Why do earth scientists think that rigid slabs of the lithosphere are descending into the mantle near the tonga islands?

The southwestern Pacific Plate has been subducting under the  Indo-Australian Plate at the Tonga Trench since the middle of the  Eocene era.

Explanation:

The lithosphere-asthenosphere boundary (hereafter it is called  as LAB), separating the lithosphere and the asthenosphere , is an important mechanical interface of the  Earth’s interior. The LAB  has also been termed as Gutenberg (G) discontinuity that defines  the top of the low velocity zone in the upper mantle.   The southwestern Pacific Plate has been subducting under the  Indo-Australian Plate at the Tonga Trench since the middle of the  Eocene era. The present convergence rate between  two plates are up to 200–250 mm/a and it is faster in the northern  part than in the southern part. The Tonga area  contains two-thirds of all deep earthquakes in the world, and comprises the zones of the deep slab subduction,  the island arc magmatism and the back-arc basin spreading.  Numerous seismologists reported  that the cold subduction slab enters into the deep mantle, lowers  mantle and induces the uplift of the 410 km discontinuity and  the depression or stratification of the 660 km discontinuity. Seismic imaging studies proposed that the   obvious low velocity zone above the subducted slabs is associated with the dehydration and de-carbonation  of the slabs.  

Learn more:  

Continental lithosphere versus oceanic lithosphere

         

Oceanic lithosphere is lighter than continental lithosphere

         

Keywords: LAB, Gutenberg discontinuity, Tonga, seismic, velocity  , subduction, magmatism