Science With Mr. Milstid

Volcanoes, by definition, are areas of the earth’s surface that allow hot, molten rock, ash, and gases to escape from below the surface.

Volcano Parts
All volcanoes have the following parts in common:

• Magma Chamber: A pool of molten rock that forms under a volcano
• Vent: Openings in the earth’s crust that magma flows through.
• Crater: The main opening at the top of a volcano.

Click the image below for a more in-depth interactive tutorial of volcano parts from the National Park Service.

Volcano Activity
All volcanoes fall into 1 of 3 main categories:

1. Active Volcanoes: volcanoes that erupt regularly, or show signs of erupting soon.
2. Dormant Volcanoes: volcanoes that have not erupted in some time, but may again in the future.
3. Extinct Volcanoes: volcanoes that have not erupted in recorded history, and probably will not again.

Where Volcanoes Form
Most volcanoes occur at or near the edges of tectonic boundaries.
The same tectonic motion that causes earthquakes, causes volcanic formations and eruptions.
The interactive map below shows the relationship between tectonic boundaries and the locations of earthquakes around the world. Click on “plate boundaries” and “volcanoes” and take note of the overlap.

Along tectonic plates, there are 3 main areas where volcanism occurs:

• Hot spots
• Rift Zones
• Subduction Zones.

HotSpots
Hot Spot volcanoes occur far from the edges of plates.
Columns of rising magma push through the surface of the earth, forming land and volcanoes

Rift Zones
Rift Zones are places where tectonic plates are moving away from one another (think: divergent boundary).
As plates separate, an opening is created for magma to move toward the surface.

The animation below shows a mid ocean ridge rift zone in action

Subduction Zones
When two converging tectonic plates collide, one (the least dense of the two, e.g. oceanic crust in a continental-oceanic collision) is pushed below the other; it is subducted.
The subducted crust melts as it moves downward through the earth, producing a pool of magma.

The animation below shows a variety of volcanic formations in action. Click the buttons at the top for an up-close view. Pay special attention to the following animations: “Hot Spot Volcanoes,” and “Continental Volcanic Arc” (which is a continetnal-oceanic subduction zone).

Types of Volcanoes
We will discuss 3 main types of volcanoes in our study of volcanism:

• Shield Volcanoes
• Cinder Cone Volcanoes
• Composite Volcanoes

Each type has unique physical characteristics, caused by the way it erupts, and, to some extent, the way it forms.

Shield Volcanoes
These volcanoes are very broad with large bases. They are made of thin layers of lava from repeated slow, gentle eruptions.
They are most common in oceanic areas. As a result, huge explosions can occur when their lava makes contact with ocean water.

Cinder Cone Volcanoes
Steeply sloped sides made of layers of ash and loose rock.
Erupt violently, releasing huge amounts of ash, gas and volcanic material.

Composite Volcanoes
Most common type of volcano.
Broad bases, get steeper toward the top.
Form from explosive eruptions, followed by quiet lava flows. This makes layers of ash and rock that get covered by layers of lava.

Volcanic Eruptions
Regardless of the “type” of volcano, there are 2 main kinds of eruptions:

• Non-explosive
• Explosive

There are many subdivisions of these categories, but we will focus on these two throughout our study.

Non-Explosive Eruptions
These eruptions are relatively calm, and produce slow, gentle flows of lava. They are associated (primarily) with shield volcanoes.

Explosive Eruptions
Explosive eruptions are violent volcanic eruptions.
During an explosive eruption, hot debris, ash and gas shoot out of a volcano - this is, obviously, incredibly dangerous.

Causes of Volcanic Eruptions
When pressure and temperature build up inside of a volcano (due to magma formation inside the volcano), eruptions occur. As the temperature of magma and the gases that occupy a magma chamber builds, their particles begin to spread out (consider the behavior of matter during a state change), and they become less dense (and more buoyant). The pressure of gas dissolved in magma, combined with its decreasing density causes it to rise, making its way to the surface and exiting a volcano in the form of lava or pyroclastic material (see below).

The type of eruption depends on the composition of the magma inside the volcano:

• If there is a lot of water in the magma and silicate in the magma, a non-explosive eruption will occur.
   
  
   
• If the magma in a volcano has a high water and silicate content, an explosive eruption is likely.
   
  

Types of Volcanic Material
When volcanoes erupt, much more than just lava may come out!
There are 4 types of lava that can be erupted from a volcano:

1. AA (ah-ah) Lava
2. Pillow Lava
3. Blocky Lava
4. Pahoehoe (Puh-hoy-hoy)

Explosive eruptions also result in pyroclastic material: lava that is blasted into the air and hardens quickly.

Pyroclastic materials include:

• Volcanic Blocks: huge lava boulders that fall back to earth.
• Volcanic Bombs: giant lava stones.
• Lapilli: lava rocks.
• Volcanic Ash: small pieces of lava that solidify in air and create dust.

Tags: Earth Science, Plate Tectonics, Volcanoes
Posted in Earth Science, Plate Tectonics, Volcanoes