In hardmode, when the player destroys an altar, new ores appear. As explained on the wiki,
Each destroyed Altar causes one type of Hardmode ore to appear in the world
First Altar: Cobalt Ore or Palladium Ore, chosen randomly.
Second Altar: Mythril Ore or Orichalcum Ore, chosen randomly.
Third Altar: Adamantite Ore or Titanium Ore, chosen randomly.
Right after this moment, is the map refreshing instantly and revealing the new ores locations?
Otherwise, I guess I must explore the entire map with my character to find the ore I'm looking for?
In the screenshot below, there is an illustration of my question. I just destroyed three altars and I noticed an Adamantite ore on my map. I was asking myself if it was a new one or if I just skipped it the first time I explored the area.
I'm playing on a large map, so depending on the answer, there is a possibility of saving a lot of time searching with the map, instead of exploring each area twice and more…
Best Answer
Does the map refresh when something changes, even if you haven't seen it since?
Usually, no. For instance, if someone on a multiplayer server created a house, your map would not update to show the house until you visit it.
This can be replicated by clearing/filling a previously explored area using map editor software (e.g. TEdit). When you look at the world map, you cannot see the changes; however if you explore the area again the changes will become evident on the minimap.
Does this apply to hardmode ores too?
Surprisingly, no! You are able to see hardmode ores instantly in the world map, even if you haven't explored the area since.
To test this, I created a brand new world using the latest version of Terraria. Next, I used TEdit to turn on Hardmode (without actually defeating the WoF), and create a hellevator so I could easily explore. Finally I spawned in 5 crimson altars for easy creation of hardmode ores.
I went in-game, then fell down the elevator holding a light-emitting item so the surrounding tiles were mapped. Next, I teleported to the surface and destroyed the Altars, blessing my world with Cobalt, Orichalcum, and Adamantite. When I entered the world map, I could see two veins of adamantite in the hellevator. Note that I have not fallen down the hellevator since I destroyed the altars!
Are the ores actually there, or do they only physically appear when I explore the area again (i.e. is the map just predicting)?
No. The ore is actually there, and was generated the instant one of the altars was broken. The map is not predicting where the ores will be.
This is proved by the following screenshot from map viewer TerraMap: