Can "plunge" be used for a "horizontal dive"?
You often plunge into the sea or plunge you hands into your pockets.
Can the word be used to describe a "horizontal immersion"?
Example sentence:
She opened the door and plunged into the blinding sun.
If not, what's a more appropriate word?
It's perfectly acceptable, but you might want to consider the "literal baggage," for lack of a better word, that the figurative term carries with it.
Take the OED literal definition that this derives from:
To thrust, throw, or drop into or in a liquid, penetrable substance, deep pit, container, etc.; to immerse, to submerge.
In any case that you use the word "plunge," readers will associate it with the literal meaning to derive its figurative meaning. If you want to invoke feelings of falling deep into something, then it's appropriate. Just be aware that the use ordinarily refers to falling or pushing in a downward direction.
The OED's first meaning of plunge involves transitively thrusting, throwing or dropping something (or oneself) into a liquid - from the French plonger - to jump into water, to submerge, or sink
However, sense 4 (intransitive) of the verb covers precisely the circumstance you relate:
- intr. To move or travel forth, on, etc., rapidly, abruptly, or recklessly; to move with a rush or sudden impulse into or out of; to hurtle, career. Also fig. 1726 W. Broome in Pope et al. tr. Homer Odyssey V. xxiii. 256 He views the strand, And plunging forth with transport grasps the land.
1806 T. Jefferson Let. 5 July in Writings (1984) 1165 If the executive is to keep all other information to himself, and the House to plunge on in the dark, it becomes a government of chance and not of design.
1834 L. Ritchie Wanderings by Seine 110 We..plunged into the high road leading to Duclair.
1885 Manch. Examiner 22 Jan. 5/2 Under a well-organised fire from the works, the Arabs plunged forth upon the square.
1891 C. Graves Field of Tares iv. vi. 241 The Norwich Express, plunging out of Liverpool Street Station.
a1902 F. Norris Pit (1903) x. 400 She had committed herself now; recklessly she plunged on.
1954 Times Lit. Suppl. 12 Nov. 714/4 As the Age of Reason plunged towards revolution, Juvenal was carried like a banner in the van of the attack.
1990 A. Stevens On Jung ii. 18 The dreamer might see a train, carrying him home, plunging in and out of a series of tunnels.
2004 Derby Evening Tel. (Nexis) 16 Mar. 6 If the Germans had captured crossings over the Trent, they could have plunged in any direction.