Perbandingan Karakteristik Lava Formasi Mandalika dan Formasi Watupatok berdasarkan Observasi Lapangan dan Petrografi di daerah Ngrayun, Jawa Timur
DOI:
https://doi.org/10.58812/jgws.v4i02.3424Kata Kunci:
Formasi Mandalika, Formasi Watupatok, Petrografi, Lava, Pegunungan Selatan JawaAbstrak
Aktivitas vulkanisme di Pegunungan Selatan Jawa menghasilkan berbagai satuan batuan gunung api yang dikenal sebagai Old Andesite Formation, termasuk Formasi Mandalika dan Formasi Watupatok yang tersingkap di daerah Ngrayun, Jawa Timur. Kedua formasi tersebut memiliki hubungan menjemari yang menunjukkan perkembangan vulkanisme pada interval waktu yang relatif sama, namun memperlihatkan karakteristik lava yang berbeda. Penelitian ini bertujuan untuk membandingkan karakteristik lava kedua formasi berdasarkan observasi lapangan dan analisis petrografi guna memahami lingkungan pembentukan dan evolusi vulkanisme di Pegunungan Selatan Jawa. Metode penelitian meliputi observasi lapangan untuk mengidentifikasi litologi, struktur batuan, dan karakteristik singkapan, serta analisis petrografi menggunakan sayatan tipis dan metode point counting untuk menentukan tekstur dan komposisi mineral batuan. Hasil penelitian menunjukkan bahwa lava Formasi Mandalika dicirikan oleh tekstur porfiritik, vesikular, struktur columnar joint, serta breksi autoklastik yang mengindikasikan proses pendinginan relatif lambat pada lingkungan subaerial. Secara petrografi, lava memperlihatkan tekstur porphyritic, glomerocryst, dan tekstur amigdaloidal. Sebaliknya, lava Formasi Watupatok menunjukkan tekstur afanitik hingga aphyric, quenched texture, mikrolit plagioklas acicular, tekstur trachyitic, rendahnya vesikularitas, serta palagonitisasi yang mengindikasikan pendinginan sangat cepat akibat interaksi langsung antara magma dan air. Perbedaan karakteristik lava pada dua formasi yang menjemari ini menunjukkan bahwa aktivitas vulkanisme di Pegunungan Selatan Jawa berkembang pada lingkungan transisi subaqueous–subaerial.
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