The magic system in my story resembles bending in Avatar TLA/LOK.
Basically, instead of needing spears/pikes/etc. to break a cavalry charge, the army's vanguard waits until the enemy charge is within a certain range, they will manipulate the soil and stone. This includes making the ground split open, exposing stones, "explosions" of dirt to frighten horses and the like.
Could this sudden creation of uneven terrain and frightened horses then cause enough turmoil to stop the charge?
Yes. I think you kinda answered your own question but:
It doesn't take a lot to scare or trip fast moving horses. With a cavalry charge the horses are trained to plough straight through into danger, you would only need to create a thin ditch in front of the horses to cause the front ones to trip and stumble, causing them to be trampled on and tripped over by the horses behind.
You would then want to create similar trenches to the side to catch those at the back who try to go around and so cause a similar effect.
Lots of small holes popping up are harder to avoid and less energy intensive for your mages than a huge hole that can be avoided. Use the momentum of the charge against them and conserve your energy.
The catalan almogavers did something similar at the battle of Halmyros. They dug hidden trenches in a swampy terrain to stop or break the line of charge, then killed all of the knights.
Doing it instantly with magic would be vastly more effective.
Link to Wikipedia Article of Battle of Halmyros
The answers here are already great, if I could suggest an addition to help your mages get away with this, the film Solomon and Sheba has charging Egyptian chariots blinded by the light reflected from polished shields, causing them to not see (and summarily fall into) a chasm in front of them.
You could either continue with this method, or if your magic is Avatar TLA style, come up with some method using fire (for blinding light) and/or water (for reflecting light) so that the enemy cavalry (especially the cavalry further back) doesn't see the trap their comrades are falling into.
The thing you want to achieve is "very fast made Trou de loup".
So as we know that type of trap is very good at stopping cavalry (and infantry) but your added element of earth movement (because the matter must be preserved its would be easy to just shift the earth to form a wall just behind the trench) would scare the horses.
This would easily work once or twice. Once cavalry commanders start to catch on, they would likely develop a counter strategy. Possible strategies include:
Desensitize the horses to the sudden sounds of earth shifting.
Including mages of their own in any cavalry charge, to keep the ground flat.
Massing a bunch of horses together to look like a cavalry charge, then while the mages are messing with that ground your real force attacks elsewhere.
Use elemental powers to alter the nature of the horses. If the horses become slightly water-based, or make horselike elementals out of non-Newtonian fluids, they could flow right over any minor chasms and still be effective.
If you can, try making a nice, deep trench. Make it deep enough, and entire horses will tumble into it. Half the enemy's charge could well end up suffocating beneath the bodies of their comrades who fell in atop them; a grisly prospect, but very effective as a morale blow to the survivors.
A deep trench can be easily seen by the front ranks unless concealed carefully, true. That is not the same as being able to stop before hitting it when you have hundreds or thousands of other horsemen behind you whose view is obstructed by...well, by the horses and men in front of them. You simply cannot arrest a charge instantly even on foot; on horseback, you'll need considerably more space and time to do that, and if this trap is executed properly they won't have that time before they hit the trench. Even if they manage to stop short of the trench, that gives your archers (which you presumably have, as any smart army would) a golden opportunity to unleash their fire at close range and shred the cavalry.
Given that this is being done spontaneously via magic, you don't really need to worry about preparation time. All that matters is the skill and power of your magic-wielders, which is defined by you the author. If they have the capability, they should excavate this trench while leaving a thin layer at ground level: enough to look like it's solid ground, but not enough to hold the weight of a horse. This gives you effectively perfect concealment, making sure that they don't catch on and pull back until it's too late. There is the problem of where all that dirt will go; shunt it off to the sides of your forces, perhaps, to raise walls cutting off any flanking strike against your forces that might be attempted, which has the bonus of funnelling the enemy cavalry more completely into your trap.
Another thing to consider that has been breifly touched on by other comments, is that the sudden terrain shift, noise, and dust most likely will not stop a charging warhorse as they are trained to rush sheild lines at full speed, whitch would be like one of us running full tilt at a brick wall. For that reason, the physical stoppin power of your barrier is going to be the most effective part. Maybe even pull the dirt out of the trench in such a way that it formes a line of stone spikes behing the trench (if your worlds magic is capable of altering the density of soil). Then, once the momentum of the charge has been fully halted, (with the back rows plowing into the trapped front rows) the mages could then shove the dirt back into the hole, burying anyone in it, and your infantry can charge the already wounded and panicking cavalry.
It sounds like you want to keep it simple while magical. All you need to do is momentarily disrupt that ability of the ground to support the horses. A horse at a gallop will stumble and fall if the ground is a foot deeper than it had anticipated.
Cause some of the soil to rise in a dust, but not all of it. That is, in a given cubic foot of dirt, only half of the dirt rises, on a speck-by-speck basis. That's it. The effect will be a shallow pit of very loose dirt/dust with a thick dust hovering above it. The loose soil will not support the horse's weight, the lower floor will encourage the horse to stumble, and the thick airborne dirt will obscure visibility of the actual surface. What's more, you can calculate the amount of energy needed to do it, if that's in your universe.
One technique used in Paris riots (19th century) was to remove pavement stones in a chessboard pattern. Very effective at stopping cavalry charges, especially if the stones were used for a barricade.
I am sure your magic could do something like it.
Yes. If the blast of earth creates a hole, the front horsemen will fall into it, tripping some of the soldiers behind them, and knocking soldiers into each other. Also the spooked horses will probably run into each other as well, and soldiers with dirt in their eyes may fall off their horses. And the rocks would hurt. But it depends on the force of the blast. A small blast would barely do anything, as it is unimpressive. A big blast may even damage the army crating the blast with rocks. So you should specify.
Hope this helps.
