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 ａｔ e ｓ, 7 o ｆ w ｈｉ ch ar ｅ ve ｒｙ ｌ arge. Theｓ e p ｌ at ｅ s m ｏｖｅ ove ｒ ｔ he s ｅ mi-mo ｌｔ en lower ma ｎ t ｌ eto pｒ oduc ｅ al ｌ o ｆ the m ａ jor to ｐ ogr ａ phi ｃ al(地形学旳）f ｅ atu ｒ es of t ｈ e E ａｒ th. A ｃ tiv ｅ ｚｏｎ e ｓ ｗ here ｉｎ tense deforma ｔｉ on ｏｃcur ａ re confi ｎ ed t ｏ the narrow, ｉｎ ter ｃ onn ｅ cti ｎ g b ｏｕｎｄ a ｒ iｅ s ｏ f contact ｏｆ the pla ｔ es. The ｒｅ are th ｒ ee m ａ i ｎ ｔ y ｐｅ s o ｆ zo ｎ e ｓ of conta ｃ t: ｓpreading contac ｔ s wher ｅ plate ｓ m ｏ ve apart, ｃｏ n ｖ er ｇｉ ng conta ｃｔ s whe ｒ e plat ｅ s move to ｗ ard ｓ each ot ｈ e ｒ,...
