They are lithophiles and therefore remain close to the Earth's surface because they combine readily with oxygen and so associate strongly with silica, forming relatively low-density minerals that do not sink down into the Earth's core.
ðeɪ ɑːɹ lˈɪθəfˌaɪlz ænd ðˈɛɹfoːɹ ɹᵻmˈeɪn klˈoʊs tə ðɪ ˈɜːθz sˈɜːfɪs bɪkˈʌz ðeɪ kəmbˈaɪn ɹˈɛdili wɪð ˈɑːksɪdʒən ænd sˌoʊ ɐsˈoʊsɪˌeɪt stɹˈɔŋli wɪð sˈɪlɪkə, fˈɔːɹmɪŋ ɹˈɛlətˌɪvli lˈoʊdˈɛnsᵻɾi mˈɪnɚɹəlz ðæt duːnˌɑːt sˈɪŋk dˌaʊn ˌɪntʊ ðɪ ˈɜːθz kˈoːɹ.