Temporal correlations between continental flood basalt eruptions and mass extinctions have long been recognized ^1. Massive carbon degassing of Large Igneous Provinces can cause catastrophic global climatic and biotic perturbations ^1–3. However, recent high precision geochronology from the Deccan Traps ^4,5 and the Columbia River Basalt Group ^6 challenges this causal link by showing that the major phase of flood basalts eruptions happened after the onset of global warming ^7–9 by several hundred thousand years. Here, we argue that major eruptions of continental flood basalts may require densification of the crust by intrusion of larger volumes of magma than are extruded. Simple models show that magma crystallization and release of CO2 from such intrusions could produce global warming before the main phase of flood basalt eruptions on the observed timescale. Our model, consistent with geological, geophysical, geochemical and paleoclimate data, suggests that the evolving crustal density has a first-order control on timing of the major phase of continental flood basalt volcanism. Degassing of CO2 from LIP-related intrusions significantly affects Earth's climate and habitability.