Advice | Should you cover exposed tree roots or leave them alone?

Q: Soil and grass keep washing away around a large oak in our lawn, exposing more tree roots plus creating more of a downward slope. A friend says to pile up mulch to cover it and create a volcano look. A neighbor says to build a wall around the outer limits and fill in with soil. I say I am not sure what is the correct method to stop further erosion. Any suggestions?