Query: Volcanoes emit around 0.3 billion tonnes of CO2 per year.

Relevant passages:
Hekla <=SEP=> Hekla ( -LSB- 3 = Is-Hekla_pronunciation . ogg -RSB- ) , or Hecla , is a stratovolcano in the south of Iceland with a height of 1491 m. Hekla is one of Iceland 's most active volcanoes ; over 20 eruptions have occurred in and around the volcano since 874 . During the Middle Ages , Europeans called the volcano the `` Gateway to Hell '' .   Hekla is part of a volcanic ridge , 40 km long . The most active part of this ridge , a fissure about 5.5 km long named Heklugjá , is considered to be the volcano Hekla proper . Hekla looks rather like an overturned boat , with its keel being a series of craters , two of which are generally the most active .   The volcano 's frequent large eruptions have covered much of Iceland with tephra and these layers can be used to date eruptions of Iceland 's other volcanos . 10 % of the tephra created in Iceland in the last thousand years has come from Hekla , amounting to 5 km3 . The volcano has produced one of the largest volumes of lava of any in the world in the last millennium , around 8 km3 .

Greenhouse gas <=SEP=> A greenhouse gas ( abbrev . GHG ) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range . This process is the fundamental cause of the greenhouse effect . The primary greenhouse gases in Earth 's atmosphere are water vapor , carbon dioxide , methane , nitrous oxide , and ozone . Without greenhouse gases , the average temperature of Earth 's surface would be about -18 ° C , rather than the present average of 15 ° C . In the Solar System , the atmospheres of Venus , Mars and Titan also contain gases that cause a greenhouse effect .   Human activities since the beginning of the Industrial Revolution ( taken as the year 1750 ) have produced a 40 % increase in the atmospheric concentration of carbon dioxide , from 280 ppm in 1750 to 406 ppm in early 2017 . This increase has occurred despite the uptake of a large portion of the emissions by various natural `` sinks '' involved in the carbon cycle . Anthropogenic carbon dioxide emissions ( i.e. , emissions produced by human activities ) come from combustion of fossil fuels , principally coal , oil , and natural gas , along with deforestation , soil erosion and animal agriculture .   It has been estimated that if greenhouse gas emissions continue at the present rate , Earth 's surface temperature could exceed historical values as early as 2047 , with potentially harmful effects on ecosystems , biodiversity and the livelihoods of people worldwide . Recent estimates suggest that on the current emissions trajectory the Earth could pass a threshold of 2 ° C global warming , which the United Nations ' IPCC designated as the upper limit to avoid `` dangerous '' global warming , by 2036 .

Volcano <=SEP=> A volcano is a rupture in the crust of a planetary-mass object , such as Earth , that allows hot lava , volcanic ash , and gases to escape from a magma chamber below the surface .   Earth 's volcanoes occur because its crust is broken into 17 major , rigid tectonic plates that float on a hotter , softer layer in its mantle . Therefore , on Earth , volcanoes are generally found where tectonic plates are diverging or converging , and most are found underwater . For example , a mid-oceanic ridge , such as the Mid-Atlantic Ridge , has volcanoes caused by divergent tectonic plates pulling apart ; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together . Volcanoes can also form where there is stretching and thinning of the crust 's , e.g. , in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America . This type of volcanism falls under the umbrella of `` plate hypothesis '' volcanism . Volcanism away from plate boundaries has also been explained as mantle plumes . These so-called `` hotspots '' , for example Hawaii , are postulated to arise from upwelling diapirs with magma from the core -- mantle boundary , 3,000 km deep in the Earth . Volcanoes are usually not created where two tectonic plates slide past one another .   Erupting volcanoes can pose many hazards , not only in the immediate vicinity of the eruption . One such hazard is that volcanic ash can be a threat to aircraft , in particular those with jet engines where ash particles can be melted by the high operating temperature ; the melted particles then adhere to the turbine blades and alter their shape , disrupting the operation of the turbine . Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth 's lower atmosphere ( or troposphere ) ; however , they also absorb heat radiated up from the Earth , thereby warming the upper atmosphere ( or stratosphere ) . Historically , so-called volcanic winters have caused catastrophic famines .

NW Rota-1 <=SEP=> NW Rota-1 is a seamount in the Mariana Islands, northwest of Rota, which was discovered through its hydrothermal activity in 2003. The volcano has been observed to be erupting underwater, the first time that submarine explosive eruptions have been directly witnessed. The summit of the seamount lies at 517 m (1,696 ft) below sea level, with a ridge capping off the seamount. It has several craters, the most active of which is located south of the summit ridge and is called "Brimstone Pit". Eruptions have been recorded in 2008–2010 and took the form of Strombolian eruptions with the emission of lava and large amounts of tephra. The erupted material is unstable and forms sediment flows and landslides; one large landslide occurred on 14 August 2009 and stripped part of the southern flank of NW Rota-1. Such landslides generate ash plumes underwater and are similar to large landslides at other active submarine volcanoes. The activity gives rise to intense hydrothermal plumes, which contain droplets of molten sulfur and bubbles of carbon dioxide, among other compounds. It is estimated that NW Rota-1 releases 400,000 ± 100,000 tonne/year of carbon dioxide. The hydrothermal activity nourishes microbial mats and these in turn many animals, although the unstable environment from the frequent eruptions limits animal diversity.