the mantle. Sometimes they collide, pull apart, or slide alongside each other; resulting in convergent boundaries, divergent boundaries, and transform boundaries.

The Pacific Ring of Fire, a string of volcanic regions extending from the South Pacific to South America. Credit: Public Domain
The Pacific Ring of Fire, a string of volcanic regions extending from the South Pacific to South America. Credit: Public Domain

In the case of the former, subduction zones are often the result, where the heavier plate slips under the lighter plate – forming a deep trench. This subduction changes the dense mantle into buoyant magma, which rises through the crust to the Earth’s surface. Over millions of years, this rising magma creates a series of active volcanoes known as a volcanic arc.

These ocean trenches and volcanic arcs run parallel to one another. For instance, the Aleutian Islands in the U.S. state of Alaska run parallel to the Aleutian Trench. Both geographic features continue to form as the Pacific Plate subducts beneath the North American Plate. Meanwhile, the Andes Mountains of South America run parallel to the Peru-Chile Trench, created as the Nazca Plate subducts beneath the South American Plate.

In the case of divergent boundaries, these are formed when tectonic plates pull apart, forming rift valleys on the seafloor. When this happens, magma wells up in the rift as the old crust pulls itself in opposite directions, where it is cooled by seawater to form new crust. This upward movement and eventual cooling of this magma has created high ridges on the ocean floor over millions of years.

The East Pacific Rise is a site of major seafloor spreading in the Ring of Fire, located on the divergent boundary of the Pacific Plate and the Cocos Plate (west of Central America), the Nazca Plate (west of South America), and the Antarctic Plate. The largest known group of volcanoes on Earth is found underwater along the portion of the East Pacific Rise between the coasts of northern Chile and southern Peru.

Transform Plate Boundary
The different type of tectonic plate boundaries. Credit: oceanexplorer.noaa.gov

A transform boundary is formed when tectonic plates slide horizontally and parts get stuck at points of contact. Stress builds in these areas as the rest of the plates continue to move, which causes the rock to break or slip, suddenly lurching the plates forward and causing earthquakes. These areas of breakage or slippage are called faults, and the majority of Earth’s faults can be found along transform boundaries in the Ring of Fire.

The San Andreas Fault, stretching along the central west coast of North America, is one of the most active faults on the Ring of Fire. It lies on the transform boundary between the North American Plate, which is moving south, and the Pacific Plate, which is moving north. Measuring about 1,287 kilometers (800 miles) long and 16 kilometers (10 miles) deep, the fault cuts through the western part of the U.S. state of California.

Plate Boundaries:

The eastern section of the Ring of Fire is the result of the Nazca Plate and the Cocos Plate being subducted beneath the westward moving South American Plate. Meanwhile, the Cocos Plate is being subducted beneath the Caribbean Plate, in Central America. A portion of the Pacific Plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate.

Along the northern portion, the northwestward-moving Pacific plate is being subducted beneath the Aleutian Islands arc. Farther west, the Pacific plate is being subducted along the Kamchatka Peninsula arcs on south past Japan.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

The southern portion is more complex, with a number of smaller tectonic plates in collision with the Pacific plate from the Mariana Islands, the Philippines, Bougainville, Tonga, and New Zealand. This portion excludes Australia, since it lies in the center of its tectonic plate.

Indonesia lies between the Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor. The famous and very active San Andreas Fault zone of California is a transform fault which offsets a portion of the East Pacific Rise under southwestern United States and Mexico.

Volcanic Activity:

Most of the active volcanoes on The Ring of Fire are found on its western edge, from the Kamchatka Peninsula in Russia, through the islands of Japan and Southeast Asia, to New Zealand. Mount Ruapehu in New Zealand is one of the more active volcanoes in the Ring of Fire, with yearly minor eruptions, and major eruptions occurring about every 50 years.

Krakatau, perhaps better known as Krakatoa, is an island volcano in Indonesia. Krakatoa erupts less often than Mount Ruapehu, but much more spectacularly. Beneath Krakatoa, the denser Australian Plate is being subducted beneath the Eurasian Plate. An infamous eruption in 1883 destroyed the entire island, sending volcanic gas, volcanic ash, and rocks as high as 80 kilometers (50 miles) in the air. A new island volcano, Anak Krakatau, has been forming with minor eruptions ever since.

Mount Fuji, Japan
Mount Fuji, Japan, as seen from the ISS. Credit: NASA

Mount Fuji, Japan’s tallest and most famous mountain, is an active volcano in the Ring of Fire. Mount Fuji last erupted in 1707, but recent earthquake activity in eastern Japan may have put the volcano in a “critical state.” Mount Fuji sits at a “triple junction,” where three tectonic plates (the Amur Plate, Okhotsk Plate, and Philippine Plate) interact.

The Ring of Fire’s eastern half also has a number of active volcanic areas, including the Aleutian Islands, the Cascade Mountains in the western U.S., the Trans-Mexican Volcanic Belt, and the Andes Mountains. Mount St. Helens, in the U.S. state of Washington, is an active volcano in the Cascade Mountains.

Below Mount St. Helens, both the Juan de Fuca and Pacific plates are being subducted beneath the North American Plate. Its historic 1980 eruption lasted 9 hours and covered 11 U.S. states with tons of volcanic ash. The eruption caused the deaths of 57 people, over a billion dollars in property damage, and reduced hundreds of square miles to wasteland.

Popocatépetl is one of the most active and dangerous volcanoes in the Ring of Fire, with 15 recorded eruptions since 1519.  The volcano lies on the Trans-Mexican Volcanic Belt, which is the result of the small Cocos Plate subducting beneath the North American Plate. Located close to the urban areas of Mexico City and Puebla, Popocatépetl poses a risk to the more than 20 million people that live close enough to be threatened by a destructive eruption.

Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com
Map of the Earth showing the relation between fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com

Earthquakes:

Scientists have known for some time that the majority of the seismic activity occurs along plate boundaries. Hence why roughly 90% of the world’s earthquakes – which is estimated to be around 500,000 a year, one-fifth of which are detectable – occur around the Pacific Rim, where multiple plate boundaries exist.

As a result, earthquakes are a regular occurrence in places like Japan, Indonesia and New Zealand in Asia and the South Pacific; Alaska, British Columbia, California and Mexico in North America; and El Salvador, Guatemala, Peru and Chile in Central and South America. Where fault lines run beneath the ocean, larger earthquakes in these regions also trigger tsunamis.

The most well-known tsumanis to take place in the Ring of Fire include the 2004 Indian Ocean earthquake and tsunami. This was the most devastating tsunami of its kind in modern times, killing around 230,000 people and laying waste to communities throughout Indonesia, Thailand, and Southern Asia.

In 2010, an earthquake triggered a tsunami which caused 4334 confirmed deaths and devastating several coastal towns in south-central Chile, including the port at Talcahuano. The earthquake also generated a blackout that affected 93 percent of the Chilean population.

In 2011, an earthquake off the Pacific coast of Tohoku led to a tsunami that struck Japan and led to 5,891 deaths, 6,152 injuries, and 2,584 people to be declared missing across twenty prefectures. The tsunami also caused meltdowns at three reactors in the Fukushima Daiichi Nuclear Power Plant complex.

The Ring of Fire is a crucial region for many reasons. It serves as one of the main boundary regions for the tectonic plates of over half of the globe. It also affects the lives of millions if not billions of people who live in these regions. For many of the people who live in the Pacific Ring of Fire, the reality of a volcanic eruption or earthquake is commonplace and a challenge they have come to deal with over time.

At the same time, the volcanic activity has also provided many valuable resources, such as rich farmland and the possibility of tapping geothermal activity for heating and electricity. As always, nature gives with one hand and takes with the other!

If you have enjoyed this article there are several others on Universe Today that you will find interesting. Here is one called 10 Interesting Facts About Volcanoes. There is also a great article about plate tectonics.

You can also find some good resources online. There is a companion site for the PBS program Savage Earth that talks about the Ring of Fire. You can also check out the USGS site to see a detailed map of the Pacific Ring of Fire and more detailed information about plate tectonics.

You can also listen to Astronomy Cast. Episode 141 talks about volcanoes.

What Causes Earthquakes?

False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA

The two main answers to ‘how earthquakes happen’ is: as a result of tectonic plates colliding and volcanic eruption. The shock waves associated with nuclear weapons testing and other man-made explosions. To be considered an earthquake a shock wave has to be of natural origin.

Earthquakes Caused By Tectonic Plates:
The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many earthquakes happen in areas where plates collide or slide past each other. The Elastic Rebound Theory applies to these quakes.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks, or a period of less frequent shocks as the two rock masses temporarily ‘stick’ and become locked together. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses settle into their new positions. Aftershocks cause problems for rescue services because they can bring down buildings that were weakened by the main quake.

Earthquakes Caused By Volcanoes:
Volcanic earthquakes are far less common than tectonic plate related ones. They are triggered by the explosive eruption of a volcano. When a volcano explodes the associated earthquake effects are usually confined to an area 16 to 32 km around its base.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly when it contacts air. This chokes the volcano’s vent and blocks the escape of pressure. The only way a blockage can be removed is by the pressure building up until it literally explodes the blockage outward.

The volcano will explode in the direction of its weakest point, so it is not always upward. Extraordinary levels of pressure can produce an earthquake of considerable magnitude. The shock waves have been known to produce a series of tsunami in some instances.

There you have the answer to ‘how earthquakes happen’. Keep in mind that there have been man-made shock waves following large explosions, but they are not considered earthquakes because of their artificial origin.

We have written many articles about earthquakes for Universe Today. Here’s an article about the biggest earthquake, and here are some pictures of earthquakes.

If you’d like more info on earthquakes, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
http://earthquake.usgs.gov/learn/topics/plate_tectonics/rift_man.php
http://www.geo.mtu.edu/UPSeis/where.html
http://www.geo.mtu.edu/volcanoes/hazards/primer/eq.html
http://news.discovery.com/earth/are-volcanoes-and-earthquakes-related.html

What are Earthquake Fault Lines?

False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA

Every so often, in different regions of the world, the Earth feels the need to release energy in the form of seismic waves. These waves cause a great deal of hazards as the energy is transferred through the tectonic plates and into the Earth’s crust. For those living in an area directly above where two tectonic plates meet, the experience can be quite harrowing!

This area is known as a fault, or a fracture or discontinuity in a volume of rock, across which there is significant displacement. Along the line where the Earth and the fault plane meet, is what is known as a fault line. Understanding where they lie is crucial to our understanding of Earth’s geology, not to mention earthquake preparedness programs.

Definition:

In geology, a fault is a fracture or discontinuity in the planet’s surface, along which movement and displacement takes place. On Earth, they are the result of activity with plate tectonics, the largest of which takes place at the plate boundaries. Energy released by the rapid movement on active faults is what causes most earthquakes in the world today.

The Earth's Tectonic Plates. Credit: msnucleus.org
The Earth’s Tectonic Plates. Credit: msnucleus.org

Since faults do not usually consist of a single, clean fracture, geologists use the term “fault zone” when referring to the area where complex deformation is associated with the fault plane. The two sides of a non-vertical fault are known as the “hanging wall” and “footwall”.

By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. Basically, when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him. This terminology has endured for geological engineers and surveyors.

Mechanisms:

The composition of Earth’s tectonic plates means that they cannot glide past each other easily along fault lines, and instead produce incredible amounts of friction. On occasion, the movement stops, causing stress to build up in rocks until it reaches a threshold. At this point, the accumulated stress is released along the fault line in the form of an earthquake.

When it comes to fault lines and the role they have in earthquakes, three important factors come into play. These are known as the “slip”, “heave” and “throw”. Slip refers to the relative movement of geological features present on either side of the fault plane; in other words, the relative motion of the rock on each side of the fault with respect to the other side.

Transform Plate Boundary
Tectonic Plate Boundaries. Credit:

Heave refers to the measurement of the horizontal/vertical separation, while throw is used to measure the horizontal separation. Slip is the most important characteristic, in that it helps geologists to classify faults.

Types of Faults:

There are three categories or fault types. The first is what is known as a “dip-slip fault”, where the relative movement (or slip) is almost vertical. A perfect example of this is the San Andreas fault, which was responsible for the massive 1906 San Francisco Earthquake.

Second, there are “strike-slip faults”, in which case the slip is approximately horizontal. These are generally found in mid-ocean ridges, such as the Mid-Atlantic Ridge – a 16,000 km long submerged mountain chain occupying the center of the Atlantic Ocean.

Lastly, there are oblique-slip faults which are a combination of the previous two, where both vertical and horizontal slips occur. Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.

Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com
Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com

Impacts of Fault Lines:

For people living in active fault zones, earthquakes are a regular hazard and can play havoc with infrastructure, and can lead to injuries and death. As such, structural engineers must ensure that safeguards are taken when building along fault zones, and factor in the level of fault activity in the region.

This is especially true when building crucial infrastructure, such as pipelines, power plants, damns, hospitals and schools. In coastal regions, engineers must also address whether tectonic activity can lead to tsunami hazards.

For example, in California, new construction is prohibited on or near faults that have been active since the Holocene epoch (the last 11,700 years) or even the Pleistocene epoch (in the past 2.6 million years). Similar safeguards play a role in new construction projects in locations along the Pacific Rim of fire, where many urban centers exist (particularly in Japan).

Various techniques are used to gauge when the last time fault activity took place, such as studying soil and mineral samples, organic and radiocarbon dating.

We have written many articles about the earthquake for Universe Today. Here’s What Causes Earthquakes?, What is an Earthquake?, Plate Boundaries, Famous Earthquakes, and What is the Pacific Ring of Fire?

If you’d like more info on earthquakes, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources: