I am reading the bestseller, "A Global History". Here is a sentence,'...These are equipped with sensors...the sensors can identify loose soils that had been prehistoric agricultural fields or were covering ancient caravan routes or architectural ruins.'Why the past tense is used here?
I assume you mean the "were covering" part. This is probably because the previous phrase ("had been prehistoric") has pushed the writer to instinctively stick with the past tense. I certainly didn't see anything wrong with it on first reading and may well have written it the same way myself, however I think "are" would be more correct than "were". The sentence "the sensors can identify loose soils that were covering ancient caravan routes" would definitely read better with "are".