Playing vs human - Modern.
Opponent played
Blood Moon.
Next turn I cast
Spreading Seas on a
Scalding Tarn.
Xmage never registered turning the
Scalding Tarn into an
Island.
Both effects operate in Layer 4 (type-changing effects). Neither is a characteristic-defining ability, and neither effect would interrupt the other (see:
Blood Moon vs.
Urborg) to create a dependency, so you apply them in time-stamp order.
Blood Moon first, then
Spreading Seas.
Rules:
613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s
card type, subtype, and/or supertype.
613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then
all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in
which effects are applied within a layer. (See rule 613.7.)
613.7. Within a layer or sublayer, determining which order effects are applied in is sometimes done
using a dependency system. If a dependency exists, it will override the timestamp system.
613.7a An effect is said to “depend on” another if (a) it’s applied in the same layer (and, if
applicable, sublayer) as the other effect (see rules 613.1 and 613.3); (b) applying the other
would change the text or the existence of the first effect, what it applies to, or what it does to
any of the things it applies to; and (c) neither effect is from a characteristic-defining ability or
both effects are from characteristic