Table 1 Description of identified seismic units by using seismo-stratigraphic standard terms⁵² and their interpretation. Each data segment is 1 km wide and 200 m high. The positions of each data segment are shown in Figure S2.

8

Strong amplitudes; parallel to slightly wavy reflections. Drapes underlying Unit 7. Occasionally with extensive chaotic facies

Hemipelagites with intercalated ash from subaerial volcanism^36,37,38,39

7

Weak amplitudes; parallel to slightly wavy reflections. Locally disrupted every few hundred metres. Drapes underlying units; sharp contrast to overlying Unit 8

Hemipelagites

6

High amplitudes; stratiform reflections. Locally developed. Gradual transition to underlying Unit 4; sharp transition to overlying Unit 7

Lava flows^{13,54,55,59,60,61}

5c

Upward-increasing amplitudes and reflection continuity; chaotic in the lower part. Overlies and fills a depression at the top of Unit 5b

Upwards coarsening volcaniclastics, possibly lava (this study)

5b

Variable amplitudes; upwardly concave and outward-converging continuous reflections, intercalated chaotic reflection packages. Downlapping underlying units; crater at the top

Volcaniclastics, formed by explosive eruptions^{8,43,44,45,46,47,62,63,64,69,70,71,72,75,76,77,78}

5a

Weak but variable amplitudes; granular in the centre, chaotic or partly outward-converging; overall mound-shaped geometry. Gradual transition to underlying Unit 4 and overlying Unit 5b

Volcaniclastics^{8,69,70,71,72,73}, possibly hydrothermal deposits^62,74

4

Variable amplitudes, generally higher than those of Unit 7; sub-parallel to chaotic reflections beneath the DWVs, becoming continuous and parallel with increasing distance

Hemipelagites, overprinted by incipient volcanic eruption (this study)

3

High amplitudes; lower part stratiform, upper part mainly continuous. Gradual transition to overlying Unit 4

Lava flows^{13,54,55,59,60,61}

2

Strong amplitude variability; stratiform; upwardly concave reflections, funnel-shaped unit, nested within Unit 1 where underneath DWVs and to the side

Hemipelagites overprinted by ascending magma and fluids (this study)

1

Weak but variable amplitudes; stratiform reflections. Lower boundary not imaged. Gradual transition to nested Unit 2 and overlying Unit 3

Hemipelagites