I. Introduction-

The composition of magma is very important for the type of eruption

- Physical properties of magma (viscosity) controls eruptive style.

Many different types of eruptions and volcanoes

- controlled by type of magma.

II. Volcanic deposits

A. Lava flows

- since lava is a liquid, it will flow down hill. Flow paths can be as long as 50 km.

- Can be divided into two major types depending composition- mafic (basaltic) and felsic material

Basaltic flows- on land:

- Two catagories of volcanic material is created by freezing basalt lava flows:

Pahoehoe (Hawaiian language for "ropey"- describes texture of material.

Aa (exclamation made when walking on sharp glass edges).

- Sequence of events:

pahoehoe erupted first, it becomes more viscous and loses gasses

forms a solid "skin" as it freezes evolves into Aa lava- very sharp, glassy material.

Basalt flow under water- pillow basalts

- Feature caused by submarine eruptions- (very common at mid-ocean ridges). The deposit looks like pillows- hence the name.

B. Pyroclastic Deposits

Large controlled by water and gases in the magma, also by higher silica content

- most common when confining pressures keep them from escaping.

- common in rhyolitic and andesitic lavas

- cause explosive volcanoes (e.g. Mt. St. Helens) when they move upward to near the surface where pressure is low.

- Deposits that are formed include ash, tuffs, breccias.

- Also include pyroclastic flows:

result when hot gases bouy up solid ash particles

The misture of gases and material at > 800°C can travel up to 160 km/hr.

III. Eruption styles and the types of volcanoes they make:

(1) Central eruptions-

discharge lava or pyroclasitic material from a central vent.

Forms cones- the most well-known type of volcano shape.

Lava eruptions- when volcano built up by lava flowing from central vent. Several types:

- Shield volcano-

formed from basaltic magma

Low viscosity so flows easily across surfaces, thus tend to form very los gradient slopes of 2 to 10°- look like a shield.

Mauno Loa on Hawaii is best example- 10 km from seafloor to top, 120 km in diameter at base.

- Volcanic dome-

formed from felsic lavas

small steep sided features.

domes commonly plug the vent, trap gas, eventually causing explosion such as at Mt. St. Helens

- Cinder cone- from pyroclastic eruptions:

Small, generally less than 500 m high piles of pyroclastic material with very steep slopes.

Their slopes controlled by "angle of repose" the steepest slope sustainable by the material- may be 30° or more.

Short lived features because they are quickly eroded.

- Stratovolcano- from composite eruption

mixtures of shield volcanoes and cinder cones- both lava and pyroclastics.

May have small cinder cones that cover their flanks.

May be many flank eruptions of lava.

Other features of volcanoes:

Craters: Thse occur at the summit of most volcanoes. They are caused by material that is blasted out of the vent of the volcano, or when the material sinks back into the vent.

Calderas: These are larger features that form after the violent ejection of large amounts of magma from a magma chamber. These features can be several km to up to 50 km across (for example at Taupo in NZ).

An exmple of a caldera is "Crater Lake" in Oregon.

- Other features

Craters- small bowl shaped pit a the summit of volcanoes,

Calderas- similar to craters, but much larger, result from catastrophic eruptions

Mechanism forming craters and claderas collapse into voided magma chamber, not that overlying material is blown away.

Phreatic (steam) explosions- caused when water (sea or rain) mix into magma chambers- e.g. Krakatoa.

(2) Fissure eruptions-

Originate from linear cracks (fractures) in the surface.

On continents they typically form what is known as "flood basalts" or Plateau Basalts.

Can form very large deposits covering 1000's of km2

Examples include the Columbia river basalts (Washington state) and the Deccan traps (India).

Very common type is the mid-ocean ridge- a "volcano" that is 10,000 km long

Note: I did not get to these "other phenomenon" or the relationship between plate tectonics and volcanism in class. I will go over the plate tectonics when we do plate tectonics at the end of the semester.

(3) Other phenomena

Lahars

- These are essentially mud slides that ocur when pyroclastic debris gets wet from melting glaciers, rivers, rain

- Can be very devastating: cover large areas, destroy towns, kill many people at one shot.

Volcanic gases-

- largely water vapor, but other things including CO2 and sulfur

- The ejection of the gases is thought to form the atmosphere and oceans

- Still important for climate- observations of decreases in global average temperature caused by eruption of volcanoes.

Hot springs and geysers

- Reflect late stage of volcanic eruption

- caused by water circulating in to ground, becomes heated to above boiling point (forms steam), increase pressure, eventually escape upward

- Hot springs when water slowly comes to surface

- geysers when water ejected upward into atmosphere (e.g. Old Faithful).

IV. Volcanism and Plate Tectonics

"Ring of Fire" the rim of the Pacific Ocean, surrounded by volcanoes

- Now realize that these volcanoes correspond to plate boundaries

- must be some link between volcanoes and tectonics

Ocean Ridge volcanism

- Mid-ocean ridges places where plates are pulled apart

- mantle beneath ridges melt (reduce pressure) and the more bouyant molten material moves to the surface

- Iceland- very good example where this can be seen. Iceland is being pulled apart as North America plate moves away from Eurasian plate.

Covergence zone volcanism

- The upper mantle melts as the plate subducts.

- Can be two types:

ocean-ocean convergence form "island arc" volcanoes, for example Philippine Islands and Lesser Antilles (e.g. Martinique)

Ocean-continent convergence form when there is subduction under a continent, for example Andes mountains and Cascade Range in Oregon and Washington.

Intraplate volcanism

- Places where there is volcanism in the middle of a plate, e.g. Hawaii

- Formed by Hot spot volcanism- volcanoes that are fixed in the mantle (i.e don't move relative to the mantle)

- As the plate moves over the hot spot, a chain of extinct volcanoes is left behind- called an aseismic ridge- because there are no earthquakes.

- Provide a very useful way of calculating the rate of motion of the plates by dating the volcanoes on the aseismic ridge.

Slides:

(1) Pre-eruption Mt. St. Helens

(2) Diagram of eruption of Mt. St. Helens- watr enters side of mountain, heated, caused landslide, reduce confining pressure, magma erupted

(3) Eruption of Mt. St. Helens

(4) Volcanic dome building in crater of Mt. St. Helens

(5) Basaltic lava flow on Hawaii, contrast style of eruption.

(6) Example of fissure eruption

(7) Basalt layers in the Columbia flood basalts

(8) Crater Lake, Oregon

(9) Model of formation of crater lake

(10) Pyroclastic flow

(11) Pahoehoe and Aa on Hawaii

(12) Submarine pillow basalts

(13) subaerial basals on Iceland