Answer: The mulch should probably be removed. You can use it elsewhere in the garden. Before laying down the lava rock, cover the area with weed barrier cloth to keep weeds from popping through the lava rock. If you leave the mulch and cover with lava rock the mulch will eventually decompose and you'll have weeds growing through the lava rock. So, remove it, cover the area with weed barrier and then put down your lava rock. Best wishes with your garden!
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