No creature without flying will be able to block that turn, including creatures that lose flying after Magmatic Chasm resolves and creatures without flying that enter the battlefield after Magmatic Chasm resolves.

Because Magmatic Chasm’s effect doesn’t change the characteristics of any permanents, the set of creatures affected by Magmatic Chasm is constantly updated. Creatures without flying that enter the battlefield later in the turn won’t be able to block.