Abstract:

More than one hundred volcanoes erupted in the northern Hainan Island(NHI)during the Pleistocene and Holocene. With varied volcanic eruption types, and well-preserved volcanic edifice, these volcanoes are ideal for studying the Quaternary volcanic geology and cone morphology. We selected a total of 38 volcanoes in NHI with eruption age less than 200000 years as samples. Based on the unmanned aerial vehicle(UAV)tilt photography technology, we got the high-precision and high-resolution digital orthophoto map(DOM)and digital elevation model(DEM)data for these volcanoes. Combined with Google Earth high-resolution remote sensing satellite image and ALSO(12.5m)digital elevation model(DEM)data, these data provide the way to quantitatively measure and study the high-precision cone morphology. Previously, the method of Hco(height of cones)/Wco(cone base diameter)is commonly used to analyze the shape of cone, which is difficult to extract the information of irregular cones. When the volcanic cone is non-circular, or the elevation of crater rims is not uniform, the morphology parameters from artificial extraction have poor reliability. So, this paper proposes a new method, the ‘Sco^h(horizontal sectional area of volcanic cones)vs H^h(vertical distance from the top of the cone to the cross section)' to accurately analyze the morphology change of cones in northern Hainan Island. This method has two advantages. Firstly, it can deal with cones of any shape to avoid artificial errors. Secondly, it can conveniently filter to the greatest extent the interference factors such as uneven weathering and cone undulation caused by surface vegetation, so that the original morphological characteristics of the cone can be quantitatively displayed. Applying the above method, we accurately analyzed the cone morphology in the NHI. It is indicated that volcanic cones could be divided into three groups in the 'Sco^h vs H^h' projection diagram. The first group with the average(H^h/Sco^h)^h>3m=0~0.6, and about 22% of the cones belong to this type. The second group with the average(H^h/Sco^h)^h>3m=1.0~6.0, and about 65% of the cones in the area belong to this group. The third group with the average(H^h/Sco^h)^h>3m=4.0~12.0, and about 13% of cones belong to this group. Field geological survey confirmed that the first group volcanic cone is composed of volcanic tuff with parallel bedding, as revealed by the Luojingpan volcano, which is a tuff ring cone. Phreatomagmatic eruption is the major volcanism in this group. The second group of volcanic cone is composed of loose scoria or agglomerate, as represented by Maanling volcano. The Strombolian-Hawaii volcanic eruption produced this type of cones. The third group of cones is similar to lava dome and made of block lavas with diameter usually greater than 30cm, as exemplified by Bijialing volcano; therefore it is inferred to be the magma extrusion origin. All lines of evidence indicate that volcanic cones formed by different eruption types exhibit different morphological characteristics, which are accurately reflected in the ‘Sco^h vs H^h’ diagram. Therefore, it is possible to use high-precision DEM data of volcanic cone to constrain volcanic eruption type through this method.

Key words: digital elevation model, volcanic cone, eruption mechanism, Late Quaternary, Qiongbei

摘要：

文中以琼北峨蔓、 海口等地200ka以来喷发形成的38座火山为研究对象, 基于Google Earth高分辨率遥感卫星影像及ALSO(12.5m)数字高程模型(DEM)数据, 并基于无人机倾斜摄影测量生成的高精度、 高分辨率数字正射像图(DOM)和数字高程模型(DEM)数据, 详细测量了锥体形态, 对其形态进行定性和定量研究。文中提出了一种新的方法: “Sco^h(火山锥水平截面积)-H^h(锥体山顶至截面垂直距离)法”, 可精确分析琼北锥体形态上的差异。研究表明, 在Sco^h-H^h投图中, 根据曲线的不同, 火山锥体总体可分为3类: 1)曲线坡度较小, Average(H^h/Sco^h)^h>3m=0~0.6, 区内约有22%的锥体属于该类型; 2)曲线坡度较陡, Average(H^h/Sco^h)^h>3m=1.0~6.0, 区内约有65%的锥体属于该类型; 3)曲线坡度最陡, Average(H^h/Sco^h)^h>3m=4.0~12.0, 区内约有13%的锥体属于该类型。野外地质调查证实, 第1类火山锥以罗经盘为代表, 由火山砾、 火山灰、 围岩碎屑构成, 具有平行层理, 属于凝灰岩环(tuff ring)型锥体, 为射汽岩浆喷发(phreatomagmatic erutpion)成因。第2类火山锥以马鞍岭为代表, 由火山弹、 熔岩饼等岩浆爆破成因的火山碎屑组成, 属斯通博利型(Strombolian)-夏威夷型(Hawaii)火山喷发成因。第3类火山锥以笔架岭火山为代表, 多由直径>30cm的熔岩块构成, 推测为侵出成因(extrusion)。研究表明, 不同喷发类型形成的火山锥具有不同的形态特征, 这些形态差异可以准确地反映在Sco^h-H^h曲线中。因此, 利用该方法基于火山锥高精度DEM数据可能能够限定火山喷发的类型, 这对于高效统计确定大型火山群的喷发类型、 评估火山喷发灾害类型和灾害预期提供了一个新的研究思路。

关键词: 数字高程模型, 火山锥体, 喷发机制, 晚第四纪, 琼北