Volcanoes

Volcanoes : 

Volcanoes And other igneous activity

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions May 18, 1980 The largest volcanic eruption to occur in North America occurred Mt. St. Helens erupted The blast blew out the entire north flank of the volcano, leaving a gaping hole The ash cloud was so thick that it turned noon day as dark as midnight in towns as far away as 80 miles

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Mt. St. Helens – Before and After

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions So, why are some volcanoes violent, while others such as Mt. Kilauea in Hawaii relatively quiet? The primary factors that determine whether a volcano erupts violently or quietly include Magma composition Magma temperature Amount of dissolved gases in the magma

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Magma Composition Viscosity is a substance’s resistance to flow The more viscous the magma, the more likely the eruption will be violent Thinner (less viscous) magmas are more likely to flow out quietly

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Magma Temperature Temperature affects viscosity The hotter the magma, the thinner the magma gets (Think of pancake syrup) So, the mobility of magma is affected by the temperature of the magma

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Chemical composition What the magma is composed of is also important Viscosity is directly related to silica content in the magma The greater the silica content, the more viscous the magma becomes Rhyolitic lavas are very viscous Basaltic lavas tend be more fluid

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Dissolved gases A vent is an opening in a volcano During explosive eruptions, gases trapped in magma provide the force to eject molten rock The gases are mostly water vapor and CO2

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Gases, magma, and explosive force Basaltic lavas (low viscosity) allow gases to escape easily, so eruptions are relatively quiet Hawaiian volcanoes High viscosity lavas slow the upward movement of gases The gases collect in bubbles and pockets that increase in size until they explosively eject molten rock Mt. St. Helens

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Volcanic Material Depending on the make up of the volcano, different substances can be ejected Lava Gases Pyroclastic materials

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Lava Hot basaltic lavas are very fluid and so form lava with smooth skin that wrinkles as the still molten subsurface lava continues to move Called Pahoehoe lava Resembles twisted braids in ropes Basaltic lava can also form a surface with rough, jagged blocks covered in dangerously sharp edges and spiny projections Called Aa lava

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Pahoehoe lava

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Aa lava

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Gases Although the amount of gas in magma is small by percentage (1% - 6% of total weight), the quantity is still huge Thousands of tons may be emitted each day Gases include: water vapor, CO2, nitrogen, sulfur, chlorine, hydrogen, and argon Sometimes gases from volcanoes are extremely deadly and fast, falling down the sides of a volcano at over 100 mph.

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Pyroclastic material Particles produced in volcanic eruptions Dissolved gases propel blobs of lava and pulverized rock to great heights. The fragments range in size from very fine dust to pieces that weigh several tons

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Anatomy of a volcano Often begins with just a fissure or crack in th ground As material flows out of the crack, the volcano begins to grow and take shape At the summit of many volcanoes is a steep walled depression called a crater

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions A Volcano is a mountain formed by lava and/or pyroclastic material The three main type of volcanoes are Shield volcanoes Cinder cones Composite craters

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Shield volcano Produced by the accumulation of fluid basaltic lava Have the shape of a broad, slightly domed structure that resembles a warrior’s shield Example is Mt. Kilauea in Hawaii

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Cinder Cone Volcano Built by ejected lava fragments that harden in the air They have a very simple shape They are normally steep sided Normally form from a single eruption that lasts a few weeks (rarely more than a few years) Lava then plugs the vent and the volcano never erupts again Example is Sunset Crater in Arizona

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Composite Cone Volcano Large, nearly symmetrical structure composed of layers of lava and pyroclastic deposits Beautiful, and dangerous Full of magma rich in gases and silica Most are found along the Pacific in the Ring of Fire Mt. St. Helens and Mt. Rainier in WA, Mt. Fuji in Japan, and Mt. Shasta in CA are examples

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Comparing the three types of volcanoes

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Other volcanic landforms In addition to building a mountain, the activity of volcanoes can also create Calderas Necks and Pipes Lava Plateaus

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Calderas A large depression in a volcano Created in one of two ways The collapse of the top of a composite volcano after an explosive eruption The collapse of the top of a shield volcano after the magma chamber is drained

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Sequence of events that formed Crater Lake, OR

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Calderas Crater Lake, OR is a filled caldera formed about 7,000 years ago when a composite cone violently erupted and collapsed

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Ngorongoro Crater is the world’s larges unfilled crater. Located in Tanzania 14 miles across at its widest point 2,000 feet deep

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Other Views of Ngorongoro

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Necks and Pipes Most volcanoes are fed magma through conduits, called pipes, connecting a magma chamber to the surface Since lava is considerably harder and more tightly bound than loose cinders, cinder cones often reveal their necks and pipes after they erode away A great example is Shiprock in NM

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Shiprock, NM

The Nature of Volcanic Eruptions : 

The Nature of Volcanic Eruptions Lava Plateaus Created when, rather than building a cone, basaltic lava flows from fissures across the land Covers wide area The extensive Columbia River Plateau in NW USA was formed this way

Intrusive Igneous Activity : 

Intrusive Igneous Activity Although volcanic eruptions are the most violent and spectacular events in nature, most magma cools deep in the Earth Intrusive igneous structures that are created deep in the Earth are called Plutons and form familiar features such as Sill Laccolith Dike Batholith

Intrusive Igneous Activity : 

Intrusive Igneous Activity Pluton Are generally classified according to their Shape Size Relationship to surrounding rock layers

Intrusive Igneous Activity : 

Intrusive Igneous Activity Sills and Laccoliths Plutons that form when magma is intruded close to the surface Sill forms when magma is injected along sedimentary bedding surfaces parallel to bed planes To form, the overlying sedimentary rock must be lifted to a height equal to the thickness of the sill They only form at shallow depths They look like buried lava flows Horizontal sills are the most common

Intrusive Igneous Activity : 

Intrusive Igneous Activity Plutons Sills and Laccoliths Laccoliths are similar to sills, but the magma that formed them was more viscous Form as a lens-shaped mass that pushes the overlying sedimentary layers upward Usually not much wider that a few kilometers

Intrusive Igneous Activity : 

Intrusive Igneous Activity Sills or Laccoliths? Located in Yellowstone National Park

Intrusive Igneous Activity : 

Intrusive Igneous Activity Dikes Magma is injected into fractures and cut across preexisting rock layers Many form when magma from a large magma chamber invades fractures in the surrounding rock

Intrusive Igneous Activity : 

Intrusive Igneous Activity Batholiths Largest of the intrusive igneous bodies Very thick, often extending dozens of miles into the crust It has a surface exposure greater than 39 square mi Example: Sierra Nevada mountains (including Yosemite National Park)

Intrusive Igneous Activity : 

Intrusive Igneous Activity Types of igneous plutons and their formations

Intrusive Igneous Activity : 

Intrusive Igneous Activity Origin of Magma Magma originates when solid rock, located in the crust and upper mantle, partially melts Three things are influential in the melting of rock Heat Pressure Water

Intrusive Igneous Activity : 

Intrusive Igneous Activity The role of heat The deeper in the Earth a rock is, or a rock travels to, the hotter it gets This change in temperature with increasing depth is known as the geothermal gradient

Intrusive Igneous Activity : 

Intrusive Igneous Activity The role of pressure If the confining pressure on a rock is increased, it increases the temperature at which the rock melts But, if the confining pressure is reduced, then the temperature at which a rock will melt is a lot lower than normal The melting of rock when confining pressure drops is known as decompression melting

Intrusive Igneous Activity : 

Intrusive Igneous Activity The role of water Water content in the rocks also affects melting Water causes rocks to melt at a lower temperature So, “wet” rock buried at depth has a much lower melting temperature than “dry” rock

Intrusive Igneous Activity : 

Intrusive Igneous Activity Magma formation summary Magma can be formed in three ways Heat may be added A decrease in the confining pressure Rocks may be wet or water added