What is the origin of throwing dust over head (as a sign of mourning) in the Bible? In Job 2:12, when Job's friends first see Job at a distance, they "did not recognize him, and they raised their voices, and wept aloud; they tore their robes and threw dust in the air upon their heads."

I know that tearing of one's clothes is a common mourning ritual as portrayed throughout the Bible, but is throwing dusts upon one's head as well? Where did the mourning ritual of throwing dusts on one's self originate? What is is symbolic of? What other accounts do we have in the Bible for it?
It would seem that is the earliest known Biblical reference to ashes being in Genesis, when Abraham accepts that he is nothing but dust and ashes before the LORD, so it is a symbol of humility before God.


  Then Abraham answered and said, “Indeed now, I who am but dust and
  ashes have taken it upon myself to speak to the Lord..."
  
  Genesis 18:27


Nevertheless, I believe there is also another symbol of great significance. It is important to note that ashes were taken from the burnt offering in the earthly sanctuary:


  And the priest shall put on his linen garment, and his linen trousers
  he shall put on his body, and take up the ashes of the burnt offering
  which the fire has consumed on the altar, and he shall put them beside
  the altar.
  
  Leviticus 6:10


Ashes could symbolize something very important, purification:


  Then a man who is clean shall gather up the ashes of the heifer, and
  store them outside the camp in a clean place; and they shall be kept
  for the congregation of the children of Israel for the water of
  purification; it is for purifying from sin.
  
  Numbers 19:9


Many times sackcloth and ashes were put on for mourning a death, but other timer for mourning over sin and showing outwardly the repentance that should have been happening inwardly.

Here is the case of Ahab...


  But there was no one like Ahab who sold himself to do wickedness in
  the sight of the Lord, because Jezebel his wife stirred him up. And he
  behaved very abominably in following idols, according to all that the
  Amorites had done, whom the Lord had cast out before the children of
  Israel.
  
  So it was, when Ahab heard those words, that he tore his clothes and
  put sackcloth on his body, and fasted and lay in sackcloth, and went
  about mourning. 
  
  And the word of the Lord came to Elijah the Tishbite, saying, “See how
  Ahab has humbled himself before Me? Because he has humbled himself
  before Me, I will not bring the calamity in his days. In the days of
  his son I will bring the calamity on his house.”
  
  1 Kings 21:25-29


And also the case of the city of Nineveh where even the animals were covered in sackcloth and ashes...


  So the people of Nineveh believed God, proclaimed a fast, and put on
  sackcloth, from the greatest to the least of them. Then word came to
  the king of Nineveh; and he arose from his throne and laid aside his
  robe, covered himself with sackcloth and sat in ashes. And he caused
  it to be proclaimed and published throughout Nineveh by the decree of
  the king and his nobles, saying,
  
  Let neither man nor beast, herd nor flock, taste anything; do not let
  them eat, or drink water. But let man and beast be covered with
  sackcloth, and cry mightily to God; yes, let every one turn from his
  evil way and from the violence that is in his hands. Who can tell if
  God will turn and relent, and turn away from His fierce anger, so that
  we may not perish?
  
  Jonah 3:5-9


And Job's repentance...


  Therefore I abhor myself, And repent in dust and ashes.”
  
  Job 42:6


They symbolize the blood of Christ.


  For if the blood of bulls and goats and the ashes of a heifer,
  sprinkling the unclean, sanctifies for the purifying of the flesh, how
  much more shall the blood of Christ, who through the eternal Spirit
  offered Himself without spot to God, cleanse your conscience from dead
  works to serve the living God? And for this reason He is the Mediator
  of the new covenant, by means of death, for the redemption of the
  transgressions under the first covenant, that those who are called may
  receive the promise of the eternal inheritance.
  
  Hebrews 9:13-15


We should take the example of how people mourned and repented and put ashes on their head, as repent in the same way. Instead of covering ourselves with ashes, claiming to be covered by the blood that Jesus spilled in the cross of Calvary for us. For that is how we are cleansed.


  But if we walk in the light as He is in the light, we have fellowship
  with one another, and the blood of Jesus Christ His Son cleanses us
  from all sin.
  
  If we say that we have no sin, we deceive ourselves, and the truth is
  not in us. If we confess our sins, He is faithful and just to
  forgive us our sins and to cleanse us from all unrighteousness.
  
  1 John 1:7-9


TL;DR - ashes are representative of Christ blood which cleanses us from our sins.