We propose a model in which chemical remanent magnetization (CRM) acquired within the first 20 m.y. of crustal evolution may account for 80% of the bulk natural remanent magnetization of older basalts. The CRM of the crust is acquired as the original thermoremanent magnetization (TRM) is lost through low‐temperature alteration. The CRM intensity and direction are controlled by the posternplacement polarity history. This model explains several independent observations concerning the magnetization of the oceanic crust. The model accounts for amplitude and skewness discrepancies observed in both the intermediate‐wavelength satellite field and the short‐wavelength sea surface magnetic anomaly pattern. It also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the Cretaceous Quiet Zones, while predicting other systematic variations with age in the bulk magnetization of the oceanic crust. The model also explains discrepancies in the anomaly skewness parameter observed for anomalies of Cretaceous age. Further, our study indicates varying rates of TRM decay in very young crust which depicts the advance of low‐temperature alteration through the magnetized layer.