The study of volcanism

• Hard to study when half of the action occurs underground and out of site
• Often the lava is too hot to approach safely
• Eruptions can be unsafe due to not knowing when it is going to blow!
• Even though, several facts are known….

Facts about Volcanoes

• Molten rock underground has been identified as magma
• Magma forms wherever temps and pressures are high enough to melt rock
• Both the asthenosphere and the plate movement can cause rock material to liquefy
• When magma rises due to extreme heat, it is known to form a Volcano on the surface of earth
• Magma helps to identify what type of eruption occurs…
• The rate at which magma moves is determined by its silica content.
• Magmas with high silica are slow moving and typically light in color: called Felsic magma.
• Magmas with low silica are fast moving and typically dark in color: called Mafic magma.

Magmas cause differing eruptions:

• All magmas have a certain amount of dissolved gases in them from forming underground
• The primary gases include carbon dioxide, water vapor, and sulfur
• The amount of gas found in a certain magma sample will influence the kind of eruption that occurs
• Thick, felsic magmas will usually trap more gases and will thus create a very explosive eruption!
• Thinner, mafic magmas will not always have an excess of gas and will not produce such an explosive eruption
• Less explosive eruptions (mafic magma) actually produce more fluid-like, high quantity lava flows (ie - Hawaii…not much boom, buts lots of lava!)

Lava and its characteristics:

• Magma that reaches the surface is called Lava
• Lava (just like magma) is influenced by the amount of silica in the liquid rock
• Lava flows often have different appearances as well:

1. Aa lava is characterized by rough, jagged surfaces
2. Pahoehoe lava looks much smoother and more rope-like in appearance

Lava Fragments

• Explosive eruptions produce solid fragments of lava called Tephra
• Tephra is very dangerous because it is erupted from the volcano and flies through the air at high speeds and very high heat!
• The smallest pieces of tephra are called Ash
• Larger pieces are called Lapilli
• The largest pieces are called either Blocks or Bombs. Blocks are solid rock fragments and bombs are liquid blocks of molten rock.
• Tephra often mixes with gases from the eruption to form a dense, super-heated, high-speed cloud called a Pyroclastic Cloud.
• A Pyroclastic Cloud hit the Caribbean in 1902 (Mt. Pelee) in which 30,000 people were instantly smothered or burned to death by the cloud!

Different Kinds of Eruptions

• Rift eruptions occur at long, narrow fractures in the crust
• Rift eruptions typically flow smoothly and are not very gas rich
• Rift eruptions will often produce Shield Cone volcanoes…very broad based and gentle sloping sides
• Rift eruptions occur at spreading centers such as the mid-Atlantic ridge. Lava's often ooze out and cool rapidly forming Pillow Lavas.
• Rift eruptions that occur on land will often flow out smoothly and form Basalt Plateaus (named Basalt after the type of rock that typically forms). Major continental growth is the result of these plateaus

• Subduction Boundary eruptions are the result of magma that forms at subduction boundaries
• Magmas tend to be thicker at the subduction boundary eruption and contain much more gases making for very explosive eruptions
• Volcanic activity around subduction boundaries usually forms a Cinder Cone volcano which is large and steep-sided
• Most of the world's active volcanoes occur at subduction boundary eruptions…such as the entire perimeter around the Pacific plate.
• Hot Spot eruptions are areas where volcanic activity forms in the middle of the lithospheric plate
• Lava erupted at hot spots is usually very fluid-like and erupts with high quantity, not high explosion
• Hot spot eruptions usually form shield cone volcanoes.
• The Hawaiian islands are a perfect example of a hot spot eruption
• Hot spot eruptions often form chain islands (ie - Hawaii)
• The actual island of Hawaii is above the hot spot right now. The other islands of Kauai, Oahu, Molokai, and Maui are old islands as they have all moved past the hot spot and are now longer active

Examples of Volcanic Eruptions

Eldfell is a volcanic shield cone on the island of Iceland

• Formed in 1973 on the upper mid-Atlantic ridge - so it is the perfect Rift eruption
• Tephra covered nearly half of the island
• Lava flow threatened to seal off the only port entry into the village of Vestmannaeyjar, which could have cut off the major source of income on the island - fishing!
• People used fire hoses to squirt millions of gallons of water on the lava to quickly cool it and prevent it from sealing off the entrance into the harbor

Mount St. Helens is a cinder cone volcano in the state of Washington

• It is one of 15 volcanoes in the Cascade Range
• Its last eruption was May 1980
• Interestingly, this volcano began to awake in March of 1980 with minor earthquake activity and noticeable cracking, etc.
• Mudflows formed from melting snow; the mountain bulged and turned into a landslide; and a giant pyroclastic cloud formed and engulfed the entire area!
• Blew trees down 20 miles away; rattled windows 120 miles away; cloud of ash was shot up over 15 miles high!
• Mount St. Helens is the result of a subduction boundary eruption
• Like other subduction boundary eruptions, this one produced major damage from the explosion of gas and ash, but very little lava/magma appeared.

Kilauea is a shield cone volcano on the island of Hawaii

• Kilauea erupts nearly once a year since 1952
• Small amounts of lava erupt almost daily
• Kilauea is the result of a Hot Spot eruption; the plate there is thought to only be about 50km thick rather than the normal 100km
• The lava in this volcano is generally very mafic - very fluid-like
• It often seeks the lowest spots to run into and settle on the gentle mountain side
• These lava lakes then take up to 25 years to fully cool! This allows a great amount of time to study the lava/magma as it cools
• Because the volcano is "low tempered" and the top of the island is sparsely populated, little damage or danger occurs when the volcano decides to blow

Other notable eruptions:

• Most famous eruptions are from subduction boundary eruptions because they are associated with dramatic and violent booms!
• Mt. Vesuvius of Italy erupted in AD 79 and wiped out three cities! Pompeii, the largest city, was buried in ash and over 30,000 people died!
• Mt. Krakatau of Indonesia (south Pacific) erupted in 1883 and destroyed over half the island. Considered the largest eruption of recorded history. 36,000 people drowned by the waves that were created from the explosion! The sound of the boom could be heard over 3000km away!
• Mt. Pinatubo in the Philippines erupted in 1991 and its ash is said to have been found EVERYWHERE on earth!
• The largest volcanic cone in the solar system is called Olympus Mons of Mars. The cone is 26km high and nearly 600km across!!!
• Io, a moon of Jupiter, also has nearly 100 volcanoes on its surface, telling us that it, too, must have plate tectonics similar to earth.

Plutonic Activity

• Volcanoes and lava flows are the activities on the surface of volcanism
• What goes on underground?
• Magma is constantly forcing its way into layers of rock and anywhere it can find a weakness in the lithosphere
• The rock masses that form when magma cools inside rocks are called igneous intrusions or simply Plutons
• Dikes are sheets of igneous rock that cut across the rock layers they intrude - typically vertical intrusions
• Sills are sheets of igneous rock that are parallel to the layers they intrude - typically horizontal intrusions
• Laccoliths form much like sills, but occur when the pressure builds in the intrusion causing a swelling of the rock layers
• The last intrusion is the Volcanic Neck, which forms when the ancient volcano begins to erode away, leaving behind the super-hard volcanic neck
• Volcanic necks are very common in prehistoric regions such as South Africa
• Volcanic necks produce diamonds!!!
• Batholiths are the largest of all igneous intrusions. They are really enlarged laccoliths that form the base for most mountain ranges.