一T h e E a r t h c o mpr ise s t h r ee princip a l l ayers: t h e dense, i r on-r ich c o re, the m a nt le m ade of sili ca t e (硅酸盐) that a re s emi-molten at d e p th, and th e thi n, sol i d-sur f ac e crust Ther e are two k i n ds of cru st, a lo w e r and d ense r ocean ic cr us t and a n upper,lig h t er con t inen t al c r ust foun d over o n l y abo u t 4 0 p ercent of the E art h's s urf a ce
T he ro cks of th e cru s t ar e of very diffe r ent ag e s
Some contine nt al rocks a re ove r 3,000 m i llion yea r s old, w h i l e th os e o f the oc e an floo r ar e le ss tha n 2 0 0 mi l lio n ye a rs o l d
The crust s and the to p, sol i d part of the ma n tle, totaling about 7 0 to 1 0 0 kil o meters in thickn es s, a t present appear to c on sist o f abo ut 1 5 r i gid p l