1. THE NATURE OF VOLCANIC ERUPTION
By: Alisha Reconada

2. Volcanic activity is commonly perceived as a process that produce picturesque, cone-shaped structure that periodically erupts in a violent manner.
Some eruptions are very explosive while many others are not.

3. MAGMA
Magma associated with an explosive eruption may be thousand of times more viscous than magma that extruded in a quiescent manner.

5. FACTORS AFFECTING MAGMA'S VISCOSITY
The primary factors include the magma's temperature, its composition, and the amount of dissolved gases it contains.
A magma's viscosity is directly related to its silica content. In general, the more silica in magma, the greater its viscosity.
The amount of volatiles ( gaseous components of magma, mainly water) contained in magma also affects its mobility.

6. WHY DO VOLCANOES ERUPT?
One of the simplest mechanisms for triggering a volcanic eruption is the arrival of a new batch of hot liquid rock into a near - surface magma reservoir.
The injection of a fresh supply of magma causes the pressure in the chamber to rise until cracks in the rock above.

7. What is extruded during eruptions?
LAVA FLOWS - because of their low silica content, hot basaltic lavas are usually very fluid
two types of lava:
(1) Typical slow-moving aa flow
(2) Typical pahoehoe flow

8. aa flow
pahoehoe flow

9. Lava tubes - hardened basaltic flows commonly contain tunnels that once were nearly horizontal tubes carrying lava from the volcanic vent to the flow's leading edge.

10. Gases - magmas contain various dissolved gases (volatiles) held in molten rock by confining pressure, just as carbon dioxide is held in soft drinks.
The gases are about 70% water vapor, 15% carbon dioxide, 5% nitrogen, 5% sulfur, and lesser amounts of chlorine, hydrogen, and argon.

11. Pyroclastic materials - when basaltic lava is extruded, dissolved gases escape quite freely and continually.
These gases propel incandescent blobs of lava to great heights.

12. Volcanic structures and eruptive styles by:Kaycel castro
After knowing the nature of volcanic eruptions, how it erupts and what are the materials extruded during eruptions, we move on to volcanic structures and eruptive styles.
We will look at the general anatomy of volcanoes, the major volcanic types and some volcanic landforms.
Volcanic structures and eruptive styles by:Kaycel castro

13. Anatomy Of a volcano Fissure
A crack that develops in the crust from which magma moves
A volcanic activity begins with a fissure, a crack that develops in the crust. Since the magma is less dense than the surrounding rocks the magma rises. The picture here shows a crack of Mt. St. Helens.
A close- up of a crack in the volcanic surface of a cooled lava flow from the crater of Mt. St. Helens

14. Anatomy Of a volcano Conduit/ pipe
A circular path where magma localizes
As the magma moves up the fissure, it localizes in a circular path known as the conduit or pipe (points to the picture).

15. Anatomy Of a volcano Vent Surface opening Fumarole
A vent that only emits gases
The magma then terminates at the vent or the surface opening. Some vent only emit gases which are called fumaroles. Continuous eruption, separated by periods of inactivity and accumulation of extruded materials will build a structure that we know as a volcano.

16. Anatomy Of a volcano Crater Funnel- shaped depression at the summit
The funnel- shaped depression at the summit is called the crater (points to the picture).

17. Anatomy Of a volcano Parasitic cone
Produced from continuous eruption from the fissures in the base
As a volcano matures, fissures that develop on the base of the volcano emits volcanic materials. Successive eruption produces a parasitic cone (points at the picture).

18. Shield volcanoes Shape resembles a warriors shield
Produced by accumulation of basaltic lava
Have grown up from the deep ocean floor to form islands
Example: Islands of Hawaiian chain, Iceland, Galapagos
We imagine volcanoes as large, symmetrical cones but not all volcanoes are like that. Let’s differentiate and look into the major types of volcanoes.
First: Shield volcanoes. Their shape resembles a warrior’s shield that’s why they are called like that. These volcanoes are formed or produced by accumulation of basaltic lava. Let’s recall that basaltic lava, as mentioned by the first reporter are less viscous or very fluid compared to rhyolitic and andesitic lava, making it flow farther than the source, thus creating a the shield shape.
Most shield volcanoes have grown up from the deep ocean floor to form islands. Examples include the Islands of Hawaiian chain, Iceland and Galapagos.

19. Mauna Loa: Earth’s Largest volcano
Base to summit: 9 km high
Volume: cubic km
Caldera: 2.6 x 4.5 km, 150 m depth
Mauna Loa, Earth’s largest volcano is a shield volcano. We can see its broad and slightly domed structure on the picture that resembles a shield. It measures 9 km from base to summit and its volume is approximately km3. Its caldera, a large circular depression at the summit measures 2.6x 4.5 km across with 150 m depth.

20. Kilauea, Hawaii: Eruption of a Shield volcano
Most active shield volcano (50 eruptions)
“curtain of fire” in 1983
Cinder and spatter cone: Puu Oo
Kilauea, a shield volcano in Hawaii, is the most active shield volcano with more than 50 eruptions recorded since Before the volcano erupts, small earthquakes are felt. The longest and largest eruption occurred in 1983, a 6km fissure extruded lava, a 100m “curtain of fire” was observed. The cinder and spatter cone Puu Oo was built.

21. Cinder cones Composed of loose pyroclastics
Most abundant of the major types of volcanoes
Have a high angle of repose and large, deep craters
very short eruptive cycles
95% formed in less than one year
Cinder cones are named because like cinders, they are built from extruded materials that hardens while in flight. They are composed of loose pyroclastic materials. They are the most abundant of the major types of volcanoes. They have a high angle of repose and large, deep craters relative to their over all size. Their structure is relatively symmetrical, elongated and higher on the side that was down wind during eruptions. Their eruptive cycle is short, when continuous eruption with short periods of inactivity stops, the magma in the pipe connected to the vent solidifies and it will never erupt again.

22. Paricutin: Life cycle of a Cinder cone
First eruptive phase: 1943 in a cornfield
June 1944 Aa flow covered the San Juan Parangaricutiro
Today, it will not erupt again
Paricutin, a cinder cone in Mexico, city began in 1943 at a cornfield. Earthquakes were felt, sulfurous gases and glowing rock fragments were felt. Large fragments built the cone and ashes were blown away. On the first day it grew 40 m and grew 100 m on the fifth day. On June 1944, an Aa flow covered the village of San Juan Paranguricutiro, leaving the church and steeple exposed. After 9 years of continuous discharge with short periods of inactivity, it will never erupt again.

23. Composite Cones Most picturesque but potentially dangerous volcanoes
Also called stratovolcanoes
The Ring of fire
Large, nearly symmetrical structure
Product of gas- rich magma, andesitic composition
Composite cones are the most picturesque but dangerous volcanoes. They are also called stratovolcanoes. They are large and nearly symmetrical in structure They are products of gas- rich magma, andesitic composition. Most are located in the ring of fire, called continental volcanoes. Some examples are our very own Mayon Volcano and Mt Fujiyama in Japan.

24. Nuée Ardente: A Deadly Pyroclastic Flow
Glowing avalanches
Hot gases with ash and large rock fragment
The destruction of St. Pierre
Eruption of Mount Pelée 1902
inhabitants died
Also known as glowing avalanches, nuée ardente is very dangerous. It is a deadly pyroclastic flow, hot gases with ashes and large rock fragments. Mt. Pelée erupted and nuée ardente was observed which destroyed St. Pierre, a place in the land of Martinique, almost all of inhabitants died. The top picture shows St. Pierre before the eruption and the bottom picture shows how it looks like afterwards.

25. Lahars: Mudflows on Active and Inactive Cones
volcanic debris becomes saturated
when snow melt during eruptions
volcanic deposits triggered by rainfall
Lahar is from an Indonesian word that refers to deadly mudflows caused by saturated volcanic debris on composite cones. It happens on both active and inactive cones. In active cones, the heat released melts the snow and ice which saturates the volcanic deposits that leads mudflows. On the other hand, inactive cones produce lahars when volcanic deposits are triggered by heavy rainfall. The picture shows a house damaged by lahar and some uprooted trees, west- northwest of Mt St. Helens.

26. The figure shows the three major types of volcanoes and their respective sizes. A shield volcano being the largest, composite cone that is larger that a cinder cone but smaller than the shield volcano and the cinder cone which is the smallest.

27. Calderas Caldaria= cooking pot
Large, collapse depressions having a circular form
1- 10 km in diameter
3 types depending on formation:
Crater lake- type calderas
Hawaiian type calderas
Yellowstone- type calderas
Here are some other volcanic landforms. First is the caldera, from the word caldaria or cooking pot. It is a large, collapse depression having a circular form km in diameter. There are 3 different types of calderas depending on formation. The crater lake- type, Hawaiian type and the Yellowstone type calderas, we will describe them one by one on the next slides.

28. Crater lake- type Calderas
Crater lake, Oregon
Collapse of the summit of a composite volcano caused by eruption
Eruption of Mount Mazama
Wizard Island
Maximum diameter: 10km, 1175 m deep
Crater lake- type calderas form when the summit of a composite volcano collapses due to eruption. When a volcano erupts extruding a lot of magma, the magma chamber partially empties which makes the upper portion of the volcano unsupported thus it collapses. Water fills out the caldera creating this type of caldera. Just like what happened to Mt Mazama (points at the picture) that erupted 50km3 of magma and other materials, creating the crater lake, Oregon with maximum diameter of 10 km, 1175 m deep and the wizard island that serves as memory of what happened to Mt Mazama.

29. Hawaiian type Calderas
Collapse of a shield volcano caused by subterranean drainage from a magma chamber
Hawaii’s active shield volcano, Kilauea
Large caldera that measures 3.3 x 4.4 km
Vertical walls, looks like a deep, flat bottomed pit
Unlike crater lake- type calderas that needs eruption to occur and only happens in a composite cone, Hawaiian type calderas are different. Even without eruption, it occurs on shield volcanoes, one example is the Kilauea, which collapsed due to subterranean drainage (the east rift zone) from a magma chamber. Leaving vertical walls, that looks like a deep, flat bottomed pit. This large caldera measures 3.3 x 4.4 km across.

30. Yellowstone type Calderas
Collapse of a large area caused by extraordinary volume of erupted material
Produce the largest volcanic structures on earth
Yellowstone National Park
100 cubic km of pyroclastic material erupted, 70 km across
Yellowstone type calderas form when a large area collapses caused by extraordinary volume of erupted material. They produce the largest volcanic structures on earth. An example is the Yellowstone National Park that extruded 100 km3 of pyroclastic materials, creating a 70 km (across) caldera.

31. Fissure Eruptions and Lava Plateaus
Columbia plateau
Basaltic lava buried the existing landscape (50 m thick)
Fissure eruptions are considered as volcanic landforms. They contribute to a change in landscape just like what happened in the Columbia plateau. The basaltic lava buried the existing landscape (50 m thick). Flood basalts or fluid lava flows are shown in the picture.

32. Columbia plateau
This is the Columbia plateau, the activity began 17 million years ago that produced a basalt plateau with an average thickness of more than 1 km.

33. Volcanic pipes and necks
Pipes- “Windows into the earth”
Necks- remains after the cone has vanished
Ship Rock, New Mexico
Volcanic pipes are considered “windows into the earth” because they may extend tubelike to depths exceeding 200 km, magmas then migrate up to these structures that produce rocks that are samples of the mantle with very little alteration due to ascent. After along time due to erosion, the cone of a volcano will vanish leaving volcanic necks. The picture shows Ship Rock, New Mexico, a volcanic neck that stands 420m.
Volcanic structures differ in size and shape because of their composition. These variations gives us an idea of how eruptive styles can affect the condition of village and the environment nearby.