Rift
The rifts or time rifts were fractures in the planar fabric of Dominaria and to an extent of other planes. In the greater scheme of things the rifts resulted from the imbalance between the primal forces of the Multiverse and the power of godlike planeswalkers.
They were each caused by some catastrophe, in almost every case a planeswalker was involved. Some time after Karona's destruction the rifts that accumulated over Dominaria's history proved to much—the damage to the plane was too severe to allow for self-repair.
While each rift had a unique "personality", linked to the circumstances of its creation, they all had some common properties: They cut of the plane from its mana and caused anomalies in space and time. They also influenced each other, and with each additional rift the others deteriorated.
Major rifts
Prior to the Mending there were eight major rifts in existence. Here they are presented in the chronological order of their creation:
Madaran rift
More than twenty thousand years before the events of the Time Spiral Cycle the first battle between planeswalkers on Dominaria took place in Madara. It was a fight to death between the elder dragon Nico Bolas and a unknown demonic Leviathan. Their battle destroyed one third of the continent and caused the first damage to the plane. The rift was located exactely at the Talon Gates, a remnant of that conflict. The rift was closed second to last by Nicol Bolas, using Leshrac, killing the Nightwalker in the process.
Yavimayan rift
Urza's Sylex Blast changed the face of the world, brought the [[Ice Age (event)|] and led to the formation of the Shard. In addition to these visible effects the plane itself was damaged, resulting in Yavimaya's rift. When the rifts worsened, the sentient forest tried to heal the damage, without success. Under the influence of Leshrac Jeska used Radha to seal the rift, this time the barbarian lost her potential to become a planeswalker.
Tolarian rift
In contrast to the other rifts the Tolarian one was felt immediately after it came into being. Urza's failed time travel experiment caused temporal disruptions throughout the isle—there were bubbles where time flowed much faster or much slower. During the Phyrexian Invasion when Tolaria was overrun Barrin obliterated the whole island, worsening the damage beyond repair. Fortunately Karn was able to travel back in time and mend the rift just bevor Barrin cast his spell.
Shivan rift
At the beginning of the Invasion Teferi phased out a part of Shiv and Zhalfir leaving a hole in Dominaria. When the time came for Shiv to return he realized that it would not fit. Teferi was successful to heal the damage, loosing his spark but survived. Closing the first rift allowed Shiv to phase back in.
Zhalfiran rift
Zhalfir was phased out along with Shiv. Under the influence of Leshrac Jeska used Radha to seal the Zhalfiran rift. Zhalfir was not able to phase back in however and Teferi remarked that it was "lost".
Urborg rift
The rift at Urborg did not involve any planeswalker, instead it was a side effect of the Rathi Overlay with Volrath's Stronghold arriving here. Phyrexians from an alternate timeline were coming through it. Eventually Lord Windgrace realized what he had to do and gave up his live to heal the rift. This was the third one that has been closed.
Skyshroud rift
To protect Skyshroud Forest Freyalise relocated it when the Rathi Overlay began. Similiar to the rift in Urborg it was linked to an alternate timeline where the Phyrexian Invasion was successful. Freyalise gave her life to seal the rift and except for a single tree the whole forest died with her. The second rift was sealed.
Otarian rift
The most recent rift was come into existence when Karona, the incarnation of Dominarian magic was destroyed. Because of Karona's nature Otaria's rift was diffuse, widespread, and the most difficult to seal. Only with the help of Venser and Radha, and by sacrificing herself could Jeska close the it.
With the last major rift sealed, the Great Mending began.