Volcanic Geomorphology: A Review of Worldwide Research

The purpose of this article is to provide an overview of academic research on volcanic geomorphology, through the use of bibliometric analysis and bibliographic visualization maps for the discernment of its growing interest by the academy in the last 30 years. It is sustained on the publications indexed in the Scopus database between 1956 and 2019, obtaining relevant information on scientific production, following the methodological structure of a rigorous bibliometric process, which ranges from the search for descriptors or keywords to the configuration of visualizations of tables and maps that allowed to consider the contributions by authors, institutions, journals and topics that have shaped the evolution of this field of study. The generations of bibliometric maps allowed understanding the intellectual structure of the field of study made up of 707 articles where the analysis of co-occurrence of author keywords showed six main lines of research that, combined with the co-citation maps, allowed understanding the breadth of intellectual structure. Extensive information is provided on the thematic that other investigations partially addressed or failed to capture their current status. Considering that the results allow us to identify areas of current interest and the potential of research in volcanic geomorphology.


Introduction
There is currently a wide interest and concern about the environment and how it works and evolves. The threats of climate change and the extinction of species are commonly highlighted; but what about changes in landscapes? [1]. There are still many unanswered questions about physical landscapes and anthropic events; how to conserve them and manage geological spaces in the best way is a great question [2]. That is why, geomorphology is the science that studies the origin and development of landforms, such as hills, valleys, caves, or volcanic spaces, that seek to explain the morphological processes on the surface, under monitoring and of those that shape the reliefs [3,4] characterized by and its relationship with geography, taking into account that worldwide this branch has been one of the first sub-disciplines to express interest in biophysical aspects for decades. Perhaps through this analysis, it is possible to know which journals and disciplines had the greatest impact on the research flow? Who are the most representative experts in the given field of research? How has the intellectual structure of the subject developed over time? Are international audit documents cited more? What are the subjects associated with this particular line of research?
Based on the above, two main objectives are considered, where the first seeks: To analyze the performance of scientific production related to published articles, researchers, main contributions and journals through a bibliometric analysis for the extraction of new knowledge and applications that contribute to the scientific development. The second is to reveal the intellectual structure of the study area, locating the central themes by co-occurrence and co-citation of cited authors and journals [32]; the purpose of which is to generate a bibliometric study about "volcanic geomorphology", which seeks to contribute significantly to the advancement of research within the geological field. Identifies the scopes that have already been addressed within volcanology, and explore others that have not yet been promoted at the scientific level; but in addition, it seeks to promote trends in topics of great importance for the development of future research that stands out within the scientific branch [33,34].

Materials and Methods
Systematic literature reviews provide a rigorous and formal methodical procedure, which seeks to minimize bias and possible errors when selecting studies to know an area of knowledge [35,36]. Bibliometric studies are considered to contemplate a similar procedure as well [37]. The proposed methodology for bibliometric analysis has been structured, and comprises a sequence of four phases that allow the proposed developed analysis to be carried out: (1) Define the search criteria; keywords and time periods; (2) data compilation; (3) adjustment and refinement of criteria; (4) export and analysis of results in order to know the structure and evolution of geomorphology given its growing interest in academia (see Figure 1).
Geosciences 2020, 10, x FOR PEER REVIEW 3 of 17 flow? Who are the most representative experts in the given field of research? How has the intellectual structure of the subject developed over time? Are international audit documents cited more? What are the subjects associated with this particular line of research? Based on the above, two main objectives are considered, where the first seeks: To analyze the performance of scientific production related to published articles, researchers, main contributions and journals through a bibliometric analysis for the extraction of new knowledge and applications that contribute to the scientific development. The second is to reveal the intellectual structure of the study area, locating the central themes by co-occurrence and co-citation of cited authors and journals [32]; the purpose of which is to generate a bibliometric study about "volcanic geomorphology", which seeks to contribute significantly to the advancement of research within the geological field. Identifies the scopes that have already been addressed within volcanology, and explore others that have not yet been promoted at the scientific level; but in addition, it seeks to promote trends in topics of great importance for the development of future research that stands out within the scientific branch [33,34].

Materials and Methods
Systematic literature reviews provide a rigorous and formal methodical procedure, which seeks to minimize bias and possible errors when selecting studies to know an area of knowledge [35,36]. Bibliometric studies are considered to contemplate a similar procedure as well [37]. The proposed methodology for bibliometric analysis has been structured, and comprises a sequence of four phases that allow the proposed developed analysis to be carried out: (1) Define the search criteria; keywords and time periods; (2) data compilation; (3) adjustment and refinement of criteria; (4) export and analysis of results in order to know the structure and evolution of geomorphology given its growing interest in academia (see Figure 1).

Figure 1.
Scheme of the methodology applied in this study.

Defining Search Criteria
The words used as search parameters consider the terms geomorphology and volcano as referents of the study area [38], those that are in common use within the volcanological field [39]. The selection of these terms allowed the compilation of the base documents, for the proposed analyzes.

Defining Search Criteria
The words used as search parameters consider the terms geomorphology and volcano as referents of the study area [38], those that are in common use within the volcanological field [39]. The selection of these terms allowed the compilation of the base documents, for the proposed analyzes.

Data Compilation
The search was carried out in March 2020, which was based on the compilation of the information on "volcanic geomorphology" through the Scopus scientific database, considered as a viable alternative when selecting a database of peer-reviewed scientific literature [40] that analyzes the behavior of citations received by journals and generates indicators that evaluate the performance of journals, authors, research groups, universities, countries and even regions of the world [41,42].
A search strategy was used considering all the documents that contain the keywords in the titles, abstracts, and keywords. The terms were used combined with a series of Boolean operators and symbols that allow the refinement of the exploration and offer more adjusted results [43]. The search is presented as follows: (TITLE-ABS-KEY ("geomorphology") AND TITLE-ABS-KEY (volcano*)), descriptors that allow us to focus on the scope of volcanic geomorphology in a specific way, resulting in a total of 1001 documents found.

Adjustment and Refinement of Criteria
The data was refined by means of inclusion and exclusion criteria of the information. The "Earth and Planetary Sciences" area and the English language were selected for their frequent use in academia. Additionally, articles are selected (article, article in press), because the research papers are presented by one or more specialists and evaluated by peers [44]. Additionally, documents published until 2019 were considered. The resulting search equation applied is: (EXCLUDE (PUBYEAR, 2020)) AND (LIMIT-TO (DOCTYPE, "ar")) AND (LIMIT-TO (SUBJAREA, "EART")) AND (LIMIT-TO LANGUAGE, "English"). Through this process, 709 documents were obtained.

Export and Analysis of Data
The records obtained from Scopus were exported in CSV (comma-separated values) format, containing data on bibliographic information, abstract, keywords, among others, which are useful when studied through bibliometric analysis [45]. This downloaded information from bibliographic sources normally contains errors, requiring a data normalization process. The process consists of verifying the information to rule out erroneous or incomplete data [46]. This process can be done by combining Microsoft Excel and Vosviewer software. The first allows a manual review of the data of authors, journal titles or affiliations, which consolidates the data collection in 707 documents. This refined database makes it possible to consolidate the use of the second software for the review of the units of analysis prior to the construction of the bibliometric maps. In the co-occurrence map, it is necessary that some keywords be integrated into acronyms and single and plural forms and in the co-citation maps the names of the Journals and the author's name should be standardized if necessary (thouret, j.-c. by thouret, jc) [47].
Vosviewer software was used to generate the bibliometric maps. This software allows the construction, exploration and graphic representation of easy-to-interpret two-dimensional maps [46,48], allowing to reveal the intellectual structure of the field of study using bibliometric networks recognized in the academic world, such as author co-occurrence maps, author co-citation and journal co-citation [49]. The Vosviewer has contributed notably in the development of bibliometric analyzes in various areas of knowledge in areas of earth science [50,51] management [52], and health [53].

Analysis of Scientific Production
The analysis of the results of the advanced search after cleaning the data of the geological term called "volcanic geomorphology", indicates that the study of this line of research has resulted in a total of 707 documents produced from 1956 to 2019, the same ones that for analysis purposes we have divided into three time periods, such as introduction , growth (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), and maturity (2010-2019). Likewise, the analysis of productivity has been proposed of a field of study using Price's law, which has served to evaluate the increase in production, adjusting to exponential growth [54]. Obtaining, as a result, the equation y = 2E−68e 0.0792x , where the value of R 2 (coefficient of determination) is 0.86, revealing the quality of the representative power of the function, therefore, we can say that the research on "volcanic geomorphology". It is in the phase of exponential growth, which guarantees that it is a growing research field (see Figure 2) Geosciences 2020, 10, x FOR PEER REVIEW 5 of 17 can say that the research on "volcanic geomorphology". It is in the phase of exponential growth, which guarantees that it is a growing research field (see Figure 2) In this period, 90 scientific articles were produced, representing 12.73% of the total. Its last three years record a higher production, reaching a peak in 1997 with 11 items; where the document "The structure and geomorphology of the Dashgil mud volcano, Azerbaijan" is highlighted [55], the work of one of the most representative authors in the area. Additionally, in this period, the citations obtained were 4261 (22.45%), announcing the beginning of this area of research that expanded over time [56]. Other articles featured in this period cover topics, such as "[k]eys to analyze active lahars from Pinatubo on SAR ERS imagery" [57] and "[t]he effect of collapse structures on ground deformations in calderas" [58].

Growth (1999-2009): Geomorphological Evolution
In this growth period, scientific production increased considerably in 242 articles (34.23%) and 9713 citations (51.19%). This demonstrated a growing interest in the subject of volcanic geomorphology in academic literature. In 2009 it reached the maximum production of the period with 33 articles; highlighting "LIDAR strip adjustment: Application to volcanic areas" [59], work that proposes a modality to analyze volcanic complexes through the adjustment of the Lidar strip. Other articles featured in this period were: "The topography and morphology of low shields and associated landforms of plains volcanism in the Tharsis" [60] and "Geomorphic and geologic settings of the Phoenix Lander mission landing site" [61].  In this period, 90 scientific articles were produced, representing 12.73% of the total. Its last three years record a higher production, reaching a peak in 1997 with 11 items; where the document "The structure and geomorphology of the Dashgil mud volcano, Azerbaijan" is highlighted [55], the work of one of the most representative authors in the area. Additionally, in this period, the citations obtained were 4261 (22.45%), announcing the beginning of this area of research that expanded over time [56]. Other articles featured in this period cover topics, such as "[k]eys to analyze active lahars from Pinatubo on SAR ERS imagery" [57] and "[t]he effect of collapse structures on ground deformations in calderas" [58].

Growth (1999-2009): Geomorphological Evolution
In this growth period, scientific production increased considerably in 242 articles (34.23%) and 9713 citations (51.19%). This demonstrated a growing interest in the subject of volcanic geomorphology in academic literature. In 2009 it reached the maximum production of the period with 33 articles; highlighting "LIDAR strip adjustment: Application to volcanic areas" [59], work that proposes a modality to analyze volcanic complexes through the adjustment of the Lidar strip. Other articles featured in this period were: "The topography and morphology of low shields and associated landforms of plains volcanism in the Tharsis" [60] and "Geomorphic and geologic settings of the Phoenix Lander mission landing site" [61].

Maturation (2010-2019): Peak of Volcanic Geomorphology
Finally, in the maturation period, the scientific production reached a total of 375 articles, representing 53.04% of the total, where its peak was registered in 2012 with 44 publications. The featured article of the period is "Circular geomorphology and geological features in the Japanese islands" [62], generating interest in the scientific world by studying the geomorphology of circular islands on the Asian continent, and analysis of the seismic and volcanic risks of them. The period registers 5002 citations, which represent 26.36% of the total citations, showing the apogee of the topic and its breadth. Other articles featured in this period "Environmental changes across the Triassic-Jurassic boundary and coeval volcanism inferred from elemental geochemistry and mineralogy in the Kendlbachgraben section (Northern Calcareous Alps, Austria)" [63] and "Eruption of kimberlite magmas: Physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on Earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)" [64].

Contribution of the Main Authors
The 707 publications have been written by 1725 authors. Table 1 shows the 15 main authors with the highest production of articles in the study area, highlighting Németh Károly from Massey University Manawatu, Palmerston North (New Zealand), with 13 articles, among which "Volcanic structures and oral traditions of volcanism" are distinguished. of Western Samoa (SW Pacific) and their implications for hazard education" [65], "Reconstructing paleoenvironment, eruption mechanism and paleomorphology of the Pliocene Pula maar (Hungary)" [66] and "Facies architecture of an isolated long-lived, nested polygenetic silicic tuff ring erupted in a braided river system: The Los Loros volcano, Mendoza, Argentina" [67]. Abbreviations: AU = Authors; AT = Articles; AF = Affiliations; CO = Country; CI = Citations; HI = H-Index.
The second place of production corresponds to the author Lahitte P., from Geoscience Paris Sud (France), with nine published articles. Highlights the work "DEM-based reconstruction of southern Basse-Terre volcanoes (Guadeloupe archipelago, FWI): Contribution to the Lesser Antilles Arc construction rates and magma production" [68]. Additionally, Table 1 presents the H-Index as an estimate of the impact of the contributions made by a researcher where Baker V.R. presents the highest H-Index (58), followed by Martí J. (43) and Németh K. (34).

Frequently Cited Documents
The use of document citation analysis evaluates its impact in a certain field of study [69]. Table 2 shows the first 15

Country and Region Contributions
The contribution of the different countries was established considering the affiliation of each of the authors of the articles. The study topic has been developed in 73 countries, but research has been concentrated in 15 countries, representing 20.55% of the total (see Table 3). Seven European countries, three Asian and American, and two countries in Oceania are ranked as the most productive. The fifteen main countries belong to the group of developed countries, showing a dominant position in volcanic geomorphology, demonstrating their economic capabilities and levels of research. The United States is the first country in the ranking with 217 documents and 6411 citations. The second place corresponds to Italy with 115 documents, but third in citations. The countries that occupy the third and fourth place with the highest production are France and the United Kingdom with 77 articles.

Author Keyword Co-Occurrence Network
It is a content analysis technique that uses author keywords to construct semantic visual maps that reveal the cognitive structure of the investigated area [45,85]. For this purpose, the VOSviewer software was used to prepare the bibliometric map using co-occurrence data and its association strength [22], managing to represent and analyze the information as an alternative to traditional techniques of multidimensional representation and network visualization by combining visualization and clustering techniques favoring analysis [86]. A total of 1266 keywords provided by the author were extracted from the information base. These words co-occurred at least five times. The network shows 31 keywords that make up six clusters (Figure 3), in which the relationships between words are represented by graphs that were connected by means of nodes and links. The nodes represent the keywords, and their size is related to the number of times the keyword appears in the documents, that is, a larger size reflects a higher frequency of appearance (and vice versa). The links (edges) indicate the relationships between a pair of nodes and the strength of this relationship is observed in the width of the link, where a greater it implies a greater relationship [46].

Author Keyword Co-Occurrence Network
It is a content analysis technique that uses author keywords to construct semantic visual maps that reveal the cognitive structure of the investigated area [45,85]. For this purpose, the VOSviewer software was used to prepare the bibliometric map using co-occurrence data and its association strength [22], managing to represent and analyze the information as an alternative to traditional techniques of multidimensional representation and network visualization by combining visualization and clustering techniques favoring analysis [86]. A total of 1266 keywords provided by the author were extracted from the information base. These words co-occurred at least five times. The network shows 31 keywords that make up six clusters (Figure 3), in which the relationships between words are represented by graphs that were connected by means of nodes and links. The nodes represent the keywords, and their size is related to the number of times the keyword appears in the documents, that is, a larger size reflects a higher frequency of appearance (and vice versa). The links (edges) indicate the relationships between a pair of nodes and the strength of this relationship is observed in the width of the link, where a greater it implies a greater relationship [46].  The most developed research area is Cluster 3 (blue) called "Geomorphology and tectonism" (see Table S1). A cluster consisting of six nodes with 117 co-occurrences, where his research focuses on knowing the determined age of geomorphological structures [87] and its tectonic implications [88], having as keywords "geomorphology", "geochronology" and "stratigraphy". Cluster 1 "Topographical methods" consists of eight nodes and 112 co-occurrences, a cluster that highlights topographic, tectonic and volcanic detection methods through photogrammetry [89], and remote sensors [90], with prominent keywords, such as "digital elevation model", and "photogrammetry".
Others, smaller clusters are: Cluster 2 (green) "Volcanic Products", made up of eight nodes and 92 co-occurrences, showing studies related to the types of geological materials [91] that have arisen as a product of volcanic processes [14]; where the keywords that stand out are: "maar", "lava flow", "scoria cone". Cluster 4 (yellow) "Volcanism" consists of four nodes and presents 75 co-occurrences. In this cluster, the studies are focused on volcanism [92], knowledge and detection of land relief modifications [93] and the conformation of new geological structures through geotechnical processes [94]; where the most representative keywords of the cluster are "volcanism" and "morphometry". Cluster 5 (purple) "Pyroclastic flows" consists of two nodes and 25 co-occurrences, represents studies related to volcanic flows that arise in an eruption [95] and of structures that make up the land relief as a result of geological processes [96].
Featured keywords include "lahar" and "caldera". Finally, Cluster 6 (light blue) "Mass movements" comprises two nodes and 14 co-occurrences, comprising flow-like mass studies [97] and especially modeling tools for large amounts of data linked to a spatial reference [98]. The relevant cluster keywords are "landslides" and "geographic information system" (see Table S1).

Co-citation Network of Cited Authors
This map allows us to study the intellectual structure of the scientific discipline using the authors of the reference works as the unit of analysis, which allows us to reflect the fields of research with the greatest impact [45,49,99]. In the construction of the map, the Vosviewer software and the similarity measurement were used to match the data, called the association force and the fractional counting method [22] 36,748 reference authors were established, determining as a parameter that the minimum number of author citations is at least 20 citations, obtaining 700 authors, who are represented by nodes, exposed in eight clusters (see Figure 4).

Geosciences 2020, 10, x FOR PEER REVIEW 9 of 17
The most developed research area is Cluster 3 (blue) called "Geomorphology and tectonism" (see Table S1). A cluster consisting of six nodes with 117 co-occurrences, where his research focuses on knowing the determined age of geomorphological structures [87] and its tectonic implications [88], having as keywords "geomorphology", "geochronology" and "stratigraphy". Cluster 1 "Topographical methods" consists of eight nodes and 112 co-occurrences, a cluster that highlights topographic, tectonic and volcanic detection methods through photogrammetry [89], and remote sensors [90], with prominent keywords, such as "digital elevation model", and "photogrammetry".
Others, smaller clusters are: Cluster 2 (green) "Volcanic Products", made up of eight nodes and 92 co-occurrences, showing studies related to the types of geological materials [91] that have arisen as a product of volcanic processes [14]; where the keywords that stand out are: "maar", "lava flow", "scoria cone". Cluster 4 (yellow) "Volcanism" consists of four nodes and presents 75 co-occurrences. In this cluster, the studies are focused on volcanism [92], knowledge and detection of land relief modifications [93] and the conformation of new geological structures through geotechnical processes [94]; where the most representative keywords of the cluster are "volcanism" and "morphometry". Cluster 5 (purple) "Pyroclastic flows" consists of two nodes and 25 co-occurrences, represents studies related to volcanic flows that arise in an eruption [95] and of structures that make up the land relief as a result of geological processes [96].
Featured keywords include "lahar" and "caldera". Finally, Cluster 6 (light blue) "Mass movements" comprises two nodes and 14 co-occurrences, comprising flow-like mass studies [97] and especially modeling tools for large amounts of data linked to a spatial reference [98]. The relevant cluster keywords are "landslides" and "geographic information system" (see Table S1).

Co-citation Network of Cited Authors
This map allows us to study the intellectual structure of the scientific discipline using the authors of the reference works as the unit of analysis, which allows us to reflect the fields of research with the greatest impact [45,49,99]. In the construction of the map, the Vosviewer software and the similarity measurement were used to match the data, called the association force and the fractional counting method [22] 36,748 reference authors were established, determining as a parameter that the minimum number of author citations is at least 20 citations, obtaining 700 authors, who are represented by nodes, exposed in eight clusters (see Figure 4).   Table S2).

Co-citation Network of Scientific Journals
This analysis visualizes the shared citations of the references of the journals [100] For this purpose, the Vosviewer software was used to generate the co-citation network, showing those journals that are cited at least 20 times, obtaining 122 journals that are related in six clusters ( Figure 5), where the nodes with the largest size correspond to the journal with the highest number of citations received.  Table S2).

Co-citation Network of Scientific Journals
This analysis visualizes the shared citations of the references of the journals [100] For this purpose, the Vosviewer software was used to generate the co-citation network, showing those journals that are cited at least 20 times, obtaining 122 journals that are related in six clusters ( Figure  5), where the nodes with the largest size correspond to the journal with the highest number of citations received.   Table S3 shows a diverse and complex co-citation group for journals: Cluster 1 (red) "Earth geological processes" consists of the Geology journals; Geomorphology; Geological Society of America Bulletin and Sedimentology, where the most relevant is "Geology" with H-index 189 considered as the representative journal of the study area for its number of co-citations; In Cluster 2 (green) "Earth and science" shows the journals Earth and Planetary Science Letters (EPSL), Bulletin Volcanology, Tectonophysics and Geophysics; as the most representative; the most relevant of this group is "Earth and Planetary Science Letters (EPSL)" with 215 H-index. Cluster 3 (blue) "Volcanic processes" are made up of the journals Volcanology and Geothermal Research, Journals Geophysics Research and Remote Sensing of Environment as the most representative; where "Journal of Volcanology and Geothermal Research" presents the highest H-index (105). Cluster 4 (yellow) "Geological Nature" presenting the journals Nature, Journal Geophysical Research, Science and Icarus; where the "Journal Geophysical Research" shows the highest H-index of the group.
In Cluster 5 (purple) "Structures and Geopatrimony" with journals (such as the Geological Society of America Bulletin, Geoheritage, and Geological Magazine) highlights "Geological Society of America Bulletin" with 134 H-index. Finally, in Cluster 6 (light blue) "Marine Geology" with a notable presence from the journals Marine Geology, Oceanography, Eos, Transactions American Geophysical Union and Marine Geophysical Research. In this cluster, the highest H-Index corresponds to "Marine Geology" (117).

Conclusions
This article exhibits a review of the intellectual structure on volcanic geomorphology during the period 1956-2019. Bibliometric analysis was used as a method of evaluating scientific production, helping to understand the intellectual structure and promoting the discovery of new topics. The analysis of scientific production reflects an exponential growth according to the mathematical adjustment of the trend curve without reaching the saturation point described by Price, where an initial period of 1956-1998 is identified with a production of 90 scientific articles, a second period from 1999 to 2009 (242 publications), showing increasing interest from the academy; and a third period 2010-2019 (375 publications) evidencing the consolidation of the scientific field.
The growing interest in the field of volcanic geomorphology is evident, considering the contribution of 707 scientific articles written by 1725 authors, from 73 countries. This international contribution has a strong responsibility on the part of developed countries, such as the United States, Italy, France, the United Kingdom, and Germany. The authors of this subject with the highest production come from European institutions; the presence of researchers from American institutions also stands out, demonstrating the expansion of information.
For the analysis of the journals, the most important were considered, the Journal of Volcanology and Geothermal Research presents the highest number of contributions (106), followed by Geomorphology (64). Analysis of bibliometric maps revealed connections, such as co-occurrence bibliometric mapping of keywords shows topics related to geomorphology, geochronology, volcanism, morphometry, indicating that most subject areas were based on analyzes of geomorphological structures, age, and volcanism. Secondly, the authors co-citation bibliometric mapping, demonstrating that the intellectual structure is related to geological processes and effects. Being Head, J.
The analysis of co-citation of journals allowed to know the different conceptual approaches that are structured in topics related to volcanic processes, geo-heritage and marine geology. Finally, the use of bibliometric analysis allows us to identify an intellectual structure and its tendency in journals on the topics of geology, geophysics, geomorphology, remote sensors, and marine geology. This research has limitations: (i) Not using more academic fields; (ii) considering a single type of document (articles); (iii) the use of a single database (Scopus) instead of considering other databases that are frequently used in the academic world like Web of Science. Later studies may choose to identify the intellectual structure from another perspective. However, the rigorous methodology used the selection of a reliable and comprehensive academic database, as well as a careful selection of keywords based on theory and the extensive analysis carried out is considered as an important study that serves as a reference point for future research in the area of volcanic geomorphology.