关键词: 数字高程模型, 火山锥体, 喷发机制, 晚第四纪, 琼北

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