I thought the Earth is rather fluid inside such that stuff gets mixed around over time. That such blobs can stay intact for billions of years seems far-fetched.
The mantle is officially "solid" (as proven, I believe, by propogating shear waves, which an actual liquid cannot do). It flows, but very slowly. Very very slowly. I'd say that big chunks of material staying intact over billions of years is, in fact, an illustration of just how slowly.
Wikipedia: "[The mantle] is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel."