I block your vision, but you need me to see. What am I?

I travel the world but have no legs, I carry your words without any pegs. I can bring joy or deliver despair, And reach you quickly from anywhere.

Should I collapse, Steel yourself, For agony of defeat. Upon the brow, In burdened back, To each I come complete. In satin cast, Or climbing blooms, Devout peck, two shall meet.

I'm born of light, yet not alive, I stretch and shrink, but never strive. I mimic forms I cannot be, A silent twin that clings to thee. I vanish when the source is near What am I, always there yet unclear?

I bite the ocean floor, but never chew, clinging to earth beneath the blue. Though born of iron, I sleep in sand, holding giants with one hand.