Could you please clarify what is Block Contention ?
Is it when 2 or more processes need to access the same Oracle block at the same time.
Does block contention occur only during updates/deletes or does it occur even during selects
The definition is perfect, occurs also for index blocks.
When a process needs to read a block, it must flag it as "being read", to prevent someone else to write it at the same time. But others can read at the same time, too (this is called SHARED "locking").
When a process needs to write into a block, it must flag it as "being written", and no other process can read or write to it (this is called EXCLUSIVE "locking").
So it happens anytime one has to read or write (in a word, access) and another is currently reading or writing (with the exception of read/read collision).