She visited a quarry in Somerset to reveal a greener way of clearing undetonated explosives left in the sea from the First and Second World Wars.
The explosions can confuse whales into lethal strandings by damaging their hearing.
First Dame Joanna, 75, and Environment Secretary George Eustice ducked into a metal bunker where they set off a ruinous bang which shot a jet of water up 60ft and shook surrounding rock.
Then a more eco-friendly blast sprayed water just 10ft high, with a more muted thud.
The Stop Sea Blast campaign, which Dame Joanna fronts, is calling for a change in how munitions left in the water from the wars are disposed of when building offshore wind farms.
Detonations can cause huge seabed disruption and threaten lives of whales and dolphins.
Dame Joanna said it was “indefensible” to use more damaging methods with half a ton of explosives because the less harmful alternative costs the same and is just as easy to use.
She said: “They are far less damaging to the seabed, marine life and to the sonar system used by whales and dolphins. It’s been tested. It’s reliable so I don’t see what the argument is.”
The Government last year called for low-noise alternatives to be prioritised over high-order detonations when developing protocols to clear underwater unexploded munitions.
In 2011 nearly 40 long-finned pilot whales entered a bay at the Kyle of Durness, Sutherland, at high tide and became stranded, with 19 dying. A report found nearby bomb disposal was the only external event with the potential to cause the whale stranding.
Marine mammals need their auditory system for navigation, feeding and communication.
Mr Eustice said: “If we’re going to build thousands of new wind farms offshore, we must deploy environmentally friendly ways to deal with unexploded ordnance.” There is about 100,000 tons of explosive material left in the sea surrounding Britain following the world wars.
The Stop Sea Blasts campaign is pushing for the Government to mandate the use of quieter deflagration, which involves firing a magnesium cone at the munitions and causing the explosive contents to burn out.