LONGSORAN RAKSASA GUNUNG API MERAPI YOGYAKARTA – JAWA TENGAH
DOI:
https://doi.org/10.33332/jgsm.geologi.v15i4.56Abstract
Longsoran raksasa merupakan longsoran sangat besar kerucut gunung api komposit ke arah tertentu sehingga membentuk kawah bukaan tapal kuda, yang dihadapannya terlampar endapan berbentuk topografi gumuk. Penyelidikan lapangan di daerah Godean dan sekitarnya, Kabupaten Sleman Yogyakarta telah menemukan endapan longsoran raksasa dari G. Merapi, yang membentuk topografi gumuk di tepi utara perbukitan batuan gunung api purba Godean. Sebaran sisa endapan longsoran Merapi itu menutupi area berukuran 2 km x 2 km dan ketinggian gumuk kurang dari 30 m di atas dataran di sekitarnya. Endapan longsoran masih sangat lepas, berupa fasies bongkah berlapis, yang tersusun oleh endapan piroklastika, aliran lava dan endapan rombakan. Seluruh endapan mengalami frakturasi sangat kuat, membentuk rekahan gergaji dan sesar minor sebagai akibat gerakan longsor. Endapan longsoran ini dapat terawetkan karena membentur dan tertahan oleh perbukitan batuan Tersier Godean. Dari G. Merapi sampai Godean endapan longsoran itu bergerak sejauh 30 – 35 km dengan volume mencapai 10 km3 dan daerah terlanda mencapai 300 km2. Ke arah selatan, material longsoran berubah menjadi aliran lahar, yang melanda daerah Bantul sampai di wilayah Pandak, berjarak 50 km dari G. Merapi. Di bagian barat lahar mengalir melalui K.Progo dan di sebelah timur mengikuti K. Bedog. Di bagian tengah endapan lahar tertahan oleh perbukitan batuan Formasi Sentolo sehingga menyisakan banyak bongkah besar andesit di wilayah Sedayu. Untuk mengantisipasi terulangnya bencana katastrofik longsoran G. Merapi pada masa mendatang diperlukan mitigasi yang lebih seksama.
Kata kunci: Godean, longsoran raksasa, lahar, Merapi, Yogyakarta
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