underwater_magma_feature.json

{
  "format_version": "1.13.0",
  "minecraft:partially_exposed_blob_feature": {
    "description": {
      "identifier": "minecraft:underwater_magma_feature"
    },
    "places_block": "minecraft:magma",
    "placement_radius_around_floor": 1,
    "placement_probability_per_valid_position": 0.5,
    "exposed_face": "up"
  }
}