function setup() {
 	createCanvas(600,600);
    fill(0, 0, 0);
//Draw a volcano
	triangle(180, 600, 280, 500, 280, 600);
    triangle(320, 600, 320, 500, 420, 600);
//Draw a neutral buoyancy line
    line(0, 300, 600, 300);
//Define starting x,y coordinates of ash    
    x = width / 2;
    y = height;
}

function draw() {
  
  // Draw a piece of ash
  stroke(50);
  fill(100);
  rect(x, y, 5, 5);
  
  // Moving up at a constant speed
  y = y - 1;
  
  // Horizontal movement at neutral buoyancy
  if (y < 300) {
    x = x + 1;
    y = 300;
  }
}