1. CHAPTER 3
Volcanoes

2. Volcanoes
Volcanoes occur when molten magma reaches the Earth’s surface due to vents or fissures in the crust.
Volcanoes are divided into one of three general categories based on their level of activity: Active, Dormant and Extinct.

3. Volcanoes
A volcano is generally said to be active if it has erupted recently, is erupting or is likely to erupt again in the near future, e.g. Mount Vesuvius, Italy.
A dormant volcano has not erupted in recent history, but is likely to erupt again in the future. For example, the volcano in Yellowstone National Park is estimated to erupt once every years.
An extinct volcano is one which has not erupted in recorded history and is not expected to erupt again in the future. Slemish in Co. Antrim is the site of an extinct volcano f

4. Volcanic Emissions Lava Pyroclast/Pyroclastic Flow Tephra Water vapour
Five general substances are emitted from volcanic eruptions:
Lava
Pyroclast/Pyroclastic Flow
Tephra
Water vapour
Gases.

5. Water Vapour
Volcanoes along subduction zones often emit large amounts of water vapour during an eruption. This water is sea water carried into the mantle by the subducting plate.

6. Gases
Dangerous gases such as carbon dioxide, chlorine and sulphur dioxide are released during volcanic eruptions.

7. Tephra Tephra is the collective name given to ash, dust and pumice.
Ash is formed during explosive eruptions as rock is pulverised inside the vent. This ash and dust are emitted from the volcano and rise high into the atmosphere, disrupting air travel.
Pumice is created when lava mixes with air, causing it to fill with air bubble

8. Pyroclast/Pyroclastic Flow
Pyroclast, or pyroclastic flow, is emitted from volcanoes at destructive plate boundaries.
Pyroclast is often referred to as volcanic bombs. It consists of rock fragments, lava and ash that are emitted from a volcano during an eruption.

9. Lava
Lava is molten rock emitted from volcanoes and fissures on the Earth’s crust.
Volcanoes produce two types of lava: Acidic Lava and Basic Lava.
Acidic lava is emitted from volcanoes at destructive plate boundaries where subduction occurs.
Basic lava is emitted from volcanoes at constructive plate boundaries and at hotspots.

10. Lava
Lava is molten rock emitted from volcanoes and fissures on the Earth’s crust.
Volcanoes produce two types of lava: Acidic Lava and Basic Lava.
Acidic lava is emitted from volcanoes at destructive plate boundaries where subduction occurs.
Basic lava is emitted from volcanoes at constructive plate boundaries and at hotspots.

11. Volcanic Distribution
Volcanoes are usually clustered together in lines along plate boundaries.
Volcanoes occur at constructive/ divergent boundaries as plates pull apart from each other.
Volcanoes occur at convergent/destructive plate boundaries as two plates collide with each other, causing subduction.
Volcanoes at hotspots form away from plate boundaries on both continental and oceanic plates. Hotspots occur due to unusually hot plumes of magma rising through the mantle beneath a thin layer of oceanic plate.

12. Volcanic Landforms
Volcanic activity creates a variety of different landforms, both on the Earth’s surface and inside the Earth’s crust.
Landforms created on the surface are called extrusive or volcanic features.
Landforms created within the crust are called intrusive or plutonic features.

13. Extrusive Landforms
Volcanic cones are the most common feature associated with volcanic activity.
Volcanic cones can form in many ways and vary in structure and composition,e.g. Composite cones & Cinder cone.
Mount Fuji in Japan is an example of a composite volcano
Mauna Loa in Hawaii is an example of a series of cinder cones.

14. Intrusive Landforms
Intrusive landforms are created when volcanic material cools and solidifies within the Earth’s crust. The material covering these landforms is removed by weathering and erosion to leave them exposed on the surface.

15. Intrusive Landforms - Batholith
A batholith is a large area of igneous rock that forms when magma cools inside the Earth’s crust. As magma rises into the crust, it melts surrounding layers of rock. Over millions of years, this magma cools to form granite rock.

16. Intrusive Landforms - Sills
Sills are horizontal layers of granite rock that form when magma moves through the crust and forces its way between layers of sedimentary rock before cooling slowly.
Slieve Gullion in Co. Armagh is an example.

17. Intrusive Landforms - Dykes
Dykes are formed when magma forces its way upwards through layers of overlying rock before cooling. Once cooled, a layer of granite is formed perpendicular to the layers of rock.

18. Intrusive Landforms - Laccoliths
Laccoliths are formed when magma forces its way through layers of rock. As more magma intrudes (forces its way) between the layers, the pressure of the magma forces the rock to push upwards. It cools slowly to form a domeshaped layer of granite rock, e.g. Pine Valley Laccolith, Utah in the US.

19. Intrusive Landforms - Lopoliths
Lopoliths are formed in a similar way to laccoliths. As magma intrudes, its weight forces the rock downwards to form a bowl-shaped layer of granite, e.g. Great Dyke, Zimbabwe.

20. Intrusive Landforms - Lopoliths
Lopoliths are formed in a similar way to laccoliths. As magma intrudes, its weight forces the rock downwards to form a bowl-shaped layer of granite, e.g. Great Dyke, Zimbabwe.

21. Measuring and predicting volcanoes
Volcanologists use a variety of instruments and tools to detect and record volcanic activity.
Earthquake activity always increases beneath a volcano prior to a eruption. Seismometers and seismographs are used to measure movements in the Earth’s crust, detecting and recording earthquakes.

22. Measuring and predicting volcanoes
Electronic distance measurement (EDM) devices, tiltmeters, GPS and satellite radar are used to check for any ground deformation.
Ground deformation is one of the most obvious signs that a volcano is close to erupting. As pressure from rising magma and gases increases, volcanic mountain peaks swell shortly before eruption.

23. Measuring and predicting volcanoes
Historic eruptions in combined with information from modern instruments may also be used to predict volcanoes.

24. Measuring and predicting volcanoes
Historic eruptions in combined with information from modern instruments may also be used to predict volcanoes.

25. Positive and Negative Effects of Volcanoes
Despite their destruct powers, volcanoes have various positive effects including: Creation of new land, Creation of fertile soils, Tourism and Geothermal energy
Negative effects of volcanoes include: Loss of life, Disruption to air travel.