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.

Mountains: How Are They Formed?

When beholding the sheer size and majesty of mountains, ancient humans could not help but feel that they were standing in the presence of something… godlike. And within the belief systems of many ancient cultures, it was generally felt that mountains were something spiritual – either serving as the home of the Gods, a result of their activity, or a place to get closer to God.

Thanks to modern geology, we now know the true story of how mountains are formed. Simply put, they are the result of tectonic forces or volcanism. But knowing this has not diminished their impressive and awe-inspiring nature. When a geological formation is created through forces that can only be described as titanic, this is to be expected. But just how are mountains formed?

In truth, there are three ways in which mountains are formed, which correspond to the types of mountains in question. These are known as volcanic, fold and block mountains. All of these are the result of plate tectonics, where compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features.

Over the course of many million years, these uplifted sections are eroded by the elements – wind, rain, ice and gravity. These gradually wear the surface of the mountains down, cause the surface to be younger than the rocks that form them, and lead to the types of formations and distributions we are familiar with today.

 Matterhorn (4,478 m, Walliser Alps, East side) mirrored in Riffelsee, photograph taken from shore of lake Riffelsee.
The East side of the Matterhorn, a fold mountain that measures 4,478 meters in height, mirrored in lake Riffelsee. Credit: Wikipedia Commons/Dirk Beyer

Volcanic Mountains:

Volcanic mountains are formed when a tectonic plate is pushed beneath another (or above a mid-ocean ridge or hotspot) where magma is forced to the surface. When the magma reaches the surface, it often builds a volcanic mountain, such as s shield volcano or a stratovolcano. Examples of this sort of mountains include Mount Fuji in Japan, Mauna Kea in Hawaii, Nyamuragira in the Democratic Republic of Congo, Skjaldbreiður in Iceland  and Mount Etna in Sicily.

At other times, the rising magma solidifies below the surface and forms dome mountains, where material is pushed up from the force of the build-up beneath it. Examples of this formation include Navajo Mountain in San Juan County, Utah; the Chaitén lava dome of Chile, Torfajökull in Iceland, and Mount St. Helens in Washington State.

Fold Mountains:

As the name suggests, fold mountains occur when two tectonic plates collide at a convergent plate boundary, causing the crust to overthicken. This process forces the less dense crust to float on top of the denser mantle rocks – with material being forced upwards to form hills, plateaus or mountains – while a greater volume of material is forced downward into the mantle.

Satellite image of the Himalayan mountain chain, as imaged by NASA'sLandsat-7 imagery of Himalayas. Credit: NASA
Satellite image of the Himalayan mountain chain, as imaged by NASA’s Landsat-7 satellite. Credit: NASA

The Jura Mountains, a series of sub-parallel mountain ridges located in the Alps, are an example of fold mountains. Other examples include the “Simply Folded Belt” of the Zagros mountains, which extends from northern Syria and southern Turkey to eastern Iran and the Persian Gulf. There is also the Akwapim-Togo ranges in Ghana and the Ridge-and-Valley Appalachians in the Eastern United States.

But perhaps most famous is the Himalayan mountain chain, located between northern India and Nepal. This chain formed as a result of the collision between the Indian subcontinent and Asia some 25 million years ago, and has given rise to the tallest mountain in the world – Mt. Everest.

Block Mountains:

Block mountains are caused by faults in the crust, a seam where rocks can move past each other. Also known as rifting, this process occurs when rocks on one side of a fault rise relative to the other. The uplifted blocks become block mountains (also known as horsts) while  the intervening dropped blocks are known as graben (i.e. depressed regions).

Examples of this type of terrain can be found in the Upper Rhine valley, the Vosges mountains in France, the Black Forest in Germany, and the Vindhya and Satpura horsts in India. There is also the East African Rift, an active continental rift zone with several active volcanoes that extends from Eritrea to Mozambique.

Satellite image of the East African Rift, December 18, 2002. Credit: NASA/GSFC/METI/Japan Space Systems, and U.S./Japan ASTER Science Team
Satellite image of the East African Rift, taken on December 18th, 2002. Credit: NASA/GSFC/METI/Japan Space Systems/U.S.-Japan ASTER Science Team

Mountain Erosion:

As noted, the final way in which mountains are formed is through erosion. This occurs during and after an uplift, where a newly formed mountainous region is subjected to the effects of wind, water, ice, and gravity. These forces actively shape the surface of mountain ranges, wearing down the exposed surfaces, depositing sediment in alluvial flows, and leading to the formation of characteristic landforms.

These include pyramidal peaks, knife-edge arêtes, and bowl-shaped cirques that can contain lakes. Plateau mountains, such as the Catskills, are formed from the erosion of an uplifted plateau. And after millions of years of erosion, mountains may cease to exist entirely.

Given the size and scale of a mountain, the immense forces involved in their creation, and the immense amount of time it takes to shape and form them, it is little wonder why they are considered such a big deal. Between their religious significance (i.e. Mount Zion, Mount Olympus, Mount Ararat, and Mauna Kea, to name a few), their scenic value, the challenge they present, and their importance to the Earth sciences, these geological formations continue to enjoy a special place in our hearts, minds and culture.

As we explore other planets, we have also found new and impressive mountain formations that have taught us much about the geological activity and composition of other worlds. For example, there the volcanic mountain on Mars known as Olympus Mons, which just happens to be the largest mountain in the Solar System. And this is merely a drop in the bucket. Wherever there’s a geologically active planet, there’s mountains to be found!

We have written many articles about mountains here at Universe Today. Here’s one on Fault-Block Mountains, one on Volcanic Mountains, one on Fold Mountains, and one on Dome Mountains. And here’s an article about The Clearest Skies On Earth.

For more information, check out NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

Astronomy Cast also has a great episode on the subject – Episode 51: Earth.

Do You See a Mountain or a Crater in This Picture?

Yesterday, we posted an image taken by the HiRISE camera on the Mars Reconnaissance Orbiter (MRO) of an unusual crater formed by a triple-asteroid. We noticed some comments on the article and on social media of people who said, “hey, that looks like a mountain, not a crater!”

Thanks to our brains, this is a common illusion! Depending on the angle of the Sun when the picture was taken, images of craters taken from overhead (i.e. from orbit) may appear to be a mountain. Here on Earth, we’re used to seeing sunlight coming from overhead, and our brain interprets what we see with the assumption that the sunlight *must* always come from above. Satellite photos of terrain, however usually only show shadows when the light source is nearly horizontal with the surface.

There’s an easy fix for this illusion: flip the image over so it appears the sunlight is coming from above. We’ve done that for you, below:

The same 1-km fresh crater on Mars, but with the image upside down. Credit: NASA/JPL/University of Arizona.
The same 1-km fresh crater on Mars, but with the image upside down. Credit: NASA/JPL/University of Arizona.

Dr. Richard Barrans from Newman University in Kansas explains this crater/mountain illusion:

When a crater is illuminated from above, the shadow inside the crater is near the top and the bright part is near the bottom. A mountain shows exactly the opposite lighting pattern: bright at the top and shadowed at the bottom. This is because the part of the crater or mountain that is most perpendicular to the incoming light reflects it the best. Anyway, your brain interprets pictures under the assumption that the light is from the top of the scene. So if a picture shows an object that is bright at the top and dark at the bottom, it interprets it as a mound lit from above, not as a crater lit from below.

We do know that any photo of anything that is concave or convex will flip in its appearance if the photo is turned upside down. Take a look at any of these crater images from HiRISE, and you’ll likely be deceived on most of them! (remember, they are ALL craters!)

And here’s the image from yesterday:

A triple crater in Elysium Planitia on Mars. Credit: NASA/JPL/University of Arizona.
A triple crater in Elysium Planitia on Mars. Credit: NASA/JPL/University of Arizona.

You can now amaze your friends. You’re welcome.