To investigate the stratigraphic features of M-type facies, we conducted acoustic surveying via a deep-sea subbottom profiler equipped with on the SHINKAI 6500 during dives 6K#1521, 6K#1542, and 6K#1544. Figure 7 shows the surface geology and subsurface stratigraphy along the track line of dive 6K#1521 as the representative of M-type facies, i.e., the case for the transition from the ferromanganese nodule field to the petit-spot volcanic field. Using the deep-sea subbottom profiler equipped on the SHINKAI 6500, observations successfully revealed the bedrock and dikes of petit-spot lava intruding into the underlying sediment beneath the seafloor. We previously reported that a small outcrop of petit-spot lava was recognized in the ferromanganese nodule field at Stop 3 (Ref7.; Fig. 7a). Around Stop 3, the "deepest" reflector (acoustic basement) abruptly becomes shallower and extends up to the seafloor (Fig. 7b). In contrast, at Stop 2, where sedimentary rock outcrops were identified (Fig. 7a), the "shallow" reflectors above the deepest reflector rise close to the seafloor (Fig. 7b). At the arrival point (Stop 1), the deepest reflector is located at a depth of approximately 10 m, but it becomes shallower near Stop 5. West of Stop 5, most of the deepest reflector lies at a depth of 5 m or less.
Our results from the survey of vessel-equipped SBP show that the O-type facies are widely distributed in the central region of the area (Fig. 6a). During submersible dive 6K#1544, typical O-type facies were observed in a deep-sea plain, and many lava flows were confirmed through both deep-sea MBES and megascopic observations (Ref.; Fig. 3). Similarly, during submersible dive 6K#1466, lava outcrops were also confirmed in flat areas. Moreover, other small seamounts within the widespread O-type facies are considered petit-spot volcanoes on the basis of our submersible dives (Ref.; Figs. 3 and 6). Therefore, the O-type facies is a critical acoustic feature indicating the distribution of petit-spot lava outcrops on the seafloor. We also recognize that acoustic reflections from the surface of petit-spot volcanoes are scattered (Figs. 4b and 5). We consider that the high roughness of the surface of petit-spot volcanoes due to the distribution of fresh lava flows, as shown by our submersible dives (Fig. 3), is the cause of acoustic scattering. In contrast, the O-type facies, which are primarily found along the slopes of large seamounts located in the eastern (Takuyo-Daigo Seamount) and northeastern (Minamitorishima) parts of the area (Figs. 5 and 6a), have smooth surfaces due to the coverage of ferromanganese crust (Figs. 4c and 5; Ref.) that contrasts with the surfaces of petit-spot volcanoes. We consider that the O-type facies with smooth topography can be an indicator of non-petit-spot seamounts.
The T-type facies is observed in the southern part of the area, whereas the L-type facies is the major facies of the northern (particularly northeastern) part of the area. Considering the distribution of L-type facies along with the results of piston core sampling reported by Ref., it is suggested that non-rare earth elements plus yttrium (REY)-rich hemipelagic sediments are present in the northeastern part of the study area. In contrast, the region containing the T-type facies has a reflection intensity greater than 5.72 dB of backscatter data reported by Ref. (Fig. 6c), which corresponds to areas where a dense distribution of ferromanganese nodules is found. Between the L- or T-type facies and the O-type facies, there is an M-type facies. Investigations by the SHINKAI 6500 submersible dives via deep-sea SBP at each representative site of the M-type facies next to the L-type (dive 6K#1544) or T-type (dive 6K#1521) facies confirmed that petit-spot lava intruded into either non-REY-rich hemipelagic sediments or ferromanganese nodule fields (Figs. 3 and 7).
In the vicinity of large seamounts, low-viscosity alkaline basaltic lava -- capable of flowing over long distances (up to ~ 50 km; Refs.) -- may form reflective interfaces within sedimentary layers. However, aside from petit-spot volcanism, all igneous activity in the broader Western Pacific Seamount Province is older than 100 Ma. At ODP Site 801, sedimentary layers exceeding 400 m in thickness are superimposed on these ancient igneous materials (e.g., lava flows). Due to the shallow acoustic penetration limit of SBP, such deep-seated structures should not be detectable. An exception is the overprinting caused by the alkaline basalt eruption that occurred between 40 and 58 Ma on the northwestern slope of Minamitorishima Island. However, we observed that the L-type facies along the southern slope of Minamitorishima Island borders the O-type facies (near 24°N and 153°30'E, as shown in Fig. 6a). This finding suggests the presence of young lava intrusions within the sedimentary layers, rather than shallow reflections of lava flow originating from Minamitorishima Island. Fundamentally, the shallow acoustic reflections obtained in the M-type facies correspond to young intrusive petit-spot lava, as traced via deep-sea SBP during dives 6K#1521, 6K#1542, and 6K#1544. Therefore, the M-type facies, located between O-, L-, and T-type facies, indicates the outer boundary zone of the petit-spot volcanic field and can be defined as a transitional area where volcanic activity gradually appears or disappears.
Based on the results of our investigation, we provide a tentative interpretative map of the study area (Fig. 6d). Facies boundaries outside the SBP survey lines were determined by tracing the lateral extension of boundaries observed on the SBP survey lines, based on changes in features of both backscatter intensity (e.g., intensity contrast) and topography (e.g., lineaments and/or inflection of slope gradients). This map further clarifies the distribution of the contrasting geological regions in the study area, as previously described. The pelagic sediment region (L-type facies) dominates in the northern part, while the ferromanganese nodule field (T-type facies) dominates in the southern part. We consider that this basic framework of geological distribution relates to oceanographic settings in terms of deep-sea current flowing into the study area from the south, originating from the Lower Circumpolar Deep Water, as discussed by Ref .
The interpretative map also illustrates the relationship between two primary regions -- pelagic sediment and ferromanganese nodules -- and the position of the petit-spot volcanic fields. Building on this, we propose criteria for determining petit-spot volcanic fields. The most critical criterion indicating the distribution of petit-spot lavas is the concurrent observation of high backscatter intensity obtained from MBES data and the O-type facies obtained from SBP data. Definitive evidence for petit-spot volcanism can be provided by ground truthing observations of a small seamount or knoll with an acoustically scattered slope surface situated in the region indicating high backscatter intensity and the O-type facies. The distribution of the O-type facies constrains the exact dimensions of the petit-spot volcanic field. In light of our results, we suggest that the distribution of the M-type facies should be taken into account when assessing the extent of the overall impact of petit-spot volcanism on seafloor geology. As shown in Fig. 7, at the seafloor where the petit-spot lava intrusion took place (recognized as M-type facies) in the southern boundary zone with the ferromanganese nodule field, we observed mounds resembling mud volcanoes (Stop 1), intrusion of sedimentary rock (Stop 2) continuing to the "shallow" reflectors, and plate-like aggregates (Stop 5) because several ferromanganese nodules had overgrown (referred to as "encrustation"). Although detailed consideration of the causal link between these geological features observed on the seafloor and petit-spot lava intrusion is needed in future studies, the M-type facies in this zone, which confirmed high backscatter intensity (Fig. 6c) and geological impact (Fig. 7a), can be included in the petit-spot volcanic field. In contrast, in the northeastern connection zone with the pelagic sediment (Fig. 6d), the presence of M-type facies suggests that petit-spot lava intrusion within the sediment layer is likely to have occurred. However, we did not observe other criteria such as high backscatter intensity nor confirm ground-truth observations for geological impact. Therefore, the M-type facies in this region is not included within the petit-spot volcanic field and is instead represented by a boundary line (Fig. 6d). It is important to consider the uncertainties associated with the geological information suggested by the acoustic data when assessing the extent of the petit-spot volcanic fields.
Direct observations, whether through cameras or the human eye, remains the most reliable method for confirming and documenting volcanic formations. However, their abilities to continuously trace lava outcrops across the full extent of seamount or knoll slopes -- or across topographically flat regions beyond these features -- are inherently limited. In contrast, although vessel-equipped SBP is constrained to imaging shallow sedimentary layers, this technique offers a comprehensive and efficient means of mapping widespread lava eruptions and intrusions. One of the key advantages of vessel-equipped SBP is its ability to detect lava bodies that have not reached or fully exposed themselves at the seafloor surface. In such cases, outcrops may be undetectable by direct observation using submersibles. However, deep-sea SBP can identify subsurface reflections potentially indicative of lava intrusions within sediment layers. By carefully examining locations where these shallow reflections approach the seafloor, we can more effectively locate lava outcrops -- as demonstrated by dive 6K#1521 in this study (Fig. 7). Therefore, direct observation of the seafloor provides definitive evidence for the interpretation of acoustic data. Essentially, acoustic observations and direct observations are complementary.
Considering the complex interactions inherent in plate metamorphism, such as reactions between petit-spot melt and wall rock and the movement of volatile substances within the plate it is conceivable that some melt could potentially freeze within the plate, without ever reaching the seafloor surface. Therefore, it is important to recognize that the distribution of magma exposed on the seafloor surface provides a lower limit to the magnitude of the impact of petit-spot melt on the entire plate. The integrated SBP approach is particularly powerful because it allows for comprehensive detection including small-scale features that might not be fully captured by conventional surveys, which involve greater resource demands and more detailed operational requirements, such as MCS. A wide-area exhaustive survey using the protocol proposed in this study can effectively guide targeted MCS investigations, particularly for imaging deeper structures, thereby achieving broader survey coverage while making more efficient use of available research resources. This study demonstrates that the integrated use of seafloor backscatter intensity and subseafloor stratigraphy, obtained from both vessel-equipped multibeam echo sounders and subbottom profilers, provides a cost- and time-effective strategy with both wide coverage and high spatial resolution for delineating the surface distribution of petit-spot lava.
In summary, the O-type facies is a key indicator of petit-spot lava, with acoustic reflections from these facies revealing the spatial extent of lava eruptions. Additionally, we identified the M-type facies as an important feature indicating the transition between different facies and the boundary of the petit-spot volcanic field. The results of our SBP surveys, complemented by submersible dive observations, show that SBP is particularly effective for detecting subsurface lava intrusions that are not easily visible through direct observation. The integration of vessel-based and submersible-based observations enables the detection of smaller-scale volcanic features and offers a more comprehensive understanding of volcanic processes. The proposed strategy -- using SBP for broad seafloor coverage, followed by targeted MCS for deeper imaging -- offers a cost- and time-effective way to map both shallow and deep structures. Our protocol lays the groundwork for identifying the extent and impact of petit-spot volcanism within the entire subducting plate.