volcano index and composition.

Cross-section through a stratovolcano (vertical scale is exaggerated):
1. Large magma chamber 2. Bedrock 3. Conduit (pipe) 4. Base 5. Sill 6. Dike 7. Layers of ash emitted by the volcano 8. Flank	9. Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud

Large, explosive volcanic eruptions inject

•  water vapor (H₂O),
• carbon dioxide (CO₂),
• sulfur dioxide (SO₂),
•  hydrogen chloride (HCl),
•  hydrogen fluoride (HF) and
• ash (pulverized rock and pumice) into the stratosphere to heights of 16–32 kilometres (10–20 mi) above the Earth's surface.
• The most significant impacts from these injections come from the conversion of sulfur dioxide to sulfuric acid (H₂SO₄), which condenses rapidly in the stratosphere to form fine sulfate aerosols.
•  The aerosols increase the Earth's albedo—its reflection of radiation from the Sun back into space – and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere.

The Volcanic Explosivity Index (VEI)

•  was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions.
• Volume of products, eruption cloud height, and qualitative observations (using terms ranging from "gentle" to "mega-colossal") are used to determine the explosivity value.
• The scale is open-ended with the largest volcanoes in history given magnitude 8.
•  A value of 0 is given for non-explosive eruptions, defined as less than 10,000 m³ (350,000 cu ft) of tephra ejected;
•  and 8 representing a mega-colossal explosive eruption that can eject 1.0×10¹² m³ (3.5×10¹³ cu ft) of tephra and have a cloud column height of over 50 km (31 mi).
• The scale is logarithmic, with each interval on the scale representing a tenfold increase in observed ejecta criteria, with the exception of between VEI 0, VEI 1 and VEI 2.
• Ash, volcanic bombs, and ignimbrite are all treated alike.

WEAKNESSES

• Density and vesicularity (gas bubbling) of the volcanic products in question is not taken into account.
• In contrast, the DRE (Dense-Rock Equivalent) is sometimes calculated to give the actual amount of magma erupted.
• Another weakness of the VEI is that it does not take into account the power output of an eruption, which makes it extremely difficult to determine with prehistoric or unobserved eruptions

The 2011-2012 El Hierro eruption is an on-going volcanic event just off the island of El Hierro, the smallest and farthest south and west of the Canary Islands (an Autonomous Community of Spain), in the Atlantic Ocean off the coast of Africa. As of 7 January 2012 the eruption is underwater, with vents located approximately 2 km to the south of the fishing village of La Restinga on the southern coast of the island.