'A lack of prior knowledge in the other discipline' is often given by geoscience instructors to explain why students struggle to apply knowledge from other disciplines to the Earth. We examine this assumption by considering the disciplinary crossing of buoyancy (physics) to isostasy (geoscience). We investigate the teachers' perspective through workshops, and use the theoretical framework of Interdisciplinary Reasoning and Communication to explore what geoscience textbooks ask of students. Most introductory textbooks rely on students applying pre-existing knowledge of hydrostatics in a geoscience context ("forwards transfer"), while intermediate and advanced textbooks develop the necessary physics explicitly in a geoscientific context ("backwards transfer"). Moreover, introductory textbooks frequently use analogies for "concrete translation" between physics and geoscience ideas, but the analogical relations and where they break down ("relational translations"), receive little attention. This places high demands on students' prior understanding of hydrostatics to successfully integrate and transfer knowledge from physics to geoscience. If students lack the relevant physics background, introductory textbooks may force them to rely on intuitive ideas of physics, potentially reinforcing old, and giving rise to new misconceptions. Our results move the focus from perceived shortcomings of students to the demands made on them in interdisciplinary learning.