关键词: 晚更新世, 射气岩浆喷发, 低平火山口, 基浪堆积物, 琼北

Abstract: This study focuses on the northern Hainan volcanoes. The volcanic region belongs to the southern part of the Leiqiong depression, which experienced extensive subsidence during the Cenozoic. The surface of this area is covered with basalts and Quaternary sediments. Cenozoic volcanic rocks cover about 12 percent of the whole Hainan Island. In general, NW-trending fractures have controlled the volcanic activities in this area since Holocene. According to the published data, the volcanic activity in northern Hainan Island since Late Pleistocene was divided into Daotang period (Late Pleistocene) and Shishan period (Holocene). Our field investigation on volcanic activities has revealed that after the effusive eruption in early Pleistocene, considerable scale phreatomagmatic eruptions occurred during late Pleistocene, and then the typical central eruptions occurred at the beginning of Holocene. Late Pleistocene is the most active period of phreatomagmatic eruptions in Northern Hainan Island, while phreatomagmatic eruptions were the important eruptive character before the Holocene volcanic activity in this area. Phreatomagmatic eruptions may produce base-surges, and base-surges are usually accompanied with "maar". Maar is originally the name that the local people of Rhine, Germany called the lake and swamp filled with water in the region. In 1921, Steininger, a Germanic scientist, defined maar as a kind of volcanoes when he studied a Quaternary small round crater at Eifel, western Germany. Chinese scholar Liu Jiaqi had researched some maars within Chinese territory during the past several years, he pointed out that maar is something caused by phreatomagmatic eruptions, and its shape is similar to some other volcanic apparatus. Maar is a kind of familiar volcanic physiognomy besides volcanic cone. Some typical maars concentrate in Longgang (Northeast China) and Leiqiong (South China) regions, which are Cenozoic volcanic area. In Longgang region, there are eight maars caused by phreatomagmatic eruptions. Local people call these maars as "Longwan", such as big Longwan, triangle Longwan, south Longwan, east Longwan and so on. There are also several maars located in Leizhou Peninsula, such as Tianyang, Jiu Touyang, Qing Tongyang and Hu Guangyan. Some native geologists have begun to study maar and have made remarkable progress. The study of phreatomagmatic eruptions and its products is an important branch of volcanology, while the northern Hainan Island provides a good position to study this phenomenon. A number of maars with typical base-surges are caused by late Pleistocene phreatomagmatic eruptions in northern Hainan Island. Typical phenomena caused by phreatomagmatic eruptions can be observed around these maars, such as large-scale and low-angle cross-bedding, slaty-bedding, current-bedding and distal facies accretionary lapilli. If basaltic red-hot magma ascends and meets with the ground water near the earth's surface or surface water, explosion must occur, resulting in the formation of base-surge deposits. This process creates Base surge. In order to get better understanding of the modern volcanic activity and tectonic environment in the northern Hainan Island and southern Leizhou Peninsula, great attention should be paid to the study of base-surge in this area. Moreover, the study of base-surge may provide significant evidence about future volcanic hazard.

Key words: late Pleistocene, phreatomagmatic eruptions, Maar, base-surge, northern Hainan Island