If it's over 50, it's a ritual (unless your SG/Troupe decides otherwise).
Maw of the Volcanic Vent (Cr Ig 40)
The ground in a targeted area flashes into molten rock in a 2 and 1/3rd pace by 2 and 1/3rd pace area to a depth of 2/3rds of a paces (7 feet across by 7 feet wide by 2 feet deep) (someone check and correct my maths please). Any target which falls into it takes +36 damage (+12 damage * 3 for partial immersion). Near proximity can cause damage due to convection, and nearby water will flash into steam (storyguide's discretion on damage)
Base 15, R: Voice (+2), D: Momentary (+0) T: Part (+1), +2 Size.
Maw of the Active Volcano (Cr Ig 45)
The ground in a targeted area flashes into molten rock in a 7 and 1/3rd pace by 7 and 1/3rd pace area to a depth of 2/3rds of a pace (22 feet across by 22 feet wide by 2 feet deep) (someone check and correct my maths please). Any target which falls into it takes +36 damage (+12 damage * 3 for partial immersion). Near proximity can cause damage due to convection, and nearby water will flash into steam (storyguide's discretion on damage)
Base 15, R: Voice (+2), D: Momentary (+0) T: Part (+1), +3 Size.
Iron is molten at 1000 to 1500 C (dependant on purity and type), and AM5 Page 181 lists that as +12 damage, with lead being +9, which is 327.5C (the jump is impressive) so I'd say +12 damage is effectively what you're after.
Base individual for a fire is a large campfire or a fire in the hearth of a great hall, which is usually at most a foot across in either direction and effectively a foot tall, in the depths of winter when you want it at it's strongest.
+2 size therefore gives is 100 cubic feet to work with. At 2 feet deep, that's 50 feet square, so approximately 7 feet by 7 feet
+3 size is 1000 cubic feet, so at 2 feet deep, that's 500 square feet, which is 22 feet to a side, approximately. Assuming a square area.
Neither of my examples are rituals unless your SG rules that they are because of their power. The next example IS a ritual. It takes 2 and a half hours to cast, and uses 10 pawns of creo or ignem vis. But let's be fair, it's also pretty much guaranteed to charboil anything but the most determined assault upon you.
Maw of the Errupting Volcano (Cr Ig 50)
The ground in a targeted area flashes into molten rock in a 23 and 1/3rd pace by 23 and 1/3rd pace area to a depth of 2/3rds of a pace (70 feet across by 70 feet wide by 2 feet deep) (someone check and correct my maths please). Any target which falls into it takes +36 damage (+12 damage * 3 for partial immersion). Near proximity can cause damage due to convection, and nearby water will flash into steam (storyguide's discretion on damage)
Base 15, R: Voice (+2), D: Momentary (+0) T: Part (+1), +4 Size.
All data on metal melting points from engineeringtoolbox.com/melti ... d_860.html
I'd argue the initial damage is magical, and thus resisted by parma, but the resulting damage as the new lava is cooled (I'd say -1 damage per round if you're being generous, as that feels appropriately mythic (solidifies after 12 minutes). Otherwise, have it cycle every 30 seconds, losing -1* the number of cycles (so -1 after 30 seconds, -2 (total -3) after a minute. Means it'll have solidified in 2 and a half minutes)) is probably mundane. Clambering out, even if you are somehow unharmed by it, is difficult, as it's cooling, and thus thickening, and starts out as a relatively thick mire of liquid. Effectively a superhot swamp.
Go for booming voice and exaggerated gestures to make sure you're far enough away 
I wouldn't be sure which way to rule on the "is it fire" issue, though as my design posits, nearby water flashes into steam, which ISN'T fire. (how do you burn to death the magus with immunity to fire? Steam jet. 3rd degree burns all the way. No I haven't been working out how to kill my local flambeau with immunity to fire, cough just being prepared.) And the deprivation resulting from falling over in it could be unpleasant, especially if you're unlucky and it sets.
The damage values in this posit that a person falls the 2 feet, and as it's both a surprise and an unpleasant shock, falls to their knees, resulting in the half immersion for *3 damage. It also fails to factor in the fall damage of +1 (halved then rounded as it's technically a soft surface). Remember to add a stress die to all damage instances. Note that this spell doesn't necessarily have to target the Earth, either. You can suddenly flash detonate the air by super heating it and ignite any flammable materials within it. Or you can flash an area of water into steam to boil your enemies alive instantly, for near enough the same effects. If you do this, however, remember that the steam is SUPER-HEATED, and results in total immersion instead of partial as the cloud overtakes them, resulting in it being +48 damage instead of +36.