Interesting Facts about volcanoes

The Earth's crust is a thin layer of solid rock forming an outer shell, which is a few dozen kilometres thick. (It's thickness is comparable to the skin on an apple). Under the crust is a deep layer of hot soft rock called the mantle, which gets as hot as 4500 deg C. Below that is the much hotter core of hot liquid iron and nickel, reaching temperatures of 6000 to 7000 deg C. When this molten rock can find a way to the surface through the crust, it erupts into the air as magma, forming a volcano. There are 1500 volcanoes worldwide; 500 of them are active. About 60 major volcanic eruptions occur every year; two or three are usually huge and violent ones.

Before you can understand how volcanoes happen, you need to know about  plate tectonics. The Earth's crust is actually a collection of solid pieces which are floating on the soft hot mantle below. (Picture an ice cream cone that has been dipped into chocolate sauce; the chocolate solidifies on the surface, but is easily broken into many pieces that can slide around on the surface of the ice cream). The crust pieces are floating on the mantle, and can move. There are about 12 large pieces, and all of the plates are moving relative to all the others.

Deep in the center of the oceans, molten rock is escaping through huge fissures in the crust, right at the boundary of several plates. This upwelling of new material causes the plates making up the ocean floor to continually spread apart. 

But the plates are solid. As they are forced to spread apart in the ocean's center, their other sides bump into other plates. It's what happens when two plates bump into each other that causes earthquakes and volcanoes.

As the ends of the plates meet each other, what usually happens is that one of them is driven down into the mantle, while the other rides up over it. 

In the diagram at the left you can see the upwelling molten rock in mid-ocean forcing the plates that form the ocean bottom apart.

At the left side of the diagram, you can see the end of a plate meeting a second ocean-bottom plate. It is forced downward; this is calledsubduction. At the same time, the plate on the left rides up, creating mountains which form islands above the water's surface; magma that escapes also causes volcanoes.

At the right, the same thing happens when the ocean plate meets a continental plate; mountains are thrust up. Some of them may be volcanic. 

At places where two plates meet on land, (on the far right, above) magma can escape; its upwelling will also cause these plates to spread apart. Instead of an ocean floor trench, you get a rift valley.

Where magma comes up all along the line between plates, the plates will spread apart. Where moving plates collide, one is forced downward and the other rides up. All of this activity can cause magma to escape to the surface; volcanoes are the result. 

Where two plates don't meet head-on, but are moving by each other in opposite directions, the plates will occasionally get stuck together. Eventually, as the constant pressure from the other end of the plates causes them to unstick, the sudden jolt as they are set free causes an earthquake. A well-known region where plates are passing by each other like this is in southern California, where earthquakes from the sticking and slipping plates are common. 

Subduction zones, where one plate is being forced downward into the hot mantle, are also the cause of deep ocean trenches and large earthquakes. But subducting plates don't descend smoothly; they do it in big and little jumps. When there is an earthquake under the ocean, it is because the descending side of the ocean floor suddenly drops downward. The resulting vertical fault will generate a tsunami, or harbor wave, in the ocean above, often with disastrous consequences.

The map at the right shows the so-called 'Ring of Fire', where edges of ocean plates meet the edges of continental plates. As the plates meet and descend into the mantle, earthquakes and volcanoes occur. 

Each red triangle on the map represents a volcano; some of the most violent volcanoes ever to erupt happened somewhere along this ring.

In 1883, the region just north of Australia saw one of the most catastrophic volcanic eruptions in recent human history, when the island of Krakatoa all but vanished in a colossal explosion. We'll tell you more about that later.

Island arcs are also caused by escaping molten rock. An example is the Aleutian Island chain off Alaska, the long chain of volcanic islands at the top of the map. 

5% of volcanoes are not near the edges of plates. They are over 'hot spots'in the Earth's crust where hot currents from the core have risen up through the mantle to burn a hole through the crust. The Hawaiian Islands, part of a chain of old volcanoes 6000 km long, were formed as the crustal plate they sit on moved slowly over a hot spot, forming volcanic islands one after the other as the plate moved over the hot spot. Hot spots can also create geysers and hot springs by heating water in the crust. 

Summary:

• The process of driving an ocean plate down into the Earth's interior creates deep ocean trenches at the point of collision. For example, when the Pacific Ocean plate encounters the Philippine Islands plate, it subducts below and results in the formation of a deep ocean trench called the Marianas Trench, 11 kilometres below sea level - the deepest known region of the world's oceans.
  As a plate bends downward, it cracks, creating earthquakes. Volcanoes formed over subduction zones are usually very explosive, as the magma gets contaminated as it burns its way up through the crust.
• Where two continental plates collide, the lighter crustal rock (not dense enough to sink) gets scraped off the plate that is descending, and is pushed up to form mountain chains.
• Mid-ocean ridges formed where plates are moving apart create deep trenches in the ocean floor, with steep edges or ridges. These ridges split apart as the Earth's surface curves; where split ridges rub against each other, undersea earthquakes occur. Molten rock oozes out of the floor of the trench into the ocean, where it solidifies into blobs called pillow lava. The sea floor spreads by 1-20 cm per year.

The crust of the Earth is constantly being destroyed (melted into the mantle below) as plates collide; at the same time, new crust is being formed in other trenches between plates where upwelling lava pushes the plates apart. 

When a volcano erupts, the magma moves upward (it's less dense than the rock above it), propelled by the pressure of gases within it. As the magma nears the surface, the pressure falls, allowing the gases within it (mostly carbon dioxide and water vapour) to boil out, pushing molten rock out of the vent.

If the level of magma in the magma chamber below the volcano drops, the top of the volcano's cone may collapse into it, forming a crater called a caldera. This too may collapse and cool, and fill with water, forming a crater lake at the top of the old volcano. 

Types of Volcanoes

Shield Volcanoes

Shield volcanoes are the largest volcanoes on Earth in terms of land area covered; in northern California and Oregon, many shield volcanoes have diameters of 6 kilometres. They are built almost entirely of fluid lava flows that build up layer after layer. Shield volcanoes almost always emit very fluid or runny lava that spreads over a wide distance, so these volcanoes are not steep; from the side they take on the shape of a warrior's shield, hence their name. However, shield volcanoes produce lots of lava. Eruptions are only explosive if water somehow gets into the lava. Often they produce fountains of lava that form cinder cones. Shield volcanoes can be created over hotspots, but can also be found where subduction occurs.

The Hawaiian Islands are the most famous examples of shield volcanoes. Mauna Loa (at the right) is also the world's largest active volcano. 

Composite (Strato) Volcanoes

Composite volcanoes are the most common kind of volcano, and often rise to several kilometres above ground level. They produce very thick lava. Because the lava is thick, gases can build up in it; when the pressure gets high enough, they cause an explosion, ejecting lava and other material. Pyroclasts are pieces of solid lava blasted into the air by the eruption. Volcanic bombs are blobs of molten magma thrown out by the volcano that cool and harden in flight. Much of the material ejected by explosive volcanoes is ash and pyroclasts or bombs; the rest is lava, which either flows through breaks in the crater wall or from fissures on the sides of the cone.

Strato volcanoes are usually found where a plate is subducting, and the magma produced by strato volcanoes is less than that of shield volcanoes. Examples of strato volcanoes include Mt. St. Helens in Washington, US, and Mt. Fuji in Japan. Strato volcanoes have caused more casualties than other types of volcanoes, probably because there are more of them, and also because they form steep piles of ash and lava, where landslides and mudslides can occur frequently. 

Mt. St. Helens' eruption put so much ash into the upper atmosphere that it darkened the skies in Alberta

Cinder Cones Cinder cones are the simplest kind of volcano. They are cones made from blobs of lava that are blown into the air, breaking apart into small fragments that cool and fall as cinders around the vent to form a circular cone. Most cinder cones are less than 300 metres or so above their surroundings.

Lava Domes Lava domes are formed by relatively small masses of lava that are too thick to flow any great distance. As a result, the lava piles up around its vent. A dome grows by expansion from within. As it grows, its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Volcanic domes usually occur within the craters or on the sides of large composite volcanoes.

Types of Eruptions

• Fissure volcanoes release lava from cracks in the ground, which often shoots into the air as lava fountains.
• Composite volcanoes are cone shaped, building up in layers after repeated eruptions.
• Cinder cones build up from ash, where the volcano releases little magma.
• Lava domes build up in one spot when thick lava cools and cracks as more lava pushes out from within.
• Strombolian eruptions release sticky magma in huge clots.
• Vulcanian eruptions are explosive eruptions of sticky magma that clots in the vents, suddenly exploding out of the volcano along with ash clouds.
• Plinian eruptions (named after the Roman Pliny who witnessed and wrote about the eruption of Vesuvius in AD 79) are the most explosive kind of eruption. Boiling gases (from below-ground water that has been superheated by the magma) eject huge clouds of ash and volcanic fragments.

Interesting Volcano Facts: 

- Earth's atmosphere was first formed by gases pouring out of volcanoes 4 billion years ago.These gases were changed as rocks and seawater absorbed carbon dioxide, and as algae in the oceans produced oxygen.

- Mars' volcano Olympus Mons is the biggest in the solar system ... three times higher than Mt. Everest!