**1. Introduction**

Studying the evolution of coastal areas is a crucial issue because coastal areas have experienced human frequentation since historical and even pre-historical times [1,2] and host many socio-economic activities [3]. Coastal areas are very dynamic environments that are subject to modifications even at short timescales. Such modifications depend on several factors, such as climate change, wave action, isostasy, geology, tectonics, and anthropic actions [4]. This is true both for sandy and rocky coasts. Moreover, among the above-mentioned factors, geology and tectonics play a crucial role in the evolution of rocky coasts. In fact, sea cliffs carved in soft sediments evolve more rapidly than sea cliffs carved in hard rock-types [5]. For example, Brooks and Spencer [6] estimated a retreat rate of 0.9–3.5 m/y since 1883 for a sector of the East Anglia sea cliff carved in soft sediments, e.g., mainly unconsolidated sands and clays. Young [7] estimated an average cliff retreat rate of 0.25 m/y since the early 1990s for a sector of the California coastline in the USA, carved in lithified Cenozoic units (e.g., mudstone, shale, sandstone, and siltstone). Epifano et al. [8] estimated an average retreat rate of 0.044 m/y in a 60-year-long period for a rocky coast carved in Jurassic marls and sandstones in Portugal. Furthermore, retreat rates of rocky coasts are also dependent on the amount and density of discontinuities, e.g., bedding, fractures, and faults, even when they affect shore platforms. Naylor and Stephenson [9] analyzed discontinuities within shore platform in Wales and Australia and derived that

**Citation:** Di Crescenzo, G.; Santangelo, N.; Santo, A.; Valente, E. Geomorphological Approach to Cliff Instability in Volcanic Slopes: A Case Study from the Gulf of Naples (Southern Italy). *Geosciences* **2021**, *11*, 289. https://doi.org/10.3390/ geosciences11070289

Academic Editors: Gianluigi Di Paola, Germán Rodríguez, Carmen M. Rosskopf and Jesus Martinez-Frias

Received: 29 May 2021 Accepted: 7 July 2021 Published: 12 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the higher the density of discontinuities the more rapidly the shore platform will erode, with consequences also for sea cliffs behind shore platforms.

In this paper we have carried out a geological and geomorphological analysis of a portion of the Campi Flegrei coastline, in the Gulf of Naples (southern Italy). The Campi Flegrei is a volcanic area located along the Tyrrhenian flank of the Southern Apennines (Figure 1). Morphoevolution of the Campi Flegrei has been characterized by the interaction of volcanism and relative sea-level variation due to climate and bradyseism ([10] and reference therein). As a result, both sandy beaches and rocky coasts are present along the coast of the Campi Flegrei. Regarding rocky coasts, they are all carved in volcanic units and are affected by mainly NE-SW and NW-SE trending faults and fractures [11]. These volcanic units include both consolidated tuffaceous deposits, e.g., the Neapolitan Yellow Tuff, hereinafter NYT, aged 15 ka [12], and unconsolidated pre- and post-NYT pyroclastics.

Several studies have been carried out on specific sectors of the Campi Flegrei coast, e.g., the Monte di Procida sea cliff [13], the Miseno Cape [14], the gulf of Pozzuoli [15], and the Posillipo promontory [16]. These studies highlighted the role of ground motions, e.g., bradyseism, besides rock-types, volcano-tectonics, and climate, in shaping the coastline. Furthermore, researches along the Posillipo promontory have been focused on very specific areas, such as the Nisida island [17], the Coroglio sea cliff [18], and the Marechiaro area [19]. Researches on the Posillipo promontory have also pointed out that this amazing portion of the Campi Flegrei coastline has attracted human frequentation since historical times [17,19]. Evidence of this includes the Roman ruins of Palazzo degli Spiriti, the archaeological area of Pausyllipon and the Roman villae of Villa Diana. By the way, despite these very detailed scale analyses, research along the Posillipo coastline has never been performed along a wider portion. To tackle this issue, we analyzed the westernmost portion of the Posillipo promontory, which extends from the Coroglio sea cliff to the NW, to the La Gaiola islet, to the SE (Figure 1).

The main goal of our paper is to provide robust geological and geomorphological field data that may allow us to characterize this sector of the Campi Flegrei coastline with the aim of identifying the areas more susceptible to cliff retreat. The assessment of cliff retreat susceptibility is not a simple task and researchers can adopt various approaches (heuristic, statistic, deterministic ([20] and references therein)) at different scales (from local to regional) [3,20–26]. Our case study is presented at a local scale and is representative of cliffs in volcanic rocks made up of alternating soft and hard pyroclastic deposits. We adopted a heuristic criterion and analyzed the main predisposing factors occurring in the study area. Our aim is not to propose a new method to assess susceptibility but only to present a case study that may be a useful example and source of data for further studies on the stability of similar volcanic coastal cliffs [13,16,18,27].
