Abstract
Monogenetic volcanoes are a type of volcano compared to polygenetic volcanoes, which are characterized by smaller volumes and shorter eruptions. Their appearance features are often flat-topped cones, low-relief craters and lava flows. In many areas, they often appear in clusters to form unique landscapes, such as in Longwanqun National Forest Park, NE China. The area where they cluster is called a monogenetic volcanic field. However, they pose no less risk of geological hazards than polygenetic volcanoes. Because more and more studies have discovered their complex volcanic origins, plumbing systems and eruption styles, it has become more difficult to study the factors that control their formation and predict their eruptions. They may have occurred at the same time as polygenetic volcanoes in many areas, which means that they may be related to polygenetic volcanoes in these areas. This thesis used geomorphological, spatial, age, and geochemical information of monogenetic volcanoes to explore the development and evolution of monogenetic volcanic systems.
Firstly, in order to better utilize the free Digital Elevation Models (DEMs) covering the world to extract geomorphological data of monogenetic volcanoes, two DEM datasets with the best quality (30 m resolution), i.e., Shuttle Radar Topography Mission (SRTM), Advanced Land Observing Satellite (ALOS) World 3D 30 m (AW3D30) were analysed for their accuracies in four different monogenetic volcanic fields using four different higher resolution reference DEMs. AW3D30 was tested to be the most valuable data with the best accuracies in terms of elevation, slope angle and monogenetic volcanic morphometric parameters. Although AW3D30 is affected by slope angle, volcanic volume, and morphological features of scoria cones similarly to SRTM by these factors, it is recommended to be the preferred DEM in remote or poor areas where no other higher quality elevation data is available. This study has important implications for advancing the use of DEM data to study monogenetic volcanoes or other small landforms, e.g., dunes, landslides/rockfalls, moraines/drumlins. Furthermore, how the volumes of individual monogenetic volcanoes are estimated and the factors that affect these volumes were studied. Scoria cone base diameter (Wco) is the best parameter to represent the volume of the cone and its associated lava flow. The volume of a scoria cone can be roughly obtained through Wco, which is a convenient and highly accurate way based on various elevation data or satellite images. There are many factors that affect the eruption volume of a monogenetic volcano, including tectonic settings, tectonic regimes, crust thickness, magma physical and chemical properties, pre-existing fault systems, and local stresses. Conditions that are conducive to magma eruption include subduction settings, thicker crust, a lower density of magma and pre-existing faults with extensional/strike-slip local stress. In addition, in Changbaishan Volcanic Area, NE China-North Korea, a standalone monogenetic volcanic field (Longgang Volcanic Field) and a monogenetic volcanic area associated with a polygenetic volcano (Tianchi) were studied to explore their similarities and differences and relation to the polygenetic volcano. It is found that the standalone volcanic field was strongly controlled by tectonic factors, while the monogenetic volcanoes associated with the polygenetic volcano were strongly controlled by magmatic factors, which indicates that the development of monogenetic volcanoes in different volcanic systems of the same area might be controlled by different factors. Finally, remnants of monogenetic and polygenetic volcanoes in the Dunedin Volcanic Group, Otago, New Zealand were used to study the transition of different volcanic types. Geochemical and spatial-temporal classification analyses suggest that the development of the volcanic system in this area has three directions, i.e., crustal differentiated, mantle differentiated and undifferentiated, which was likely controlled by both tectonic and magmatic factors. The increase in magma supply and the transition from an extensional environment to a compressional environment are the reasons for the shift of monogenetic volcanoes to polygenetic ones.
This study suggests that the developments of different volcanic types are affected by different factors in different tectonic settings. In subduction settings, volcano types are mainly controlled by magmatic factors, in rift settings, they are mainly controlled by tectonic factors, and in intraplate settings, they may be controlled by both tectonic and magmatic factors. This provides insights into the development of global magmatic volcanic systems and volcanic risk assessment.