We all know what a nuclear explosion looks like.(fireball and mushroom cloud). What would an orbital beam cannon, fired at the ground continuously for a minute, look like?

A few views here: https://www.google.com/search?rlz=1C1GCEB_enUS800US800&biw=1755&bih=1027&tbm=isch&sa=1&ei=PLzYW6KsGOGb0gKU04DwBg&q=rogue+one+death+star+firing&oq=death+star+rogue+&gs_l=img.3.3.35i39j0j0i8i30l8.108464.112049..117621...0.0..0.65.180.3......1....1..gws-wiz-img.......0i67.46tMYq5Liuc#imgrc=_

Let me do a test.
I’ll be back.

Here you go.

What is it shooting?

Like a big ass flashlight, pointed directly at the ground, with massive amounts of smoke and steam being vented from the hole being created.

I bet it would look about the same
How do go the next level above Nature’s top shelf?

Here’s a long detailed answer – What if all of the sun’s output of visible light were bundled up into a laser-like beam that had a diameter of around 1m once it reaches Earth?

“Whose” beam cannon did you have in mind? Or rather, what is the beam of choice and its intensity?

Like your local chippie after the chip fat catches fire.