If I update a column (i.e. change the value contents for a given name in a given key), is the physical disk operation equivalent to delete followed by insert.
Or is it just insert somehow making the last value marked as stale ?

In the definite guide, it says the following about SSTable:
All writes are sequential, which is the primary reason that writes perform so well in Cassandra. No reads or seeks of any kind are required for writing a value to Cassandra because all writes are append operations.

It made me wonder how updates work, if it is append only.

I am new to cassandra so please let me know if I am missing something basic.