This infobox is meant to be used on pages about volcanic eruptions and on the sections of articles about a volcano about an eruption, but not as the main template on the pages of the volcanoes themselves (for that, use {{Infobox mountain}}). The completely expanded form of the template is as follows:

{{Infobox eruption
 | name        = 
 | image       = 
 | image_size  = 
 | caption     = 
 | date        = 
 | start_date  = 
 | start_time  = 
 | end_date    = 
 | end_time    = 
 | volcano     = 
 | type        = 
 | location    = 
 | coordinates = 
 | VEI         = 
 | map         = 
 | map_size    = 
 | map_caption = 
 | impact      = 
}}
Field name Input
name Name of the eruption. The conventions for naming relatively modern eruptions is to use "[Year] eruption of [volcano]". See for instance 1980 eruption of Mount St. Helens. If the timing is incomplete (common for extremely old eruptions), use as specific a time period as you may in place of the year. Many eruptions have names given to them by the scientific community, if one exists, use it!
image Photo of the eruption. A painting or other representation may be used for eruptions preceding the invention of photography. Precede filename with "File:" but do not use [ ] brackets.
image_size Size of the photo; leaving this blank will cause it to default to 250px.
caption Caption for the photo.
date Date of eruption; should be used for one day eruptions or when the start and end dates of an eruption are unknown.
start_date Start date of eruption; should be as specific as allowable.
start_time Start time of eruption; should be as specific as allowable.
end_date End date of eruption; should be as specific as allowable.
end_time End time of eruption; should be as specific as allowable.
volcano Name of the volcano where the eruption occurred.
type Type of eruption, see Types of volcanic eruptions for the different types.
location Location of the volcano, e.g. "Boulder Springs, Colorado"
coordinates Coordinates of the volcano. Use Template:Coord.
VEI Volcanic Explosivity Index, a measure of an eruption's strength similar to the Richter scale; the type of eruption can be inferred off its VEI, and this field also automatically adds the volcano to the respective "VEI-level volcanoes" category.
map Map of the eruption, if there is one. You can use a {{Location map}} template in place of a "hard" image map (use width = 250, with caption = )
map_size Same as image_size, above.
map_caption Caption for the map.
impact What was the impact of the eruption? How many lives were lost? How much property damage? What was the impact of the eruption on the local and global landscape? Basically, why is the eruption notable?

Here is an example usage:

1980 eruption of Mount St. Helens
Photograph of the eruption column, May 18, 1980
ज्वालामुखीMount St. Helens
सुरुवात समयMarch 27, 1980
सुरुवात समय12:36 p.m. PDT
प्रकारPhreatic, Plinian, Peléan
स्थानSkamania County, Washington
४६°१२′१″उ॰ १२२°११′१२″प॰ / ४६.२००२८°N १२२.१८६६७°W / 46.20028; -122.18667
ज्वालामुखी विष्फोटक सूचकाङ्क5
प्रभाव57 deaths, $1.1 billion in property damage; caused a partial collapse of the volcano's flank, deposited ash in 11 states
Map of eruption deposits

created with this source:

{{Infobox eruption
 | name = 1980 eruption of Mount St. Helens
 | image = MSH80 eruption mount st helens 05-18-80-dramatic-edit.jpg
 | image_size = 200px
 | caption = Photograph of the [[eruption column]], May 18, 1980
 | start_date = March 27, 1980
 | start_time = 12:36 p.m. PDT
 | volcano = [[Mount St. Helens]]
 | type = [[Phreatic eruption|Phreatic]], [[Plinian eruption|Plinian]], [[Peléan eruption|Peléan]]
 | location = [[Skamania County, Washington|Skamania County]], [[Washington (state)|Washington]]
 | coordinates = {{coord|46|12|1|N|122|11|12|W|display=inline}}
 | VEI = 5
 | map = St helens map showing 1980 eruption deposits.png
 | map_size =
 | map_caption = Map of eruption deposits
 | impact = 57 deaths, $1.1 billion in property damage; caused a partial collapse of the volcano's flank, deposited ash in 11 states
}}

This template can also be used inside an article, in a section dedicated to an important volcanic eruption. In these cases, the volcano, location, coordinates, and map should be left out, as they are redundant to the main article infobox (presumably Template:Infobox mountain). For instance:

1914 eruption of Sakurajima
The city of Kagoshima was covered by deep ash during the 1914 eruption of the Sakurajima, which is visible across the bay.
सुरुवात समयJanuary 1914
प्रकारPeléan
ज्वालामुखी विष्फोटक सूचकाङ्क4
प्रभावPre-eruption earthquakes killed at least 35 people; caused an evacuation and significant changes to the local topology.

Source:

{{Infobox eruption
 | name = 1914 eruption of Sakurajima
 | image = Kagoshima 1914.jpg
 | image_size =
 | caption = The city of [[Kagoshima]] was covered by deep ash during the 1914 eruption of the Sakurajima, which is visible across the bay.
 | start_date = January 1914
 | start_time =
 | volcano = 
 | type = [[Peléan eruption|Peléan]]
 | location =
 | coordinates =
 | VEI = 4
 | map =
 | map_size =
 | map_caption =
 | impact = Pre-eruption earthquakes killed at least 35 people; caused an evacuation and significant changes to the local topology.
}}