## 1 Introduction  
### Mapping Lava Flows  
Lava flow-forming eruptions are a frequent hazard that impacts populations and properties at a number of volcanoes worldwide [1; 2; 3; 4]. Mapping of active lava flows is therefore a valuable source of information to document the ongoing eruptions [5], while yielding insights into how lava flow fields develop that are necessary for predicting future flow behavior [6; 7; 8; 9; 10]. Although lava fields are mapped usually through ground-based surveys [11; 12] or more recently with Unmanned Aerial Vehicles (UAV) in the air [13; 14], satellite remote sensing offers an overall view of active lava flows, while avoiding the difficulties of working in hazardous locations [15; 16]. The areal extent of active lava flows is challenging to record with traditional field methods because of the large size of flow fields and the often-limited accessibility of volcanic areas. Moreover, analyzingsmall portions of a lava flow field may give unrepresentative results of the entire lava flow during the course of the eruption [17]. Thus, remote sensing techniques are considered a safer and more robust alternative approach capable of providing a more comprehensive survey of the whole lava flow field, which is essential when attempting to monitor lava flow hazards during volcanic eruptions [18; 19]. The multispectral nature of the data and the repeated coverage of extensive volcanic terrains are major advantages of satellite remote sensing techniques [20]. An ever-increasing number of space borne sensors continue to provide a wealth of high-quality multispectral data. Optical images from satellites have been widely exploited to map lava flows and their associated spectral properties [21; 22; 23]. Satellite data in the visible, infrared, and thermal portions of the electromagnetic spectrum contain reflectance and emittance information about lava surfaces of different age and nature [18; 24; 25], and therefore can be used to discriminate fresh lava flows in terms of mineralogical, textural and compositional differences [26; 27]. However, in optical images, it can be difficult to distinguish between weathered lava surfaces and assess their relative age due to the similarity in their chemical and surface characteristics. Furthermore, using optical satellite imagery requires that affected area be clouds-free and this often does not apply. Radar satellite data provide a solution to this visibility disadvantage by enabling the detection of morphological changes associated with volcanic activity at any time, including nighttime or when the area of interest is covered by clouds [28; 29]. However, the mapping of extensive lava flows from satellite radar data requires specific acquisition strategies. A widely-adopted strategy is based on the coherence analysis of Synthetic Aperture Radar (SAR) images, where two images recorded at two different times are compared and the lava flows are identified from low coherence areas [30; 31; 32]. An alternative strategy is based on the combination of SAR images with visible, infrared, and thermal satellite data [33; 34]. However, using SAR images presents some disadvantages, such as the ambiguity of what de-correlated regions represent, persistent decorrelation masking new activity, and problems with mapping flows that enter vegetation [31]. Another remote-sensing technique that can be used to extract recent lava flow fields is differencing of pre-eruptive and post-eruptive digital elevation models (DEMs) produced using high-resolution stereo optical imagery [15; 35; 36]. Unfortunately, pre-eruption high-resolution topography is rarely available at poorly monitored volcanoes, and lava flow fields can only be detected if their thickness is higher than the minimum uncertainty associated with the DEMs [22]; therefore, this technique is generally not suitable during effusive crises.