Question: where does new crust come from in sea floor spreading
[{'answer': 'volcanic activity', 'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 1, 'start_character': 107, 'end_paragraph_id': 1, 'end_character': 124, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'basaltic magma', 'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 5, 'start_character': 378, 'end_paragraph_id': 5, 'end_character': 392, 'bleu_score': 1.0, 'section': 'Section::::Significance.\n'}]}, {'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 930, 'bleu_score': 0.765563096853569, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13130441694399979483}, 'section': 'Section::::Significance.\n'}]}, {'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 0, 'bleu_score': 1.0, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13130441694399979483}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '44062009', 'title': 'Subduction zone metamorphism', 'section': 'Section::::Oceanic crust.\n', 'start_paragraph_id': 2, 'end_paragraph_id': 2, 'meta': {'evidence_span': ['Arc magmas are produced by partial melting of metasomatic domains in the mantle wedge, which was reacted with liquid phases derived from dehydration melting of minerals contained in the subducting oceanic crust formed at mid-ocean ridges. The subducting oceanic crust consists of a four major units. The topmost unit is a thin cap of pelagic sediments up to 0.3\xa0km thick composed of siliceous and calcareous shells, meteoric dusts, and variable amounts of volcanic ash. The next unit is composed of 0.30.7\xa0km thick pillow basalts, formed by the quenching of basaltic magma as it erupts into ocean water. Under the pillow basalts is a basaltic sheeted dike complex, that represent cooled magma conduits. The bottom units represent the crystallized magma chamber, feeding the mid-ocean ridge at which the crust was formed. It is composed of 15\xa0km thick layered gabbro atop <7\xa0km thick layer of ultramafic rocks (e.g. wehrlite, harzburgite, dunite, and chromite). Oceanic crust is referred to as a metabasite.', 'Arc magmas are produced by partial melting of metasomatic domains in the mantle wedge, which was reacted with liquid phases derived from dehydration melting of minerals contained in the subducting oceanic crust formed at mid-ocean ridges. The subducting oceanic crust consists of a four major units. The topmost unit is a thin cap of pelagic sediments up to 0.3\xa0km thick composed of siliceous and calcareous shells, meteoric dusts, and variable amounts of volcanic ash. The next unit is composed of 0.30.7\xa0km thick pillow basalts, formed by the quenching of basaltic magma as it erupts into ocean water. Under the pillow basalts is a basaltic sheeted dike complex, that represent cooled magma conduits. The bottom units represent the crystallized magma chamber, feeding the mid-ocean ridge at which the crust was formed. It is composed of 15\xa0km thick layered gabbro atop <7\xa0km thick layer of ultramafic rocks (e.g. wehrlite, harzburgite, dunite, and chromite). Oceanic crust is referred to as a metabasite.', 'The next unit is composed of 0.30.7\xa0km thick pillow basalts, formed by the quenching of basaltic magma']}}]}, {'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ['Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.', 'Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.']}}]}]