Volcanology is the study of volcanoes, lava, magma and related geological, geophysical and geochemical phenomena. The term volcanology is derived from the Latin word vulcan. Vulcan was the ancient Roman god of fire.

There are about 169 volcanoes in the United States that scientists consider active. Most of these (about 130) are located in Alaska, where eruptions occur virtually every year.

A volcanic winter is a reduction in global temperatures caused by volcanic ash and droplets of sulfuric acid and water obscuring the sun. The worst volcanic winters have caused crop failures resulting in catastrophic famines.

Mount Tambora's 1815 eruption was the largest in recorded human history. The explosion was heard on Sumatra island, more than 2,000 kilometres (1,200 miles) away, and the maximum elevation of Tambora was reduced from about 4,300 metres (14,100 ft) to 2,850 metres (9,350 feet). Heavy volcanic ash rains were observed as far away as Borneo, Sulawesi, Java and Maluku islands, and the eruption contributed to global climate anomalies in the following years, with 1816 becoming known as the "year without a summer" due to the impact on North American and European weather.

Magma is the name given to hot liquid rock inside a volcano. Once it leaves the volcano, it's known as lava.

Fresh lava can reach temperatures of 2,200° F (1,200° C). Depending on its exact temperature, lava glows either red (hot), orange (hotter), or yellow (hottest).

In general, lava flows slowly, with typical speeds of 0.25 mph (0.40 km/h) and maximum speeds of 6 to 30 mph (9.7 to 48.3 km/h) on steep slopes. An exceptional speed of 20 to 60 mph (32 to 97 km/h) was recorded following the 1977 collapse of a lava lake at Mount Nyiragongo in the Congo.

Pompeii, along with Herculaneum and many villas in the surrounding area, was buried under 4 to 6 m (13 to 20 ft) of volcanic ash and pumice in the eruption of Mount Vesuvius in AD 79. Incredibly, the ash deposits preserved the town and the remains of the people within it. Today, the excavated city is one of Italy's most popular historical sites, offering a unique snapshot of Roman life, frozen at the moment it was buried.

It was heard 1,300 miles away in the Andaman and Nicobar islands ("extraordinary sounds were heard, as of guns firing"), 2,000 miles away in New Guinea and Western Australia ("a series of loud reports, resembling those of artillery in a north-westerly direction", and even 3,000 miles away on the Indian Ocean island of Rodrigues, near Mauritius ("coming from the eastward, like the distant roar of heavy guns.")

According to the official records of the Dutch East Indies colony, 165 villages and towns were destroyed near Krakatoa, and 132 were seriously damaged. At least 36,417 people died, and many thousands more were injured, mostly from the tsunamis that followed the explosion.