Physical volcanology of Tseax Volcano, British Columbia, Canada

Tseax volcano erupted ∼ 250 years ago in NW British Columbia, Canada producing tephra deposits and lava flows. Field mapping has defined the stratigraphy of Tseax and the lava flow morphologies. Aerial photogrammetry and bathymetry surveys were used to create a high resolution digital elevation model of the volcano to facilitate mapping and estimates of erupted material volumes. Tseax volcano (∼ 10.4 ± 0.7 × 10 m) comprises an outer breached spatter rampart and an inner conical tephra cone. Tseax is associated with a 32 km long and 0.49 ± 0.08 km basanite-to-tephrite lava flow field covering ∼ 36 km and divided into 4 distinct lava flows with heterogeneous surface morphologies. We present a volcanological map of Tseax volcano at a scale of 1:22,500. This will serve as supporting information for further research on the eruptive history of Tseax volcano and the lava flow field emplacement. ARTICLE HISTORY Received 23 October 2019 Revised 16 April 2020 Accepted 17 April 2020


Introduction
Tseax volcano (pronounced 'See-Ax', Wil Ksi Baxhl Mihl in the Nisga'a language) also known as Aiyansh volcano (Sutherland-Brown, 1969), Aiyansh River volcano or Tseax River Cone (Hanson, 1924;Hickson & Edwards, 2001) is a volcano located in northwestern British Columbia (55.11085°N, 128.89944°W; see Main map). The volcano is associated with a 32 km long lava flow and forms the Nisga'a Memorial Lava Bed Park (Anhluut'ukwsim Laxmihl Angwinga'asanskwhl Nisga'a). The eruption of Tseax volcano, occurred in the late eighteenth century and is the second youngest volcanic eruption in Canada after Lava Fork (Elliot et al., 1981;Hickson & Edwards, 2001). This eruption is believed to be responsible for the destruction of 3 villages and the deaths of up to 2000 people (Barbeau, 1935;Higgins, 2009;Nisga'a Tribal Council & B.C. Parks Committee, 1997;Sutherland-Brown, 1969) making it one of Canada's deadliest natural disasters (Hickson & Edwards, 2001;Higgins, 2009).
Tseax volcano and its associated lava flow field was initially mapped by Sutherland-Brown (1969) who provided a preliminary morphological map of the lava flow. Recently, new mapping has been conducted on Tseax volcano including extensive fieldwork, bathymetric surveys, high resolution mapping aerial photography and photogrammetry. Here we present a 1:22,500 volcanological map (Main map) of the volcanic field supported by detailed descriptions of the volcanic landforms and deposits. The aim of this detailed map is to provide the context to help with understanding the eruptive dynamics of Tseax volcano (i.e. sequence and duration of the volcanic events).

Regional and local geology
Tseax volcano lies in the Northern Cordillera Volcanic Belt (NCVP; Main map; Edwards & Russell, 1999. The NCVP is an approximately 1200-km-long and 400-km-wide volcanic province extending from northwestern British Columbia, Canada, to eastern Alaska, USA. Volcanism in the NCVP is attributed to continental rifting of the North American plate due to changes in the relative motion of the Pacific and North American plates. More than 100 Neogene to Quaternary volcanic centres (ranging from individual tephra cones and lava flows to stratovolcanoes) with a dominantly basaltic and high alkali compositions have been mapped (Edwards & Russell, 1999. Tseax volcano is the southernmost volcanic centre of the NCVP located near the towns of Gitwinksihlkw and Gitlaxt'aamiks, about 60 km north of Terrace and ∼1200 km north of Vancouver, British Columbia ( Figure 1, Main map). The region surrounding Tseax volcano is part of the Coast Belt plutonic complex (Edwards & Russell, 1999;Woodsworth et al., 1991) traversed by the Nass River which is 3rd the largest river in British Columbia (Roden, 1967). Tributary to the Nass River, Tseax River flows in a narrow 15 kmlong V-shaped valley that separates the Vetter peak massif from the Hazelton Mountain Ranges (Mt Hoeft, Mt Phillipa; Figure 1). Tseax is located at ∼600 m above sea level in the steep sided Crater Creek valley, a 5 km-long E-W tributary valley to Tseax River valley. The surrounding area is heavily vegetated and most of the mountain slopes are covered by a thick and dense rainforest.
Tseax volcano rests on the sedimentary Bowser Lake group (dated at Jurassic-Cretaceous, JK Bu and JK BCR on Figure 1; Evenchick et al., 2008;van der Heyden et al., 2000). These rocks are grey sandstones intercalated by dark grey and black siltstones, mudstones and conglomerates. Some Eocene plutonic rocks (ETg, Figure 1), mostly granites and granodiorites belonging to the Coast Belt plutonic complex, are present near Tseax volcano. A few occurrences of tonalites and monzogranites have also been documented by Carter (1981). Pleistocene volcanic products including basaltic and andesitic deposits (Pb, Figure 1; Evenchick et al., 2008;van der Heyden et al., 2000) are found within 50 km around Tseax volcano. They are columnar-jointed basalts and basaltic breccias assumed to be 175-50 ka old and not associated to the volcanic history of Tseax. A dataset of aerial photos was acquired (see Appendix A.1 for details) and was used to build a 3D model of the Tseax lava flow field using the Agisoft PhotoScan software. It is an advanced image-based solution for creating professional quality three-dimensional content from images (Agisoft LLC, 2019). PhotoScan is widely use in geology (e.g. Bemis et al., 2014;Gwinner et al., 2000;James & Robson, 2012) to reconstruct and visualize objects or topographic surfaces in 3 dimensions. The 3D model supported the investigation of the different surface morphologies and the calculation of the lava volumes.

Methodology
The calculation of the lava flow volumes of Tseax are challenging as the base of the flow field was never observed. The volumes of the lava flows were calculated by multiplying the thickness with the area. The thickness of the lava flows in the Nass Valley and in the Tseax River and Crater Creek Valleys were estimated differently. The calculation details are in Appendix A.4. The pyroclastic edifices observed are tephra cones and a spatter rampart. We simplified the tephra cones as truncated cones with a summit circular crater and the spatter rampart as a semi-elliptical truncated cone with a semi-cylindrical inner. The calculation details are in Appendix A.4.
The lava flow field is partially submerged at the northern end of Lava Lake and Melita Lake (see Main map). In order to fully map the extent of the volcanic products, we collected bathymetry data for these two lakes using a Lowrance Elite 5Ti chart plotter with a 10 Hz internal high-sensitivity GPS receiver with DGPS and WAAS correction.
The GIS processing of the field and remote data allowed for the production of the main map. Figure 2 shows an example of the datasets used for mapping.

Tseax volcano and other pyroclastic vents
Tseax volcano (Wil Ksi Baxhl Mihl) has been described as two imbricated yet asymmetric volcanic cones: an outer horseshoe shaped cone and an inner tephra cone. Our mapping shows that there is, in fact, one main tephra cone partially surrounded by a spatter rampart ( Figure 3). A Satellite cone is located north of Tseax volcano. A few other pyroclastic vents present near these main cones are also described below (Figure 3).

Spatter rampart
The spatter rampart sits on the southern slopes of Crater Creek Valley and has a horseshoe shape that opens northwards (maximum opening is ∼280 m). This horseshoe feature consists of two walls ∼15-25 m high (the base is not visible) and ∼75-125 m long. The eastern spatter wall is oriented N170-180°and the south-western wall is oriented N150-160°. The spatter ramparts are not fully visible as they are partially collapsed and covered by dense forest, especially along the southernmost parts. The spatter edifice is 550-600 m wide and 360 m long maximum with a ∼320 m wide caldera. The estimated volume is ∼1.1 ± 0.3 × 10 6 m 3 (Equation A.2). The spatter walls consist of reddish to brownish and black to grey agglutinated spatter, scoriaceous spatter and scoria. The spatters are generally more coalesced at the bottom of the rampart forming thick and dense layers. The outermost deposits are completely covered by forest.

Tephra cone
The main tephra cone is located towards the northern end of the eastern spatter wall. This cone is ∼65-75 m high and ∼350-400 m in diameter, consisting mainly of black tephra (lapilli, scoria and ballistics). The summit defines a circular crater ∼30 m deep and 80 m in diameter. The pyroclastics around the crater are oxidized (brownish colours). The flanks of the cone are between 30 and 35°. On the south and west flanks, several active scarps are present. Other small concentric scarps (less than 3 m high) are located on the south and west flanks. This may indicate diverse collapse events during construction of the cone. The volume of the cone is estimated at 2.8 ± 0.4 × 10 6 m 3 (Equation A.1) and the volume of ejected tephra is ∼6.5 × 10 6 m 3 (Equation A.3).

Satellite cone and other pyroclastic features
About 150-200 m north of Melita Lake, there is a ∼20 m high tephra cone, called Satellite cone. This cone has a diameter of ∼50-55 m. A small crater (4 m deep, 7 m in diameter) occupies its summit. This small cone is asymmetric and the vent opens towards the Northwest. The summit is topped by a small layer of oxidized agglutinated scoria and accretionnary lapilli. Descending towards the base, the cone is progressively covered by black tephra. We estimate that the Satellite cone has a volume of 1.7 ± 0.1 × 10 4 m 3 (Equation A.1).
Southwest of the Satellite cone, there is an alignment of 3-4 small tephra cones a few metres high. This cone alignment may represent a volcanic fissure oriented N10-20°. The small cones are completely covered by black and iridescent tephra. Nevertheless, red oxidized tephra are consistently found stratigraphically below the black tephra. The limit of the distribution of the red oxidized tephra was not found, however, they are only present on and in close proximity to the Satellite cone. This suggests that the oxidized red fountain deposits are only linked to the building of the Satellite cone. Assuming this hypothesis, the volume of ejected red tephra by the Satellite cone is ∼2 × 10 4 m 3 (Equation A.3).

The Tseax lavas
The large-scale elements of the lava flow field are most visible from aerial and satellite images. We used field mapping in combination with aerial photos to describe and understand the structural organization of the lava field.

The Tseax lava flow field
The term 'lava flow field' is used to designate all effusive products at Tseax. Due to its young age (∼250 years), the Tseax lava flow field has undergone minimal post-emplacement erosion. Nevertheless, most of the lava flow field surface is covered by a few The lava thickness is ∼12 m according to Purssell (1993). Surface features are discussed in the text.
centimetres of green to yellow lichens and mosses. Furthermore, large portions of the lava flows are already covered by dense rainforest, e.g. the vast majority of the lava in Tseax River Valley. Moreover, in some places the lava underlies rivers, streams (e.g. Tseax River, Vetter Creek) and a few small lakes or ponds such as Ross Lake in the Tseax River Valley. The outline of the Tseax lava flow field is nevertheless easily identified from field observations and aerial or satellite imagery.
The lava flow field originates from the vent area in Crater Creek Valley, where it forms a dam (made of a few small lava islands) separating Lower and Upper Melita Lake (T'aam Baxhl Mihl). Investigation of the Upper Melita lake by canoe and paddle board has shown that the lava does not go more than ∼150 m to the east. Bathymetry shows that Lower Melita lake is very shallow (∼2 m deep on average), with the deepest part (reaching 3.5 m) found near north-western shores (Figure 4 (b)). The chaotic floor of the lake suggests that it consists of lava. The lava-filled Crater Creek Valley is about 5 km long on a slope of ∼4°. Based on the topographic slopes on both sides of Crater Creek Valley, we estimate that the average depth of the lava in this valley is 30.5 ± 5.5 m. At the bottom of Crater Creek Valley, the lava flow debouches in the Tseax River Valley. There, the lava creates a significant dam on Lava lake. Bathymetry measurements performed on Lava lake show that the lava ends 250 m south of the shore at a depth of about 25-30 m (Figure 4(c)). The lava descends northwards along the narrow Tseax River Valley (average slope of ∼1.5°on 15 km), reaching about 22 km in length with an estimated average lava depth of 21 ± 3 m. The lava then spreads out towards the southwest in the Nass Valley flood plain. There the lava forms a ∼11 km long and up to 3.8 km wide lava plain (slope of 0.1-0.2°). Eastwards, the lava flow is bordered by Tseax River and Spencer Lake. The Nass River marks the northern margin of the lava flow with no lava found on the northern side of the Nass River. A 4-7 m high lava cliff about ∼5 km in length is one of the most notable and spectacular features of the Tseax lava flow field, forcing the Nass River to form a narrow canyon near Gitwinksihlkw (Figure 2  (a)). The western margin of the lava field is entirely covered by rainforest, but easily mapped in the field and from aerial photos. The southern margins in the Nass Valley corresponds to Ksi Ts'oohl Ts'ap creek and marshlands. A significant part of the lava flow field in the Nass Valley is covered by recent forest growth in the area where Vetter Creek disappears beneath the lava. The depth of the lava in the Nass Valley is difficult to estimate, however, geotechnical drilling indicates a lava depth of 12 m near the road bridge to Gitwinksihlkw (Figure 2(a); Purssell, 1993). Therefore, we consider an average thickness of 12 ± 2 m for the lava in the Nass Valley as some parts of the flow may have been emplaced on irregular topography or may have undergone different magnitude of inflations.
In total, the Tseax lava flow field covers ∼36 km 2 . Using our thickness estimates in Crater Creek, Tseax River and Nass Valleys, we calculate a volume of 0.49 ± 0.08 km 3 for the entire lava flow field, most of the volume being in the Nass Valley. This correlates with the 0.5 km 3 previously estimated by Hanson (1924) and Higgins (2009) and the 0.455 km 3 estimated by Sutherland-Brown (1969).

The 4 Tseax lava flows
We define 'lava flow' as an individual outpouring of lava from the vent, likely corresponding to an individual eruptive event. The margins of a lava flow are generally steep and easily identifiable in the field. Despite being emplaced during a single eruption, the Tseax lava flow field can be divided into 4 distinct lava flows, named Flow 1, Flow 2, Flow 3 and Flow 4 (see Main map). The main geometrical characteristics of the lava flows are reported in Table 1.
Flow 1 is the longest, most voluminous and stratigraphically earliest lava flow. It is a phenocryst-poor pāhoehoe lava 31.6 km in length with an average thickness of 12 ± 2 m in the Nass Valley. Flow 1 forms the entire lava plain in the Nass Valley, 2.8 km wide on average and ∼12 km long. We suggest that the thickness of Flow 1 increases to 14 ± 2 m in Tseax River and Crater Creek Valleys due to narrowing. This results in a volume of 0.41 ± 0.07 km 3 . Flow 1 thus accounts for ∼84% of the total flow field volume.
Flow 2 is a 21.2 km long phenocryst-poor pāhoehoe lava flow. Located along Crater Creek Valley, it ends near the mouth of Tseax River Valley where it forms two distinct ∼2 km long lava lobes. We estimate a thickness of 7 ± 1 m. Despite the length, we estimate a volume of 0.06 ± 0.01 km 3 , representing ∼13% of the total flow field volume.
Flow 3 is a 7.2 km long phenocryst-rich 'a'ā lava flow descending from the volcano in Crater Creek Valley and terminating in Tseax River Valley near Ross lake. We estimate an average thickness of 3 ± 1 m in Tseax River Valley and 4.5 ± 1 m in Crater Creek Valley. Therefore, the volume of this flow is 0.009 ± 0.002 km 3 , accounting for < 2% of the total flow field volume.
Flow 4 is a phenocryst-rich 'a'ā lava flow and the stratigraphically uppermost flow of the Tseax lava flow field. It is 5.3 km long and located only in Crater Creek Valley. This 'a'ā flow ends where Crater Creek Valley debouches in Tseax River Valley but does not reach Lava Lake. We estimate a thickness of 4.5 ± 1 m which gives a volume of 0.006 ± 0.001 km 3 . This corresponds to ∼1% of the total flow field volume.

Subaqueous lavasbathymetry
Melita Lake (T'aam Baxhl Mihl, Figure 4(b)), near Tseax volcano, was formed after damming of Crater Creek by the lava flow. Melita Lake consist of two lakes, Lower and Upper Melita Lake (∼0.05 km 2 and ∼0.2 km 2 , respectively). Lower Melita lake is separated from Upper Melita Lake by a series of small and heavily vegetated lava islands. The shore of Lower Melita Lake consists of large blocks of pāhoehoe, in some places completely covered by clay and silt. Field observations suggest all the lava exposed along the shoreline and on the islands separating Lower and Upper Melita Lakes are from the same flow unit. Petrographic analysis of a lava block sampled along the northern shore of Melita Lake shows a phenocryst-poor lava, similar to Flow 1 and Flow 2. The deepest part of the lake is found near the western shores (∼3.5 m deep). The eastern part of the lake is shallower although two deep spots are located close to the central and northeast shores. The bathymetry results show a relatively chaotic floor with no particular volcanic features. Collapsed lava blocks and agglutinated tree logs are present along the western and northern shores of the lake.
Lava Lake (Sii Tax, Figure 4(c)) is located in Tseax River Valley at the bottom of Crater Creek Valley. The Tseax River debouches from the northwestern extremity of the lake. While Lava Lake already existed before the eruption of Tseax (D. Nyce, personal communication), it's depth increased due to damming of Tseax River by Flows 1, 2 and 3. Pressure ridges of Flow 3 form the northern shores of Lava Lake. We estimate that Flow 3 ends ∼20-50 m from the northern shore at a depth of about 7 m. The flow field terminates ∼250-270 m from the northern shores at ∼27 m depth.

Lava flow surface morphologies
Lavas of the Tseax flow field can be classified according to their surface morphologies. From field observations, we distinguished pāhoehoe and 'a'ā flows with numerous distinctive surface morphologies. In this section, we present short definitions of these surface morphological features as well as their location. We refer the reader to the Main map for a better understanding. Figure 5 shows photographs of representative surface morphologies seen on the Tseax lava flow field.
The main categories of surface morphologies are 'a'ā (Flow 3 and Flow 4) and pāhoehoe lava flows (Flow 1 and Flow 2, see Main map). We identified hummocky (Hm), lava-rises (Lr), slabby (Sl), hummocky-to-lava-rises (Hm-Lr) and slabby-to-rubbly (Sl-Rb) pāhoehoe surfaces. The general characteristics of the lavas are described in details in Appendix B. The area occupied by these surface morphologies are indicated in Table 2.

Conclusions
A 1:22,500 map of Tseax volcano and its immediate surroundings is presented here and provides the first detailed and comprehensive description of the volcanic products. The mapping and characterization of the volcanic products was conducted over three field seasons with the help of high-resolution aerial photographs, photogrammetry and bathymetric surveys. The map is designed to document and characterize the lava features and the lava flow surface morphologies. Tseax volcano consists of two imbricated volcanic structures (a central tephra cone and an outer spatter rampart) located at about 600 m a.s.l. in Crater Creek Valley. The pyroclastic products cover a surface of > 10 km 2 . Tseax volcano is associated with a 0.49 ± 0.08 km 3 and 32 km long lava flow field. We identified 4 lava flows, likely corresponding to 4 different eruptive events. Stratigraphically, we have from bottom to top: 2 long and voluminous pāhoehoe lava flows (Flow 1 and Flow 2, 0.41 ± 0.07 and 0.06 ± 0.01 km 3 , respectively) and 2 shorter 'a'ā lava flows (Flow 3 and Flow 4, 0.009 ± 0.002 and 0.006 ± 0.001 km 3 , respectively). The pāhoehoe flows show several distinctive surface morphologies namely: hummocky, lava-rises, slabby, hummocky-tolava-rises, slabby-to-rubbly pāhoehoe and rafted plates. Usually, we observe a complete transition between these surface morphologies in a lava lobe unit within a flow. The main sharp disturbances are marked by major breakouts on the large lava-rises that disrupted the smooth upper crust into slabby pāhoehoe.
The volcanological map for the Tseax volcanic field represents fundamental data required to understand the sequence and duration of events that ultimately led to the Nisga'a fatalities. Furthermore, the described lava flow surface morphologies are critical information for quantification of eruption parameters, key for numerical modelling of lava flow emplacement. This work is also an important contribution towards hazard assessment as forestry and fishery industries, tourist attractions and ∼1000 people are located in close proximity to the volcano.

Software
The Digital Elevation Model of the Tseax lava flow field was created with AgiSoft PhotoScan Professional 1.2.4.2339 and CloudCompare v2.8. Bathymetric data was analysed in Reefmaster underwater mapping software. Bathymetry and geological maps were produced using ESRI's ArcGIS 10.7. The maps were edited and structured using Adobe Illustrator CC 2015.