# The Survey of Lava Tube Distribution in Jeju Island by Multi-Source Data Fusion
## 2 Test Site and Data Sets  
### Geological Background of Lava Caves  
Jeju Island was overall investigated in this study, where lava tubes distributed throughout the island are shown in Figure 1. The United Nations Educational, Scientific, and Cultural Organization (UNESCO) evaluated the unique geological landscape and designated nine geological attractions in 2010 and three more in 2014. The presence of lava tubes and involved volcanic cones are highly distinguished among them. It is worth noting that there are conflicting theories about the origin of volcanism on Jeju Island. One theory interprets the entire island as a shield volcano [33], while another suggests that it is a composite volcano [34; 35; 36]. Also, according to a study conducted by [37], Jeju Island was formed based on the accumulation of outpoured lava flows and pyroclastic rocks erupting from a polygenetic composite volcano. The materials were intercalated or underlain by the Seogwipo Formation [38]. According to the extensive fieldwork and interpretation of stratigraphy, the alkali-basalts lava effusion on land started about 1 Ma years ago and continued until the Holocene [39; 40; 41; 37]. Furthermore, [37] demonstrated that there have been extensive post-depositional volcanic activities on land after the termination of sedimentation of the Seogwipo Formation since about 0.5 Ma. Such studies conclusively proposed that volcanic eruptions from 0.3 Ma to 0.1 Ma must have shaped the body of Jeju Island. Also, volcanic eruptions from 0.1 Ma to the Holocene constructed the present topography of Jeju Island and its various types of lava tubes. The rocks distributed at lower land areas in the eastern and western regions are predominantly transitional basalts, tholeiitic basalts, and andesite in composition, compared with those in the northern and southern regions. This petrological evidence proposed that lava tubes were primarily formed due to volcanic activities in 1.88-0.5 Ma and the post-depositional period (0.5 Ma-Holocene) [37; 42; 43], which implies that the lava tubes might have originated from the numerous lava events in the Quaternary (see Figure 1a). Although the number of lava tubes is different and has gradually increased according to survey works, Jeju volcanic Island exhibited more than 179 well-documented lava tubes, including 144 lava tubes and 35 sea caves since February 2016 [18]. Among these 179 caves, 122 caves were scattered in the eastern (22%) and western areas (51%) of Jeju City and the eastern areas of Seogwipo (19%). A total of 200 lava tubes have been identified in Jeju Island in the recent research conducted by [16]. It has been proven that lava tubes only exist with basalt low-viscosity lava flow, known as pahoehoe lava (see Q1 unit in Figure 1b). However, the progressive discovery of lava tubes since 1990 somehow conflicted with such findings. Some major lava tubes cross the basaltic andesite lava flow, known as 'a' lava, presented as Q2 in Figure 1b, and have far higher viscosity (refer to [5; 14; 44] for a detailed explanation of the evolution processes of Jeju lava tubes). Lava tubes are usually connected to cinder cones, as shown in Manjang cave and Geomun Orum cinder cones systems illustrated in Figure 1c. Photos and TLS results in Figure 1d present a few lava tubes morphologies, including shallow caves populated in 'a'a lava flows (in Geamcheon and Chogiwak caves) and other ordinary cave developments in pahoehoe lava flow (in Gammandap and Manjang cave, see following sections). Research conducted by [44] showed that 27 of the 37 examined lava tubes faced severe collapse and destruction problems. Most lava tubes in Jeju Island show that internal and/or external failure and destruction problems resulted from disconnected lava tubes. The investigation was conducted on 27 out of 179 roads over the lava tube.