Sandstar eruptions are events unique to Japari Island. The mysterious material known as Sandstar flows from the island's volcano and causes mutations to the wildlife of the land, creating Friends. To date there have been three major eruptions documented, with a fourth one predicted to happen.
Prior to the Kemono Friends anime, the Sandstar was not stated to have come from a volcano, but rather fell mysteriously from the sky. The anime retconned this by establishing the volcano and Friends referencing the events as eruptions.
While there have only been three major eruptions documented so far, smaller bursts of Sandstar coming from the volcano inbetween the larger ones are fairly common. In episode 11 of the Kemono Friends anime they confirm that recently small eruptions have been gonig on for several days in a row. These small eruptions can also leak Sandstar Low, leading to an increased Cerulean threat unless it's filtered out.
The First Eruption created the first Friends on Japari Island after Sandstar made contact with organic bodies and the first Cerulean after it made contact with non-organic bodies. Serval, Ezo Red Fox, Tsuchinoko and many other Friends were born from this event. As such, it led to humans researching Sandstar and Friends becoming residents at Japari Park. But the Cerulean threat meant the park eventually had to be closed.
In the Kemono Friends game, the player helps Mirai and the first generation of Friends clear out the Cerulean threat to the park to make it safe for visitors again. After defeating the Cerulean Queen, the park was finally safe and proceeded to be reopened. Leading to the events of the Kemono Friends: Yokosou Japari Park e! manga.
Kemono Friends: Yokosou Japari Park e! tells the story of Nana, a caretaker in training at Japari Park. Though no year is specified, it's suggested the First Eruption took place several years before 2015 as Nana talks about having dreamed of working at Japari Park since she was a child and the manga hints at being set during 2015 and 2016.
The Second Eruption created a second Cerulean outbreak on Japari Island after Mirai had saved the park the first time around. The events following the Second Eruption were depicted in Mirai's memo-logs shown in the Kemono Friends anime. Mirai's mission was ultimately unsuccesful as the park remained dangerous and the team had to evacuate once again.
The Third Eruption took place on Japari Island several years after humans had left once again. No animals were documented to have come into contact with Sandstar, instead, the only one who suddenly found themselves inside Japari Park after the Third Eruption was Kaban, a human without any memories of herself. Despite being human, Kaban seems to have been created from the Sandstar coming into contact with something new.
The Kemono Friends anime starts the day after the Third Eruption and chronicles the journey of Kaban and Serval trying to find out exactly who Kaban is and why she suddenly found herself on Japari Island.