How do magmas form at convergent plate boundaries? What will ...

[Frost2351]

How do magmas form at convergent plate boundaries?
 
  What will be an ideal response?

Question 2

The rock cycle describes recycling on a planetary scale. How might our 4.5-billion-year-old Earth differ if there was no rock cycle? Here are some ideas to consider in your answer: land elevation, depth of the oceans, and variability of rocks.
 
  What will be an ideal response?

[Jevvish]

Answer to Question 1

The melts that form at a convergent plate boundary mostly originate in the asthenosphere above the subducting plate, because the melting temperature of the asthenosphere peridotite is reduced by injection of water released from metamorphism of the subducted crust.

Answer to Question 2

Little topographic difference (no mountains forming; any primitive mountains were weathered away); oceans accumulated great volumes of sediment (e.g., average continental margin sedimentation rate is 5 cm/ k.y. over 4.56 b.y. equals 228 km, enough to fill the oceans over 40 times). There would be little variation in igneous rocks because of the lack of mixing of magma and subducted plate and sediments.

[Frost2351]

Excellent

[Liamb2179]

TYSM