An examination of the mechanisms of brittle fracture in solids is conducted via molecular dynamics simulations of the behavior of a model silica glass and crystal under applied uniaxial strain. The results show an effect of applied strain rate on the mechanism of fracture and differences between the crystal and glass. The crystal exhibits failure by bond extension until local rearrangements, made possible by the thermal motions of the atoms, can construct a fracture surface. The glass shows a pronounced variation in fracture mechanism as a function of strain rate. If the strain is applied so fast that no thermal motions are allowed, the glass fractures similarly to the crystal by bond extension. If the strain rate is sufficiently slow to allow thermal motions to take place before fracture, then the deformation mechanism is dominated by rotation of the silica tetrahedra. The fracture strength at slow strain rates is 60% less than the strength at high strain rates. All values of the glass strength are lower than the crystal strength (by at least 30%) despite using the same interatomic potential for both phases. Fracture in the glass appears to result from a coalescence of void spaces under strain, around regions of the glass that exhibited a lower density of atoms than average. In effect, the regions of low density observed in the glass phase appear to act as internal flaws, able to initiate failure by acting to collect strain.