Download Volcano Study Guide

Volcano Study Guide
Volcano – an opening in Earth’s crust through which an eruption takes place.
The nature of a volcanic eruption depends on the magma.
Magma – hot, liquid rock beneath the earth’s surface.
Lava – hot, liquid rock that reaches and goes above the Earth’s surface.
Possible heat sources that can occur below the surface
1. Radioactive decay – a breaking apart of the nucleus of an atom. When the nucleus breaks
apart it releases energy which is changed into heat.
2. Original heat – heat that is inside the earth that may have been trapped when the earth first
formed.
3. Friction – results from the movement of lithospheric plates
4. Fusion in the core – nuclear fusion, similar to the sun is occurring in the core.
Determining if a volcano is active, dormant or extinct
Active volcano – 1300 volcanoes on land around the world are though to be active, have
erupted within the last few hundred years.
Dormant volcano – “sleeping” volcano but capable of being activated
Extinct volcano – has not erupted for the last several thousand years.
Types of Volcanoes are determined by the strength and explosiveness of the eruption.
Eruption – depends on temperature, composition and the amount of dissolved gases.
Temperature – the hotter the runnier (less viscous)
Viscosity – the ability of magma to flow.
More viscous –harder to flow
Less viscous – easier to flow
Composition- the movement of magma depends on the amount of silica. The more silica the
greater the viscosity (which means that the lava runs slowly).
There are two types of magma/lava felsic and mafic
Felsic – high silica –granitic – 70% silica – light in color
 Felsic lava is sticky which means that it flows slowly and has gas held within it. It does
not allow gasses to escape readily – which results in an EXPLOSIVE eruption.
Mafic – low silica – basaltic – 30% silica – dark in color
 Mafic lava is more fluid and allows gasses to escape readily – which results in a NON
EXPLOSIVE eruption.
The amount of dissolved gases affects the magma’s viscosity. The dissolved gases provide the
force of an eruption.
As magma moves closer to the surface (such as in a volcano) the pressure is reduced and the
dissolved gases are released – sometimes suddenly as in a soda bottle.
The gases are
Water vapor
Sulfur
Sulfur dioxide
Carbon dioxide
Carbon monoxide
Hydrogen sulfide
Tephra – lava that is blasted into the air by violent volcanic eruptions and solidifies as it falls to
the ground as ash, lapilli, cinders, blocks and volcanic bombs.
Anatomy of a Volcano
Vent – in volcanic regions, an opening in the Earth’s surface through which lava, ash and steam
flow.
Crater - opening at the top of a volcano
Dike – igneous rock that forms when magma is squeezed into a vertical crack and solidifies
(crosses layers)
sill – small body if igneous rock that forms when magma is squeezed into a horizontal crack
and then solidifies (parallel to layers)
Evidence of Volcano
Volcanic neck – the core of a volcano’s vent that remains after the outer layers of lava and
tephra have been eroded away from an extinct volcano.
Caldera- the large opening formed at the top of a volcano when the crater collapses into the
vent following an eruption.
Types of Volcanoes
Cinder cone – steep sloped sides (angles close to 40 degrees)
- relative to other volcanoes –small few hundreds of meters high
- small explosive eruptions
- made up of pyroclastic material (ash and tephra)
Shield volcanoes
- volcanoes that have a very broad base, with gently sloping side (like a shield)
- quiet eruptions
- make up of layers of hot, mafic (basaltic) lava
- Hawaii is a good example of a shield volcano
Composite Volcanoes
- steep sloped sides
- very tall 1000s of meters
- very explosive eruptions, sometimes quiet eruptions (alternating for the most
part)
- made up of alternating layers of lava flows and silica rich (granitic) pyroclastic
material
- Mt. St. Helens is a good example of a composite volcano.
Kinds of eruptions
- shield volcanoes – broad base, low angle slope
- pillow lava – lava that occurs in mid-ocean ridges
- basalt plateaus – lava spreading evenly over a large area
- rift eruptions – opening in the crust “spreading centers”
o sea-floor spreading (ocean)
o rift valley (continent)
o lava “oozes” out because of its mafic or basaltic composition
Areas of Volcanic Activity (Kinds of eruptions)
Same regions as Earthquakes, which often serve as warning signs that a volcanic eruption might
occur.
1. Ring of Fire – around the rim of the Pacific Ocean, subduction zone
2. Ocean Ridge system
3. Hotspots
Ring of Fire – subduction boundary eruptions (cinder cones and Composite Volcanoes)
Explosive eruptions usually young mountain chains – like around the Ring of Fire
Hot spots – a place in the mantle where great amounts of heat are rising through the
lithosphere.
Causes of hot spots are still unknown – the source of the heat or “spot” remains in the same
location while plates move over it. – (Hawaii)
Plutonic – Igneous Rocks
When masses of magma cools beneath the surface, it forms the cores of mountains
These “igneous intrusions” are called plutons
Plutons have different names depending on their size and shape.
Small intrusive igneous formations are
-dikes
-sills
-volcanic necks
-laccoliths – magma that bulges upward and forms dome mountains (Henry Mountains, Utah,
and Black Hills, South Dakota)
Batholith – a large body of intrusive igneous rock. Forms the core of most major mountain
ranges (Sierra Nevada)
Stock- small batholith, less than 100km cubed