Between 385,000 and 315,000 years ago, volcanic activity in the Lassen volcanic center shifted dramatically from building andesitic stratovolcanoes to producing lava domes made of dacite. These eruptions formed the Lassen dome field, staged as andesite lava flows surrounding two sequences of dacitic lava domes. The first sequence of lava domes, known as the Bumpass domes, formed between 300,000 and 190,000 years ago, while the production of the younger Eagle Peak domes began about 70,000 years ago. The andesite lava flows form the older and younger Twin Lakes sequences, and date to between 315,000 and 240,000 years ago and between 90,000 years ago and present, respectively. Beginning 190,000 years ago, eruptions ceased in the Lassen Volcanic center for 100,000 years. Dacitic magma at the Lassen center formed from mafic (rich in magnesium and iron) magma meeting silicic (high in silicon dioxide) magma chambers with felsic (rich in feldspar and quartz) phenocrysts. Some dacitic crystals were partially reabsorbed as a result of mixing of hot mafic magma with cool dacitic magma, and this along with undercooling of mixed magma led to phenocryst variation within certain domes exceeding variation between the domes. All three sequences — Bumpass, Eagle Peak, and Twin Lakes — formed from lava subjected to magma-mixing processes, accounting for their heterogeneous appearance and composition. Because of these mixing mechanisms, lavas may have different compositions but similar appearances, or similar compositions with different appearances. The eruption that produced the Chaos Crags consisted of more than 90% mixed magma, and likely resulted from the interaction of felsic and mafic magmas.