Planet Earth/6c. Earth’s Volcanoes: When Earth Goes Boom!

Subduction

Diagram of the geological process of subduction.

As points of lithospheric spreading, mid-ocean ridges are the divergent boundaries where new crust is formed. Over long geological intervals, this new crust on the ocean floor pushes continents apart. If new crust is formed from these mid-ocean ridges, there must be other places where crust is equally destroyed or recycled into the interior of the Earth. Although, some crazy theories early in the 1900s proposed that the Earth expanded over time, ever growing bigger, maps of the occurrence of earthquakes and volcanoes revealed other places where Earth’s crust appeared to be destroyed by a process of what is called Subduction. Subduction is the downward movement of a lithosphere plate into the deeper molten asthenosphere, and this downward motion of the lithosphere plate is a result the collision of one lithospheric plate overriding another downwelling lithosphere plate. Subduction was first discovered by two scientists on opposite sides of the Pacific Ocean closely listening to the music that comes from deep within the Earth.

Each orange dot is the depth of an Earthquake in the Lesser Sunda Islands region of the Pacific Ocean, showing deep subduction and movement of lithosphere crust, this is called the Wadati-Benioff Zone.

A piano produces sound by hammers that strike long strings of various length. Each string produces a sound at a specific frequency, which sends vibrations through the gas particles at a matching frequency. These sound waves are heard in the ear, as the particles of gas ripple with the waves of motion that vibrate the ear drum. Geologist Hugo Benioff would listen to the Earth in the same way as he did to music: listening to vibrations coming from the Earth’s deep, and mapping these in three dimensions. Sound waves change with distance, losing energy as they travel. Low frequency sound waves travel farther than high frequency sound waves, and by listening to the Earth, Benioff could map the places that sounds originated from deep in the interior of the Earth. In certain zones, the origin of these sounds is very deep inside Earth extending down below the surface to depths of nearly 670 kilometers. Benioff pin-pointed one of these regions of deep Earthquake producing sounds north of New Zealand below the Islands of Tonga in the South Pacific in the 1940s.

The Pacific Ring of Fire, where subduction results in numerous earthquakes and volcanoes.

Subduction zone features.

Independently, a Japanese researcher named Kiyoo Wadati was also listening to Earth producing sounds, and who also discovered regions of super deep Earthquakes in the Pacific, this time near his home in Japan. Today these regions are known as Wadati–Benioff zones, and they mark areas where the crust is destroyed by the downward movement of one layer of lithosphere sinking below another layer of lithosphere. As these two gigantic layers of brittle rock rub against each other, vibrations from these deep Earthquakes, radiate from these Wadati–Benioff zones in the subsurface. Mapping the depth of Earthquakes marks the boundary between the two plates of lithospheric crust as one layer is subducted or pushed below the other. Such areas exhibit the most dangerous earthquakes and explosive volcanoes. In the Pacific Ocean this ring of earthquakes and volcanoes is known as the Ring of Fire!

Subduction is the downwelling of cold brittle lithosphere below another thick layer of lithosphere. This process subjects the plunging downward lithospheric plate to extreme heat and high temperatures. The downward moving layer melts into molten magma, which is less dense and rises, forming a zone prone to both massive earthquakes and gigantic explosive volcanoes. One place this is manifested is along the northwestern coastline of North America, as a chain of large volcanoes, including the famed Mount St. Helens that tower above the Cascade Mountain Range.

Saint Helens Erupts!

David Johnston just before the Saint Helens eruption in 1980.

Cascade Volcanic arc, sits above the subduction of several Pacific lithospheric plates of oceanic crust, including the Juan de Fuca Plate.

Mount Saint Helens in 1916

On May 17, 1980, four scientists stood on the edge of camp, looking out over a view of an active volcano in the Cascade Range of Washington. The Juan de Fuca Plate is a small lithospheric plate that arises from a divergent plate boundary from a mid-ocean ridge in the ocean off the coast of the Northwest Pacific shore. The plate’s motion is east, where it plunges under the edge of the North American continent in a subduction zone. Above this region, massive volcanoes like Mount Baker, Mount Rainer, Mount Hood, and Mount Adams poke up above the clouds, with snow capped peaks. Together they form the Cascade Mountain Range. One of the most picturesque of these mountains is Mount Saint Helens, which was often referred to as the Mount Fuji of America, because of its once symmetrical shape. In 1980, the mountain was a dormant volcano that last erupted nearly 150 years before in 1842, although the last major eruption was well back in time, around 1482. The four scientists had come to see the volcano and camp on the edge of an evacuation zone. Harry Glicken was the youngest member, only 22 years old, he was an enthusiastic lover of science. Described by those who knew him as socially awkward and obsessive about volcanoes, he was skinny with thick glasses and a rough scraggly beard. He had been camping at the spot for the last two weeks, and was leaving to travel to the University of California, where he hoped to continue his education in graduate school. David Johnston had arrived at the camp to continue monitoring the volcano, in Glicken’s soon to be absence. Johnston was 30 years old, and worked for the United States Geological Survey, originally the team leader Don Swanson was going to stay at the camp, but he had a meeting with a visiting student, so Johnston made the trip up the mountain from Vancouver, to continue observations of the volcano.

David Johnston was interested in the volcanic gasses that are emitted from volcanoes, and was hoping to monitor the eruption from the distant camp, which provided a beautiful view of the volcano’s rise into the sky. The camp was joined by two young female scientists, Mindy Brugman and Carolyn Driedger who had driven up to camp nearby for the night. Mindy Brugman had designed a laser surveying tool that could measure distances. The laser surveying tool had been set up on the ridge at the camp with an unobstructed view to the volcano six miles away, and was collecting data as to the distance to the mountains edge, as it appeared to bulge upward. They had arrived to study its movement, and help if they could in data collection. They were going to camp near by Johnston’s trailer in tents.

The arrival of the scientists was a result of a series of events that started on March 15th. On that day, in 1980 a series of earthquakes were detected by seismographs that had been put into place by the United States Geological Survey (USGS) in 1972, these earthquakes indicated that the volcano might erupt soon. USGS volcanologist David Johnston had started visiting the mountain and began measuring both temperatures and collecting air samples. Helicopters carried him out to remote ledges on the volcano. From these drop off points, he quickly scaled dangerous slopes to carry out his experiments and data collection. On March 27th a plume of volcanic ash and gases erupted nearly 7,000 feet into the air, but several weeks later the eruption bizarrely ceased. The volcano appeared to return to a dormant condition. At the camp, named Coldwater II, Mindy Brugman’s laser surveying tool demonstrated that the mountain was in fact bulging upward and outward, as gas began to build in the subsurface. The laser was reflected off mirrors that had been placed on the side of the mountain by Don Swanson. Astonishingly, the mountain was growing between 5 to 8 feet higher each day, based on their remote laser measurements. That night the weather was eerily clear. Bright stars glittered in the night sky. The camp was an idyllic location to watch the mountain and observe a geological process in action. The four young scientists were all trained geologists, but young, and hence willing to risk their lives for a spectacular view of a once in a lifetime volcanic eruption. David Johnston knew more than the others of the danger they were all in. He believed that the volcanic gasses were building below the rising dome, and the lack of a recent phreatic eruption belied a ticking bomb beneath the mountain. Phreatic eruptions are explosive venting of volcanic gasses and hot steam that are ejected from a volcano during an eruption, these explosive eruptions produce large amounts of ash and pyroclastic rocks high into the sky above a volcano. They are extremely dangerous.

Before and after photographs of Mount Saint Helens.

Mountain Saint Helens today, with its blown top.

What makes volcanoes explosive?

Easy flowing lava, such as this lava flow in Hawaii is a result of the paucity of silica in the molten rock.

Explosive volcanic eruption of the Galunggung volcano in 1982, due to a richer silica content of the molten rock.

Volcanos can be characterized by how much silica (SiO₂) the magma contains. Both Silicon (Si) and Oxygen (O) are lithophile elements that are common in continental rocks, and within the crust and lithosphere on Earth. Silica is the same molecule that is found in glass, and like glass, solid SiO₂ will shatter into tiny pieces at Earth’s surface pressure and temperature. If you have ever broken a glass or ceramic bowl, or cut yourself on a jagged piece of glass in the kitchen, you realize how brittle silica can be. Silica is enriched in the crust above subducting plates in large part because of its melting temperature, and interaction with water (H₂O). As H₂O and SiO₂ are subducted together, the water from the overlaying ocean fills in pore spaces in the rock and is superheated with depth into steam. This steam or very hot water vapor works to lower the melting temperature of surrounding silica rock, resulting in molten liquid and magma chambers at lower temperatures than would be possible in the absence of water. The magma as it rises also melts due to the decreasing pressure, crossing the melting point as the material moves upward through the crust. Magma containing a majority of silica is called Rhyolitic magma. Rhyolitic magma is the most explosive type of magma found in volcanoes. The opposite type of magma is Basaltic magma, which contains less silica, and as such tends to be less viscous; this type of magma will often flow out of volcanoes as slow-moving lava. Lava is molten liquid magma that has come to the surface and flows out of a volcano or volcanic vent, sometimes very quickly, other times very slowly. Basaltic magma, which has low amounts of silica, tends to be less explosive but can cause significant damage as lava encounters buildings and roads, in the form of molten rock, which burns everything that it encounters; wooden houses, steel cars, and concrete buildings.

Silica can react with other elements to form a variety of minerals called silicates. These are predominately quartz in continental crust; one of the most common minerals on Earth’s surface. Volcanologists measure the amount of silica in volcanoes to determine how explosive the volcano would be. Volcanoes with rocks that contain 50% or less silica are known as Basaltic. Basaltic volcanoes are the least explosive, and include many volcanoes that erupt out of basalt-rich ocean crust. Basaltic volcanoes, like those in Hawaii, produce flowing lava. Lava is liquid molten rock at the surface, while magma is the term used for liquid molten rock buried below the surface. Both are incredibly hot, with temperatures between 800° up to 2,000° Celsius (most are between 1,000° to 1,200° Celsius). Runny smooth lava will cool to form pahoehoe. Pahoehoe is basalt rocks that form smooth undulating or ropy masses, due to low viscosity molten lava. Aa (pronounced Ah-Ah) is basalt rocks that form very rough and rugged crumbly masses, due to high viscosity molten lava.

Eruption of Eyjafjallajökull in 2010, in Iceland.

Explosive volcanoes that contain large amounts of silica and water will produce enormous amounts of volcanic ash during eruptions. This is due to the increase in volcanic gasses that are released during an eruption in these types of volcanoes. Pyroclasts are molten rocks and magma that is ejected out of a volcano, often propelled into the air by the buildup of volcanic gasses in the subsurface. Tephra is the name of thick deposits of pyroclastic material that accumulates near volcanoes. Volcanic ash, is composed of tiny pyroclastic particles that are ejected into the sky, and because of their tiny sizes can be blown by winds great distances. The eruption of Eyjafjallajökull in 2010, and Grímsvötn in 2011 both in Iceland produced enormous amounts of volcanic ash that caused the cancellations of thousands of flights in Europe. Such enormous eruptions of volcanic ash can cause a slight global cool down, such as the major eruptions of Tambora in 1815, Krakatau in 1883, and Mount Pinatubo in 1991, all in the southeastern Pacific. These major eruptions resulted in large releases of CO₂ and SO₂, but the darkened sky resulted from the release of large amounts of volcanic ash, which dimmed the amount of sunlight striking Earth’s surface, temporarily cooling the planet by around 1 degree Celsius.

Types of Volcanoes

Volcanoes are any geological feature on the planet that molten material erupts from. This includes submarine volcanoes along the mid-ocean ridges of the ocean floor, as well as volcanoes that erupt on the surface of the land. Volcanoes can also be located under ice sheets in Antarctica, and result in glacier melt at depth. Volcanoes are classified based on their size and shape.

Stratovolcanoes

Mount Fuji in Japan is a stratovolcano.

Stratovolcanoes are volcanoes that have steep conical mountains, composed of layers of tephra or lava, and are the classic volcano shape, with a circular crater at its summit. These volcanoes are supported by a stock of magma that moves upward, with lava that flows out from the crater, or pyroclastic material ejected from the high crater. Mount Fuji in Japan and Mount Saint Helens are examples of this type of classic volcano.

Shield Volcanoes

Mauna Loa, on the Big Island of Hawaii is an example of a shield volcano.

Shield Volcanoes are much larger broadly shaped volcanoes that form a broad dome-shape topography. They are formed by the progressive build-up of flowing lava that cools, and less by the ejection of pyroclastic materials, and as a result will often be less steeply sloped. Mauna Loa, the volcano on the Big Island of Hawaii is an example of a shield volcano.

Tephra Cones

A tephra cone in the Lassen Volcanic National Park, California.

Tephra cones or Cinder cones are smaller volcanoes that form from the ejection of large amounts of pyroclastic tephra that builds up along the volcanic vent forming a pile of material. An example of a tephra cone is the Shit Pot (sometimes referenced as the S P Volcano) located north of Flagstaff, Arizona. These tend to be rather small, and often closely associated with other larger volcanoes. These can form from fissure eruption where a vent opens up within a volcanic field.

Caldera Volcanoes

Crater Lake Oregon is an example of a caldera volcano.

Caldera volcanoes are the largest of active volcanoes, and include enormous areas measuring many kilometers in diameter. They form a large magma chamber that is so large it is unsupported by a volcanic stock, and will sink down, with center craters forming lakes, as they fill with water. Because of their larger size, caldera volcanoes can be the most explosive and dangerous due to the extent they can reach from their centers. Eruptions are less frequent, but there are histories of such mega-eruptions in the past. An example of a caldera volcano is Yellowstone, which sits on top of this super volcano, a smaller caldera volcano is Crater Lake in Oregon.

Flood Basalt Volcanoes

Thick layers of basalt were formed from flood basalts that covered large regions with lava, which cooled into basalt rocks. Moses Coulee, Washington.

Flood Basalt Volcanoes are large volcanoes that flood the surrounding landscape with lava flows that can encompass enormous regions. Evidence of flood basalt volcanoes are found in the widespread layers of basalt that form during these eruptions. The Columbia River Flood Basalts are an example of this type of volcanic eruption pattern, where thick layers of lava flowed over a large province in Oregon.

Large Igneous Provinces

These extremely thick layers of basalt were formed from a massive volcanic eruption 66 million years ago in India. Such ancient massive volcanic eruptions are called large igneous provinces.

Large Igneous Provinces (LIPS) are volcanic eruptions that are of a massive scale, and result in very large eruptions of lava, with magma traveling through extensive dykes and sills. Dykes are vertical magma chambers while sills are horizontal magma chambers. Large Igneous Provinces encapsulate large regions of the Earth (thousands of kilometers), and often are associated with major biological extinctions. These eruptions produced the Deccan Traps in India and Siberian Traps in Russia. The term trap is used because the terrain of basalt rocks they leave behind is very steep and complex.

Where do volcanoes form?

The majority of volcanoes are typically found in regions of subducting lithospheric plates, but not all volcanoes form along these plate boundaries. Many volcanoes, especially submarine volcanoes are found along mid-ocean ridges. Other volcanoes are found in rifting valleys, where the overlaying lithospheric plate is stretched and thinned (sometimes pulled apart and breaking into normal faults), this allows the upward motion of molten magma below these thin regions in the crust of the Earth, while other volcanoes are associated with hot spots.

Hot spot cross section of the formation of the Hawaiian Islands.

Hot spots are a geologically intriguing features which are believed to be caused by a shallow convection of magma deep in the asthenosphere rising upward, or even coming from deeper layers in the Earth from the rising of heat from deep in the lower mantle. This extra heat results in melting the lithospheric plate that passes above. Like a candle held under a piece of paper passing above it, the hot spot will result in scorched line of volcanoes, fading from most active to dormant to extinct, as the lithospheric plate passes over this hot spot. The Hawaii Islands are believed to be formed by a hot spot that exists in the Pacific Ocean, while Yellowstone is also believed to be a result of a hot spot passing under the interior of North America. The idea of hot spots is very contentious, as some geologists suggest that they form not from mantle plumes of magma driven by convection, but by lithospheric plates cracking or breaking apart in these regions. The plume versus plate debate over the origin of hot spots is very active in geology.

Volcanic Hazards

Volcanoes pose major hazards to those that live in close proximity to them. Cities can be consumed by fast moving pyroclastic flows, poisonous gases can kill animals and people when released from volcanic vents, tephra can result in slope failure and major landslides, sweeping villages away, and associated earthquakes can trigger a tsunami. Thousands of human lives have perished due to the volatile nature of volcanoes. Over 23,000 people were killed during an eruption of Nevado del Ruiz in Columbia in 1985, when massive volcanic mud flow (called a lahar), swept away the city of Armero. The 1982 eruption of El Chichón in Mexico covered large areas in volcanic ash and resulted in major local agricultural loss, as well as killing 4,000 people that lived near the volcano. Closely monitored seismographs are important to give warning to evacuate nearby towns and cities, when earthquakes start, volcanic eruptions quickly follow. Drainage ravines from steep volcanoes are also prone to pyroclastic flows and should be abandoned for long term habitation because of the dangers they pose to people living in these dangerous places. Since volcanoes erupt infrequently, often on scales longer than people live, memories of their destruction are short, and often people are falsely lulled into a sense of safety.

The eruption of Mount Saint Helens on May 18, 1980 from a helicopter.

David Johnston knew the danger he was placing himself that night at the camp overlooking Mount Saint Helens. He told Mindy Brugman, Carolyn Driedger and Harry Glicken that they should drive back to Vancouver. They wanted to stay for the night, but realized the concern in his voice when he told them to all leave. They drove down the mountain, as only David Johnston stayed to watch the stars and waited for the mountain to erupt. At 8:32 a.m. on May 18th 1980, the morning sun shone down on his camping trailer as David Johnston suddenly watched the gigantic mountain before him begin to collapse. The bulge on the side of the mountain fell, and volcanic ash shot laterally directly toward his ridge. The entire side of a mountain was ejected upward and outward toward him, crossing the six miles in seconds. He reached for the ham radio and shuttered his final words, “Vancouver! Vancouver! This is it!” His last moments of his life were observed by Gerry Martin, a local Radio Amateur Civil Emergency Service operator who had camped at an observation post higher up. He messaged out “Gentlemen, the camper and car that’s sitting over to the south of me is covered. It’s going to hit me, too.” He was never heard of again. The blast reworked the topography of the terrain as the entire side of the mountain collapsed and was blasted outward toward the ridge where David Johnston was camped. About 300 square miles was utterly destroyed, and 57 people died during the eruption, including both David Johnston and Gerry Martin. The three survivors, Mindy Brugman, Carolyn Driedger and Harry Glicken who had driven back to Vancouver, would all dedicate their lives to studying volcanoes, and warning the public about their dangers.

The Earth is a dynamic planet, one that is being reformed by processes that work on both fast and slow scales of time. The intense heat and pressure locked within Earth’s interior can be unleashed, and result in devastation.

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b. Plate Tectonics: You are a Crazy Man, Alfred Wegener.	c. Earth’s Volcanoes, When Earth Goes Boom!	d. You Can’t Fake an Earthquake: How to Read a Seismograph.