During the 1990 calendar year, no rock album hit No. 1 on the Billboard 200 Album chart. The year was owned by female vocalists (Madonna, Mariah Carey), crossover rap albums (by Vanilla Ice and M. C. Hammer), pop acts (Wilson Phillips, Phil Collins), and teen sensations New Kids On the Block. The erosion of rock's popularity could be attributed to a number of causes. One was the fragmentation of radio formats. There were three main rock formats: classic rock played older groups from the 1960s and 1970s - such as the Beatles, the Doors, the Rolling Stones, Led Zeppelin, the Who - exclusively. Album rock stations also played music from the 1960s and 1970s, including new records by a handful of "core" artists; and alternative rock, which at that time had come to be defined as mostly synthesizer-driven postpunk dance music.