Geometry and topology of string junctions

We study elliptic fibrations by analyzing suitable deformations of the fibrations and vanishing cycles. We introduce geometric string junctions and describe some of their properties. We show how the geometric string junctions manifest the structure of the Lie algebra of the Dynkin diagrams associated to the singularities of the elliptic fibration. One application in physics is in F-theory, where our novel approach connecting deformations and Lie algebras describes the structure of generalized type IIB seven-branes and string junction states which end on them.