At this point, you can ask your carpenter to use butterfly splines to keep the crack from getting worse.
The root cause is likely the moisture as it was mentioned above... The wood might have been dried a little too fast, possibly leading to case hardening. I am assuming the slab is thicker than 1.5", though from your photo, it looks more like 1.25, which is pretty thin for a counter. What finish did they use?
Is it possible to find out if this was kiln dried or air dried?

In your last photo, it looks like there was already a crack that was eventually patched. That looks like end check. End checks are normally cut off because where there is one, there is more that you can't yet see.

I don't know how much you paid for this, but it shouldn't have been more than $500 for a 6' length and about 3 ' wide I assume.
As for the cracks, I wouldn't blame them on the support..

Love!

HI -- Nothing to do with supports or installation ... simply young wood that was not dried properly before use .... that would be my guess . Having a humidifier might help a little . The wood just needed to age or be kiln dried more before using it . It will just continue to crack until it stops at some point .

He used Emmet's "Good Stuff".
I'll ask what drying method he used and report back. It was a 76.5"x39" piece (so your estimate was pretty close).

HI -- Do you also know that walnut will darken over time too.

@lefty47 - I didn't know that for sure but figured it would age over time. I like it as it is but don't mind a little "character" added over time. Cracks definitely scare me more though.

lefty, walnut actually lightens over time. We have several pieces that my husband made and they indeed lighten.

We looked at Emmet's stuff and they don't really say what it is. There was a complaint on oldtownhome.com that within a month the wood had gone back to raw after being finished with Emmet's good stuff. So, it basically leaves the wood without finish. That being said, I'd be curious about the finishing regimen for this top.

@printesa The carpenter said that it was kiln dried. Interesting note about Emmet's Good Stuff... I was told I don't need to oil the top for another 18 months but sounds like it needs to be reapplied a lot more than that?

@lefty47 what do you suggest to be done to fix the cracks and prevent it from getting worse?

Given all the details, I would ask for the money back from the carpenter. It's his fault for giving a faulty and badly finished piece. Since you life in SF area, I am sure that the piece was at least 3K (I lived in San Jose for about 6 years so I am familiar with the local costs a bit). I would look for other carpenters and get their opinion as well

@crytalbathurst........Sorry for the slow response and thanks for your comments on our counter tops. I have looked over the many comments to your post and I would have to say that I agree with the majority of opinions that the wood expanding and contracting due to moisture is probably the culprit for the cracks and it has nothing to do with the overhang of the top. The top most likely was not dried out completely prior to glue up and natural drying out process has brought out the cracks. Looking at the board (I.E. the filled portion on the end) I would say that these cracks were naturally occurring and the board should not have been used in the construction of the counter top. These things happen with a wood counter top but the manufacturer should take the top off, cut that board out and replace it with a new board.

As far as the finish I can not comment on Emmets, we use only a Waterlox Tung Oil finish and apply 5 coats of the tung oil to both the top and underside of the top...this is important in an area where there is alot of natural humidity because the top will shrink and expand with the different environmental conditions.

I also noticed a comment on cost of the top. If you only paid $500 for the cost of this top I would consider that a steal!!! For us to build this counter top we would order at a minimum of $500 worth of lumber to get the right boards (I.E. no knots, no cracks etc). That top in New England would cost nearly $2,000 with our hand rubbed finish.

Good luck with your top.....unfortunately the only way to fix this correctly is to remove the top, cut out the bad board and replace it with a new one.

She didn't say anything about the price,,I said that. Based on the dimensions, it looks like she has 30 bdft of 6/4 in the raw. $8 is the most expensive I can find 6/4 walnut, and that's retail to customer not to the maker. Joint, send through planer, glue, send through drum sander, and then apply finish of choice. It's a simple process.I don't think it takes more than 5 effective hours of work. Whatever the shop rate is, that's a different story. In any case, the guy who picked the wood did a bad job and he should take it back and return the money

carolyn, from the photos it looks like there was a problem with the slab from the beginning. Any carpenter would know that such slab needs to be cut and new piece added..otherwise, it cracks and you see what happened