d04f9e2c_r__The_Last_Airbender_resource__Aang_s_experience
[Chakra] Thought Chakra [Deals with] Pure cosmic energy [Blocked by] Earthly attachments [Location] Crown of the head [Aang's experience] 
Aang was asked to meditate on what attached him to the earth, only images of Katara came mind. Pathik stated that Aang needed to let go and forget his attachments. Taken aback, Aang questioned how he could let go of Katara when he loved her and wondered why he would choose cosmic energy over Katara, stating that the attachment was perfectly acceptable "three chakras ago". Reluctantly, Aang tried to let go of Katara and his earthly attachments. However, as Aang was meditating and about to clear the final chakra, he had a disturbing vision of Katara being held captive. Aang immediately ceased his meditation and was thus disengaged from his potential entrance to the Avatar State. Pathik warned Aang that if he quit his training before unlocking this chakra, he would ultimately be unable to enter the Avatar State at all. Aang ignored the warning and left to rescue Katara. When he was later forced on the defensive while under attack in Ba Sing Se, he attempted to unlock the chakra again. In order to save Katara, he was successful in letting her go and opened up the last barrier that separated him from the vast pool of cosmic energy of the Avatar Spirit. However, the chakra was locked once more when Azula shot him down with a lightning attack. It was eventually unlocked for good when Aang was shoved by Ozai into a jagged rock; its point pushed against his scar and released the pent up energy locked there, allowing him to enter the Avatar State.
Chakras - Avatar Wiki, the Avatar: The Last Airbender resource
Aang's experience