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(Abridged) We use 8 micron Spitzer GLIMPSE images to make extinction maps of10 IRDCs, selected to be relatively nearby and massive. The extinction mappingtechnique requires modeling the IR background intensity behind the cloud, whichis achieved by correcting for foreground emission and then interpolating fromthe surrounding regions. The correction for foreground emission can be quitelarge, thus restricting the utility of this technique to relatively nearbyclouds. We investigate three methods for the interpolation, finding systematicdifferences at about the 10% level, which, for fiducial dust models,corresponds to a mass surface density Sigma = 0.013 g cm^-2, above which weconclude this extinction mapping technique attains validity. We examine theprobability distribution function of Sigma in IRDCs. From a qualitativecomparison with numerical simulations of astrophysical turbulence, many cloudsappear to have relatively narrow distributions suggesting relatively low (<5)Mach numbers and/or dynamically strong magnetic fields. Given cloud kinematicdistances, we derive cloud masses. Rathborne, Jackson & Simon identified coreswithin the clouds and measured their masses via mm dust emission. For 43 cores,we compare these mass estimates with those derived from our extinction mapping,finding good agreement: typically factors of <~2 difference for individualcores and an average systematic offset of <~10% for the adopted fiducialassumptions of each method. We find tentative evidence for a systematicvariation of these mass ratios as a function of core density, which isconsistent with models of ice mantle formation on dust grains and subsequentgrain growth by coagulation, and/or with a temperature decrease in the densestcores.

Mid-Infrared Extinction Mapping of Infrared Dark Clouds: Probing the Initial Conditions for Massive Stars and Star Clusters