The cliff is the result of abrasion caused by large waves.
Interestingly, undulating land that formed that cliff is the result of an eruption of Mount Samalas in 1257.
The two eruptions were more powerful than Mount Tambora, which effected climate change in Europe.
The coastal is a black sand beach.
The rocks from the eruption of Mount Samalas can be traced and found in The North and South Pole.
Info from: Page FB Royal Rinjani Trek.