The continents are made of the rocks (e.g. granite) which are lighter than the average rocks of the Earth, so they float above the mantle.
Those "blobs" are heavier than the average rocks, so unlike the continental rocks that have more aluminum and alkali metals than the average, the heavier rocks must have more iron and manganese than the average.
As others have already mentioned, the most likely hypothesis is that they are remnants of the bodies which have collided with the Earth soon after its formation.
The continents are made of the rocks (e.g. granite) which are lighter than the average rocks of the Earth, so they float above the mantle.
Those "blobs" are heavier than the average rocks, so unlike the continental rocks that have more aluminum and alkali metals than the average, the heavier rocks must have more iron and manganese than the average.
As others have already mentioned, the most likely hypothesis is that they are remnants of the bodies which have collided with the Earth soon after its formation.