1. VOLCANIC ACTIVITY

2. What is a volcano ?
A volcano is a rupture on the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

3. Why does a volcano form?
You have already learned about tectonic plates. Beneath the Earth surface, powerful forces are at work. These forces move lithospheric plates and produce huge chambers of magma, molten rock beneath the Earth crust.

4. How does a volcano form:
When tectonic plates collide, one plate can be pushed beneath another. As it sinks it heats up and begins to melt. This molten rock then rises and erupts on the surface as lava, building up a volcano.

5. How does a volcano form 2:
Figure 8.2: A volcano is a vent through which molten rock and gas from beneath the Earth surface escape.
(1) Large magma chamber (2) Bedrock
(3) Pipe (conduit) (4) Layers of ash
(5) Layers of lava (6) Lava flow (7) Vent
(8) Lava (9) Ash cloud (26)
As you can see in this picture, magma begins in the lava chamber and comes to the surface through the throat of the volcano. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other.

6. Where does a volcano occur?
Magma is a molten rock that has been heated because of high temperatures and pressures beneath the Earth crust. This pressure mostly occurs where the tectonic plates meet and subduct.
So, volcanic activity tends to occur along the subduction plate boundaries, where one plate slides
underneath another one.

7. The ring of Fire:
The edges of the Pacific Plate make up a long subduction boundary.
The Pacific Ring of Fire is where the majority of the volcanic activity on the Earth occurs.

8. Volcanic hot spot:
Although most volcanic activity on Earth occurs at plate boundaries, there are some volcanically active spots that are in the middle of a tectonic plate. These areas are called hot spots.
The islands of Hawaii formed over a hot spot and are not located on the Pacific Ring of Fire. It was formed by volcanic activity fed by mantle plumes.

9. Volcanic eruption: All volcanoes share the same basic features.
The magma collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface.
As the rock heats, it expands, which creates even more pressure.
As a result, the magma seeks a way out pushing toward the surface, the magma seeps through cracks in the Earth crust called vents.
Eventually, the magma reaches the surface; when it comes out, we call it an eruption.

10. Types of eruptions: Explosive Eruption Effusive Eruption
Their composition and construction depend on so many factors, that it would be impossible for two formations to be exactly alike.
In the same way, each volcano and its eruptions are unique.
However, we tend to see two major kinds of eruptions:
Explosive Eruption
Effusive Eruption

11. Explosive Eruption:
An explosive eruption is a volcanic term to describe a violent, explosive type of eruption.
Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ashes when pressure is suddenly lowered at the vent.
(in the picture Mount Saint Helens explosive eruption on July 22nd, 1980)

12. Effusive eruption:
An effusive eruption is a volcanic eruption characterized by the outpouring of lava onto the ground .
Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge, slope of the ground over which the lava travels, and duration of the eruption.
For example, basalt lava may become ʻaʻā or pāhoehoe.[2] Andesite lava typically forms blocky lava flows,[3] and dacite lava flows often form steep-sided mounds called lava domes due to their viscosity.[4]
An ʻā lava flow from Mauna Loa during its 1984 eruption.

13. link https://www.youtube.com/watch?v=WgktM2luLok
SUMMARY