Teide is currently dormant; the last eruption occurred in 1909 from the El Chinyero vent. Historical volcanic activity on the island is associated with vents on the Santiago or NW rift (Boca Cangrejo 1492, Montañas Negras 1706, Narices del Teide or Chahorra 1798 and El Chinyero 1909) and the Cordillera Dorsal or NE rift (Fasnia in 1704, Siete Fuentes and Arafo and 1705). The 1706 eruption from the Montañas Negras vent on the Santiago rift destroyed the town and principal port of Garachico, plus several smaller villages.
Historical activity associated with the Montaña Teide - Pico Viejo stratovolcanoes occurred in 1798 from the Narices del Teide on the western flank of Pico Viejo. Eruptive material from Pico Viejo-Montaña Teide-Montaña Blanca partially fills the Las Cañadas caldera. The last explosive eruption involving the central volcanic centre was from Montaña Blanca ~2000 BP. The last eruption within the Las Cañadas caldera occurred in 1798 from the Narices del Teide or Chahorra (Teides Nostrils) on the western flank of Pico Viejo (Old Peak - which is actually younger than Teide). The eruption was predominantly strombolian in style and mostly a'a lava was erupted. These lavas are visible alongside the Vilaflor - Chio road.
The explorer Christopher Columbus reported seeing "... A great fire in the Orotava Valley...," as he sailed past Tenerife on his voyage to discover the New World in 1492. This was interpreted as indicating that he had witnessed an eruption in the Orotava Valley.
Unfortunately radiometric dating of possible lavas disproved the eruption theory. However, radiometric dating indicates that an eruption did occur in 1492 from the Boca Cangrejo vent.
About 150,000 years ago, a much larger explosive eruption occurred, probably of Volcanic Explosivity Index 5. This eruption created the Las Cañadas caldera, a large caldera, at about 2,000 m above sea level. The caldera is ~16 km across east-west and ~9 km north-south. At Guajara, on the south side of the structure, the internal walls rise as almost sheer cliffs from 2,100 m to 2,715 m. The 3,718 m summit of Teide itself, and its sister stratovolcano, Pico Viejo 3,134 m, are both situated in the northern half of the caldera, and are derived from eruptions subsequent to this prehistoric explosion.
Further eruptions are possible at some future unascertainable date, including a risk of pyroclastic flows and surges similar to those that occurred at Mount Pelée, Merapi, Mount Vesuvius, Etna, Soufrière Hills, Mount Unzen, etc. During 2003, there was an increase in seismic activity at the volcano. Many volcanoes e.g. Mount St Helens, Soufrière Hills had similar seismic activity prior to becoming active. Such activity is considered as being indicative of magma rising into the edifice.
Teide is considered to be unstable and has a distinctive bulge on its northern flank. This bulge is not believed to be associated with an influx of magma, but the result of a slow northwards collapse of the edifice. Seismic evidence suggests that Teide may be constructed over the headwall scarp of the infilled Icod Valley, a massive landslide valley formed by edifice failure in a similar manner to that of the Güímar and Orotava Valleys. The summit of the volcano has a number of small active fumaroles emitting sulfur dioxide and other gases including low levels of hydrogen sulfide.