VOLCANOESGrade 9, Module 3 Lesson 1.1

(Teachers’ Guide for Discussion)

Author:MA. RACHEL B. ESPINO

Teacher I, ScienceBuhatan NHS, Sorsogon City

What is a volcano?

• A volcano is a mountain where lava (hot, liquid rock) comes from a magma chamber under the ground. A volcano usually has a summit, a slope and base.

• Most volcanoes have a volcanic crater at the top. When they are active, materials pour out of it. This includes lava, steam, gaseous compounds of sulphur, ash and broken rock pieces.

• Volcanoes erupt when magma and pressure come together, and the pressure blows off the top of the solid rock, and the magma pours out.

Types of Volcanoes: SHIELD VOLCANOES

• Shield volcanoes are built out of layers of lava fromcontinual eruptions (without explosions). Because thelava is so fluid, it spreads out, often over a wide area.Shield volcanoes do not grow to a great height, and thelayers of lava spread out to give the volcano gentlysloping sides. Shield volcanoes can produce huge areasof basalt, which is usually what lava is when cooled.

• Even though their sides are not very steep, shieldvolcanoes can be huge. Mauna Kea in Hawaii is thebiggest mountain on Earth. If it is measured from itsbase on the floor of the sea, Mauna Kea is even tallerthan Mount Everest, the tallest mountain on land.

Types of Volcanoes: STRATOVOLCANOES

• A stratovolcano, also known as a composite volcano, is a tall, conical volcano. It is built up of many layers of hardened lava, tephra, pumice, and volcanic ash.

• Unlike shield volcanoes, stratovolcanoes have a steep profile and periodic eruptions. The lava that flows from stratovolcanoes cools and hardens before spreading far. It is sticky, that is, it has high viscosity. The magmaforming this lava is often felsic, with high-to-intermediate levels of silica, and less mafic magma. Big felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Types of Volcanoes: CALDERA

• A caldera is what is left when a huge stratovolcano blows its top off. It leaves a crater where the top of the volcano was before. Krakatoa, best known for its catastrophic eruption in 1883, is much smaller now.

How are volcanoes formed?

There are two main processes.• Volcanoes are made when two tectonic plates come

together. When these two plates meet, one of them(usually the oceanic plate) goes under the continentalplate. This is the process of subduction. Afterwards, it meltsand makes magma (inside the magma chamber), and thepressure builds up until the magma bursts through theEarth's crust.

• The second way is when a tectonic plate moves over a hotspot in the Earth's crust. The hot spot works its waythrough the crust until it breaks through. The caldera ofYellowstone Park was formed in that way; so were theHawaiian Islands.

FORMATION OF VOLCANOES

http://youtu.be/6Z4as_imJfM

Classification of Volcanoes

A traditional way to classify or identify volcanoes is by its pattern of eruptions. Those volcanoes which may erupt again at any time are called active. Those that are now quiet called dormant (inactive). Those volcanos which have not erupted in historical times are called extinct.

ACTIVE VOLCANOES

• An active volcano is currently erupting, or it has erupted in the last 10,000 years. An example of an active volcano is Mount St. Helens in the United States (US).

DORMANT VOLCANOES

• A dormant volcano is "sleeping," but it could awaken in the future. Mount Rainier in the United States is considered dormant

EXTINCT VOLCANOES

• An extinct volcano has not erupted in the past 10,000 years. Edinburgh Castle in Scotland is located atop an extinct volcano.

What determines the nature of eruption?

• There are many primary factors affecting the volcanoes eruptive style, namely: the magma’s temperature, its chemical composition, the amount of dissolved gases it contains. These factors can affect the magma’s viscosity in different way.

VISCOSITY

• VISCOSITY is the property of the material’s resistance to flow. It is also described as the liquid’s thickness and stickiness. The more viscous and thicker the material is, the greater is its resistance to flow.

Effect of Magma’s Temperature to Viscosity

• The viscosity of the magma decreases withtemperature. The higher the temperature ofmagma is, the lower is its viscosity. As lavaflows, it cools and begins to harden, its abilityto flow decreases and eventually stops.

Effect of Magma’s Composition to Viscosity

• Magma’s with high silica content are moreviscous than those with low silica content .The magma that contains less silica isrelatively fluid and travels far beforesolidifying.

Effect of the Amount of Gases Contained in Magma to Viscosity

• Gas (mainly water vapor) dissolved in magma tends to increase its ability to flow. Therefore, in near-surface environments, the loss of gases makes magma more viscous a dome or a columnar.

Lava with low amount of gas as it rises has high

viscosity that piles up at a vent resulting into a dome.

Lava with less silica content is too viscous to travel far, and tends to

break up as it flows

Lava with less silica content has less silica

content has low viscosity that it can travel a great

distance, fow=rming a thin sheet

Volcanic Eruption

• Eruption of Mayon Volcano last 2006 in Albay, Philippines

http://youtu.be/MEI1BTjxmok

• Eruption of Mt. Kilauea in Hawaii, USA

http://youtu.be/488BkTUsMa4

Types of Volcanic Eruption

• PHREARIC OR HYDROTHERMAL- is a stream-driveneruption, as the hot rockscome in contact withwater. It is short livedcharacterized by ashcolumns but may be anonset for a largereruption. Example is Taalvolcano eruption inBatangas (as shown onthe right)

Types of Volcanic Eruption

• PHREATOMAGMATIC

- is a violent eruption dueto the contact betweenwater and magma. As aresult, a large column ofvery fine ash, high speedand side-way emission ofphyroclatics called basedsurges are observed.Example is the eruptionof Mt. Fukutoku-Okanobain Bonin Islands, Japan

Types of Volcanic Eruption

• STROMBOLIAN

- a periodic weak toviolent eruptioncharacterized byfountain lava. Exampleis Mt. Irazu Eruption inCosta Rica

Types of Volcanic Eruption

• VULCANIAN

- characterized by talleruption coulmns thatreach up to 20 km highwith pyroclastic flowand ash fall tephra.Example is Mt. Paricutinin Mexico.

Types of Volcanic Eruption

• PLINIAN

- excessively explosivetype of eruption of gasand pyroclastics.Example is Mt. Pinatuboin Zambales.

Examples of Volcanoes found in the Philippines

Suggested Videos

• Naked Science: Volcanoes (it provides discussion about the formation of new volcanoes in Yellowstone National Park, USA)

http://youtu.be/G1pa6afh8o4

• BBC: Iceland Volcanoes Erupts (it shows eruption of Iceland volcanoes, its effects and how people in Iceland adapts and survive along the volcanic areas)

http://youtu.be/MlH7pCK4H-s