International Value of the potential Al-Medina volcanic Global Geopark in Saudi Arabia revealed from multi-satellite remote sensing data

The Global Geopark must be of international importance in terms of their scientific quality, rarity, aesthetic appeal and education value. The Saudi Arabia, as one of the important node countries along the Belt and Road, has developed at least nine Cenozoic basaltic volcanic lava fields in the western part, with a total area of 180,000 km 2 . In this paper, the geological and geomorphic features of volcanic lava fields in western Saudi Arabia induced by the expansion of the Red Sea are interpreted using multi-source satellite images, such as Moderate-resolution imaging spectroradiometer (MODIS), Landsat-8 OLI (Operational Land Imager) and Gaofen-2 data. Our results show that the Al-Medina volcanic field (AMVF) has great advantages to become the potential volcanic Global Geopark in Saudi Arabia. This study indicates that : 1) AMVF has special international geo-scientific significance and rare natural attribute in geodynamic mechanism and lithologic characteristics; 2) AMVF has abundant volcanic landscapes, including completely preserved cones and craters and multi-phase lava flows from different eruption stages, which have great aesthetic appreciation and education values to attract the general public; 3) AMVF is close to the second holy city of Islam-Medina city, which has convenient transportation and is suitable to develop tourism to promote the local economy development; 4) AMVF had erupted repeatedly (the latest eruption was in 1256 AD), establishment of the Global Geopark can not only prevent potential geological hazard risks to Medina city from future volcanic eruptions, but also provide better protection and conservation to geoheritage sites being damaged by human activities.


Introduction
. The distribution of Global Geoparks around the world is presented in Fig.1. It shows that current Global Geoparks are mainly distributed in southwestern Europe and eastern Asia. Figure 1 The distribution of UNESCO designated Global Geoparks around the world [2] The Saudi Arabia, as one of the important node countries along the Belt and Road, has a land area of 2,250,000 km 2 , ranking the 14 th in the world. Induced by the expansion of the Red Sea, Saudi Arabia has developed at least nine Cenozoic basaltic volcanic lava fields in the western part, with a total area of 180,000 km 2 . However, 3 there is still no geoheritage site in Saudi Arabia has been inscribed on UNESCO Global Geoparks.
According to the criteria for UNESCO Global Geoparks [3] , we interpreted the geological and geomorphic features of volcanic lava fields in western Saudi Arabia by using multi-source satellite images, including Landsat-8 OLI and Gaofen-2 data, in this study. We attempt to explore the international value of the Al-Medina volcanic field (AMVF) in Saudi Arabia according to the selection criteria of UNESCO Global Geoparks, and evaluate whether it has great advantages becoming the potential volcanic Global Geopark.
Following the introduction, detailed description of the study area Saudi Arabia is presented in Section 2. In Section 3, the methodology to interpret the geological and geomorphic features of Al-Medina volcanic lava field by using multi-source satellite images and the results are described. In Section 4, we briefly discussed the reasons for AMVF whether be able to become a potential volcanic Global Geopark. The conclusions are drawn in the last section. Saudi Arabia occupies most of the Arabian Peninsula, topographically which is higher in west and lower in east. Along the Red Sea, there is a "backbone" of the Arabian Peninsula, named "Tuwaiq Mountain chain", extending in a north-south direction in general. In east of the mountain, the terrain gradually declines until the eastern plain, where a large portion covered by sand dunes and rocky regolith [4] .
The Red Sea Rift, a 2000 km-long NNW-SSE striking depression, forming a broad zone of active deformation between Africa and Arabia [5] . Rifting of the Rea Sea began about 30 Ma ago, separating the western edge of the Arabian Plate from Africa [6] ，as shown in Fig.3. Following that, the late Cenozoic geodynamic evolution of the Arabian Shield area was mainly controlled by (1) extensional processes as a consequence of the complex kinematic interactions that developed the Red Sea Basin [7][8][9][10] , which are moving the Arabian Plate towards NE with a speed of 1.6-2 cm/yr. 5 [11][12][13] ; (2) continental collision between Arabia and Eurasia to east since the middle Miocene [14][15][16][17] . In response to this complex tectonics of the Arabian Plate, a series of large Cenozoic volcanic fields and volcanic rocks mainly developed along its western margin [19][20][21][22][23][24][25][26] . At least nine intracontinental Cenozoic basaltic lava fields parallel to the Red Sea Rift are developed in western Arabian Shield (Fig.2), with a total area of 180,000 km 2 . These volcanoes and lava fields form a 600 km-long unique volcano chain from north to south in western Arabian Shield, named "Mecca-Medina-Nafud Volcano Line (MMN Line)". Among the volcanoes, the Al-Medina volcanic field (AMVF), part of the Harrat Rahat in northwest of the Arabian Plate, is the worth 6 notable active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD [5] .

Interpretation of AMVF by remote sensing data
Based on the geological background, the geological and geomorphic features of the Al-Medina volcanic lava field in western Saudi Arabia are interpreted using Landsat-8 OLI (Operational Land Imager) and Gaofen-2 data, respectively.  Table   1.   Table 2. In this paper, we collected two Level 1-A GF-2 remote sensing images over AMVF area which were acquired on February 10, 2018.

Interpretation of the Landsat-8 data of AMVF
The spectral reflection characteristics of volcanic lavas with different lithological components or eruption periods are different in the remote sensing image. According to the previous study, the spectral reflectance of different weathering degree basalt volcanic lavas differs obviously in thermal infrared bands [27] . Therefore, Band 10 or  Fig.4. The pseudo-color synthesis of the Landsat-8 data shows that the volcanic lavas of Medina volcanic field can be divided into at least three periods: 1) the dark red part 9 (marked by a in Fig.4); 2) the black part (b in Fig.4) and 3) the lighter red part (c in Fig.4).

Interpretation of the GF-2 data of AMVF
In order to judge the eruption periods of the Medina volcanic lavas more clearly, we obtained the Gaofen-2 data of the same area (Fig.5). The pseudo-color synthesis of the Gaofen-2 data shows that volcanic landscapes are widely distributed with different colors in Medina volcanic field, and it is speculated that at least four eruption periods can be identified: 1) the purple part 10 (numbered 1 in Fig.5); 2) the dark green part (2 in Fig.5); 3) the lighter green part (3 in Fig.5) and 4) the off-white part (4 in Fig.5). There are about 50 craters of various shapes in the Medina volcanic field, and the most of them are cone-shaped, while some are shield-shaped craters.
Combining the results of previous research [5,19,28] , we analyzed the GF-2 data of the Medina volcanic field and generated a 1:50,000 volcanic geological interpretation maps, as shown in Fig.6. The four eruption periods are interpreted as following.
1) the red pattern (numbered 1) in Fig.6, corresponding to the purple part (1 in    [29] can be seen clearly in some craters as shown in Fig.9, which is a unique volcanic type among the world.

Discussion
The interpretation results of multi-source satellite images indicate that the Medina volcanic field in western Saudi Arabia has the potential and advantage to become a volcanic Global Geopark for its uniqueness compared with other existing volcanic Global Geoparks: 1) Tectonically, the Medina volcanic field belongs to the hot rising mantle plume formed by the expansion of the Red Sea after the continental collision between Arabia and Eurasia to east since the middle Miocene (Fig.3). Its geodynamic mechanism is completely different with the back-arc volcanoes, such as the Japanese islands, and the intracontinental volcanoes, such as the Wudalianchi volcanoes in China [27] . The former are induced by the collision between oceanic and continental plates [18,30] , while the Wudalianchi volcanoes are formed by the deep subduction and dehydration of the west Pacific stagnant slab, possibly through hot and wet upwelling in the big mantle wedge under the NE China [27,[31][32][33][34] .
2) Concerning the volcanic landscape, Medina volcanic field has developed abundant volcano lavas, including completely preserved scoria cones and craters, as well as multi-phase lava flows. Compared with the existing volcanic Global Geoparks, such as Wudalianchi Global Geopark [27] and Leiqiong Global Geopark [35] , the AMVF not only has kinds of well-conserved volcanic geological sites, but also its

Conclusions
The interpretations and analyses of geologic and geomorphic features of volcanic lava fields based on multi-source remote sensing data can effectively identify the scientific, aesthetic and educational values of the potential volcanic Global Geopark.
And help to recognize the suitable location and boundary of the potential Global Geopark. The results indicate that the Medina volcanic field in western Saudi Arabia has the potential and advantage to become a volcanic Global Geopark due to its unique international geo-scientific significance and rare natural value in geodynamic mechanism, volcanic landscape and lithologic characteristics. Furthermore, the AMVF has great aesthetic appreciation and education values to attract the general

Availability of data and materials
The Landsat-8 remote sensing data used in this study are available in the https://www.usgs.gov/.

Competing interests
The authors declare no conflict of interest.