Polka northern Mexico and Texas, and are also called "Norteño". Traditional dances from this region reflect the influence of polka-dancing European immigrants. In the 1980s and 1990s, several American bands began to combine polka with various rock styles (sometimes referred to as "punk polka"), "alternative polka", or "San Francisco-style". There also exist Curaçaoan polkas, Peruvian polkas (becoming very popular in Lima). In the pampas of Argentina, the "polca" has a very fast beat with a time signature. Instruments used are: acoustic guitar (usually six strings, but sometimes seven strings), electric or acoustic bass (sometimes fretless), accordion (sometimes piano accordion, sometimes button
Types of volcanic eruptions Types of volcanic eruptions Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series. There are three different types of eruptions. The most well-observed are magmatic eruptions, which involve the decompression of gas within magma that