The Plate tectonics is a theory that explains the movement of the Earth’s lithospheric plates and consequent formation of major landforms like mountains and events like volcanoes and earthquakes. The theory states that the Earth’s lithosphere, which is the outermost layer of the planet, is broken into several large plates that move relative to each other.
The lithospheric plates rest on the underlying asthenosphere. These plates move due to the convection currents in the mantle, which are driven by the Earth’s internal heat sources, such as the radioactive decay of elements.
Formation of Himalayan Mountains:

• About 80 million years ago, India was Separating the two was the Tethys Sea. approximately 6400 km south of the Eurasian plate.
• The Indo-Australian tectonic plate – containing the continent of Australia, the Indian subcontinent, and surrounding ocean – was pushed northward by the convection currents generated in the inner mantle.
• For millions of years, India made its way across the sea toward the Eurasian plate. As India approached Asia, around 40 million years ago, the Tethys Sea began to shrink and its seabed slowly pushed upwards.
• The Tethys Sea disappeared completely around 20 million years ago and sediments rising from its seabed formed a mountain range.
• When India and Tibet collided (Indus-Tsangpo Suture Zone; ITSZ), instead of descending with the plate, the relatively light sedimentary and metamorphic rock that makes up the subcontinent of India pushed against Tibet, forcing it upwards, and created a massive mountain fold – the Himalayas.
• The process has not stopped yet. The Himalayas continue to rise by an average of 2 cm each year.

Formation of Andes Mountains:

• The Andes has been formed by the subduction of the Nazca Plate under the South American Plate.
• The denser oceanic lithosphere of the Nazca Plate is forced down and under the more buoyant continental lithosphere of the South American Plate.
• As subduction of the Nazca Plate continues, some of the sediments that accumulated on the ocean floor (together with some of the ocean crust) are scraped off and forced onto the South American Plate..
• The effect of the collision of the two plates deforms the leading edge of the South American Plate by folding the rocks. This crustal shortening increases the vertical thickness whilst reducing the width of the lithosphere in the collision zone (and so produces the Fold Mountains of the Andes.

Therefore, there is a fundamental difference in the formation of Himalayas and Andes. While Himalayas are formed due to collision of two continental plates, the Andes are formed by the convergence of oceanic plate and continental plate (subduction of one plate under another). This subduction leads to formation of volcanoes in Andes such as Ojos Del Salado, Cotopaxi, Chimborazo etc., which are absent in the case of Himalayas.