Question: When hittite old kingdom , minoan eruption is the  ubaid period in mesopotamia what is the copper age?
Schema: create table table_23537091_1 (copper_age varchar, ubaid_period_in_mesopotamia varchar, PRIMARY KEY (copper_age))
SQL: select copper_age from table_23537091_1 where ubaid_period_in_mesopotamia = "hittite old kingdom , minoan eruption"