 Some eruptions were violent and produced large volumes of volcanic ash that can be found in between the lava flows.Other strata between the basalt layers contain volcaniclastic and other sediments that indicate the long time intervals between eruptions, with rich vegetation taking root in a sub-tropic climate, and with local erosion or deposition of sediments in rivers and shallow lakes.One volcanically silent time period was especially long and resulted in the deposition of several sedimentary layers of varied composition, including strata rich in organic material that subsequently have generated considerable volumes of coal.