Can the Erupting Earth spell be cast somewhere that isn't on "ground"?

Question

The description of the spell Erupting Earth says:

Choose a point you can see on the ground within range.

Does the use of the word "ground" limit this spell to floors that would be considered an "exposed surface of the earth"? Or can it be cast on other kinds of surfaces?

Other possibilities include:

1. An indoor room on an upper level or over a basement
2. An outdoor deck or patio
3. A raised platform like a stage or pulpit
4. A roof
5. A balcony
6. The deck of a ship

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Related: Does the spell Erupting Earth disrupt the surface it's cast on?

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Commenter Hobbamok found an excellent related question that may reduce this one to an unnecessary duplicate: What is "the ground"?

Answer 1

Yes, otherwise dozens of spells would not function as intended.

For example, look at Tenser's Floating Disk, which states that it "floats 3 feet above the ground" It later says that it can move up or down stairs, which typically aren't made of exposed earth.

Another example, if you look at the 2nd level spell Levitate, it ends with: "When the spell ends, the target floats gently to the ground if it is still aloft." If the 'ground' meant only exposed earth, if you were above wood you simply crash when the spell ends.

For more examples look at Shapechange, Tsunami, Phantom Steed, Meteor Swarm and many other spells that would not function properly if 'ground' only meant exposed earth.

Answer 2

Yes, it can be cast on other kinds of surfaces as well

In this case, the term "on the ground" only exists to make sure it's not in the air/floating. The rules don't clarify "ground" any further.

While it may not make sense for the ground to be covered with dirt after casting the spell inside a dungeon, on top of a roof, or inside a gazebo, it is easy to reflavor the spell to have the ground be covered in rocks, chunks of wood, or another material that creates the same effect.

Answer 3

It's up to the DM; however it could be quite reasonable to rule that the Erupting Earth spell could not be cast or would fail if it was cast on a "floor".

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Note that this is in response specifically to the Erupting Earth spell found in XGtE, and not other spells found in the PHB that refer to "ground" - like Shapechange, Tsunami, Phantom Steed, Meteor Swarm.

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While the rules in the PHB have not differentiated between "ground" and "floor", it can be seen in XGtE that the rules are now making a distinction between the two.

The description of the Snare spell indicates the following (emphasis mine):

As you cast this spell, you use the rope to create a circle with a 5-foot radius on the ground or the floor. (XGtE pg 165)

Whereas the description of Erupting Earth indicates the following (emphasis mine):

Choose a point you can see on the ground within range. (XGtE pg 155)

This seems to strongly imply that there is now a difference between "ground" and a "floor".

Presumably for the purposes of the Snare spell, the examples listed in the question (indoor room on an upper level, outdoor deck or patio, raised platform, roof, balcony, deck of a ship) would make sense to qualify as "floor" and there would be no issues with casting that spell in those locations.

However, for the purposes of Erupting Earth, a raised platform or roof etc. appears to be problematic at best (consider the wording of the Bones of the Earth spell, and what would happen to the roof of a building if six pillars of stone each 5 feet in diameter and 30 feet high were to appear bursting from that level) while the deck of a ship far off in the ocean makes it even moreso.

Ultimately, it is a magical world and up to the DM to apply their decision-making as consistently as possible, but it certainly appears there is an argument that - in the case of Erupting Earth - more than a "floor" could be required.