Magma under the crust is under very great pressure. When folding and faulting occur, cracks or fractures appear. These are lines of weakness.
m æ ɡ m ə WORD_BOUNDARY ʌ n d ə ɹ WORD_BOUNDARY ð ə WORD_BOUNDARY k ɹ ʌ s t WORD_BOUNDARY ɪ z WORD_BOUNDARY ʌ n d ə ɹ WORD_BOUNDARY v ɛ ɹ i WORD_BOUNDARY ɡ ɹ eɪ t WORD_BOUNDARY p ɹ ɛ ʃ ə ɹ WORD_BOUNDARY w ɛ n WORD_BOUNDARY f oʊ l d ɪ ŋ WORD_BOUNDARY æ n d WORD_BOUNDARY f ɔ l t ɪ ŋ WORD_BOUNDARY ə k ɜː WORD_BOUNDARY k ɹ æ k s WORD_BOUNDARY ɔ ɹ WORD_BOUNDARY f ɹ æ k t̠ʃ ə ɹ z WORD_BOUNDARY ʌ p ɪ ɹ WORD_BOUNDARY ð iː z WORD_BOUNDARY ɑ ɹ WORD_BOUNDARY l aɪ n z WORD_BOUNDARY ʌ v WORD_BOUNDARY w iː k n ə s WORD_BOUNDARY