Definisi
GEOMORFOLOGI • Ilmu yang mendeskripsi (secara genetis) bentuklahan dan proses-proses yang mengakibatkan terbentuknya bentuklahan tersebut serta mencari antar hubungan antara prosesproses dalam susunan keruangan (Van Zuidam, 1977)
Geomorphology • Kajian tentang bentuklahan (landform) yang menyusun permukaan bumi dengan tekanan utama pada sejarah asal mula (genesis). Dalam mengkaji genesis tidak dapat lepas dari kajian mengenai material penyusun dan prosesproses yang bekerja selama pembentuklahan bentuklahan Colima Volcano, Mexico. ASTER satellite image, Visible/Near Infrared (VNIR) from 26 February 2007. Source: pitt.edu/~ajc44/
1. Proses-proses dan hukum-hukum fisik yang sama yang bekerja sekarang bekerja pula pada waktu geologi, walaupun tidak selalu dengan intensitas sama seperti sekarang, 2. Struktur geologi merupakan faktor pengontrol dominan dalam evolusi bentuklahan dan stuktur geologi dicerminkan oleh bentuklahannya’ 3. Proses-proses geomorfik meninggalkan bekasbekasnya yang nyata pada bentuklahan dan setiap proses geomorfik yang berkembang akan mempunyai karakteristik bentuklahan tertentu.
4. Karena perbedaan tenaga erosi yang bekerja pada permukaan bumi, maka dihasilkan urutan bentuklahan yang mempunyai karakteristik tertentu pada tahap dan perkembangannya, 5. Evolusi geomorfik yang kompleks lebih umum dibanding dengan evolusi yang sederhana, a. Simple form b. Compund form c. Monocyclic form d. Multicyclic forms e. Exhumed.
6. Sebagian kecil relief bumi lebih tua dari Tersier, dan kebanyakan dari relief tersebut lebih muda dari Pleistosen, 7. Interpretasi bentanglahan yang sekarang tidak mungkin dilakukan tanpa memperhatikan perubahan-perubahan geologi dan iklim selama Pleistosen,
8. Apresiasi iklim dunia adalah perlu untuk mengetahui bebagai kepentingan suatu proses geomorfik yang berbeda’
9. Walaupun geomorfologi menekankan pada bentanglahan sekarang, namun untuk mempelajarinya secara maksimum perlu sejarah perkembanganya
Tujuan Klasifikasi Bentuklahan Menyederhanakan bentanglahan di permukaan bumi yang kompleks menjadi unit-unit sederhana yang mempunyai kesamaan dalam sifat dan perwatakannya. Sifat dan perwatakan tersebut mencakup 4 hal: 1. Struktur geologis/geomorfologis 2. Proses geomorfologi 3. Kesan topografis (daratan, perbukitan, pegunungan), 4. Ekspresi topografik (misal: kemiringan lereng, bentuk lereng tunggal maupun majemuk, panjang lereng, bentuk lembah,
Proses Geomorfologi 1. Endogenic Processes: – Volcanism – Plate Tectonics – Diastrophism: Folding, Faulting, Warping 2. Exogenic Systems: – Weathering – Mass Wasting – Erosion, Transportation, and Depositional Processes – Alluvial/Fluvial (flowing water) – Glacial (ice) – Eolian (wind) – Coastal (waves)
Image Source: geoinfo.amu.edu.pl/wpk/pe
PERMUKAAN BUMI
R. ORDE I
BENUA (DARATAN)
LEDOK LAUTAN
R. ORDE II
PEGUNUNGAN
DATARAN
Bentuklahan inisial Struktural Konstruksional Endogen
R. ORDE III
BENTUK EROSIONAL
BENTUK DEPOSISIONAL
BENTUK RESIDUAL
Bentuklahan sekuensial Proses Destruksional Eksogen
Dasar Pemikiran Klasifikasi Bentuklahan
PROSES dan TENAGA GEOMORFOLOGI
Proses Geomorfologi: Semua proses baik fisik maupun khemis yang mengakibatkan modifikasi konfigurasi/ bentuk permukaan bumi Tenaga Geomorfologi: Semua medium alami yang mampu merusak dan mengangkut material bumi
Application • Resource Distribution – Soils and Agriculture – Water resources – Forest and Biological
• Natural Hazards – Monitoring – Prediction
• Planning – Transportation – Development Image Source: casoilresource.lawr.ucdavis.edu
PROSES GEOMORFOLOGI
EKSOGEN
DEGRADASI
AGRADASI
ENDOGEN
EKSTRA TERESTRIAL
VOLKANISME
DIASTROFISME
Pelapukan
Mekanis
Khemis
Mass Wasting
Organism
Erosi
1.
Pelapukan: pecahnya batuan akibat disintegrasi dan dekomposisi; belum ada gerakan massa (tidak termasuk pelepasan dan pengangkutan)
2.
Mass wasting: semua pengangkutan massa puingpuing batuan menuruni lereng akibat pengaruh langsung tenaga gravitasi
3.
Erosi: proses terlepas dan terangkutnya material bumi oleh tenaga air.
1. Permulaan penyebab terjadinya gerak massa batuan dan erosi’ 2. Faktor perendahan permukaan lahan secara umum 3. Pengaruh terbentuknya berbagai bentuklahan 4. Proses utama dalam pembentukan regolit dan tanah.
• Aliran lambat:
creep : soil creep.
talus creep rock creep rock glacier creeps. solifluction
• Aliran cepat: Earth flow Mud flow Debris avalance Land slides: • • • • •
• Subsidence
Slump Debris sloipe’ Debris fall Rock slide Rock fall
1. 2. 3. 4. 5. 6. 7.
Land slide Debris avalance Earth flow Mud flow Sheet flow Slope wash Stream
LONGSOR CROWN
Jalan Putus Oleh Longsoran Akibat Banjir
Bangunan Rumah Rusak Terkena Tanah Longsor
BENTUK LERENG ASLI
BENTUK LERENG SEKARANG
PROSES LERENG MASA LAMPAU
PROSES LERENG SEKARANG
Hubungan timbal balik masing2 bentuk lereng yang dapat diukur
BENTUK LERENG BARU (YAD)
Kemungkinan pengukuran pada periode tertentu
1
2
3
1. 2. 3.
Main slope retreat Main slope decline Main slope shortening
• 1. 2. 3.
Hal ini tergantung pada: Bentuk lereng asli Karakteristik internal lereng Seluruh karakteristik lereng utama (internaldan eksternal)
A. Menurut W. Penk
B. Menurut W.M. Davis
3 Faktor yang mempengaruhi perkembangan landscape (W.M Davis) 1. Struktur 2. Proses 3. Stadia (waktu) Dengan waktu terdapat adanya tingkat (stage) perkembangan: 1. Stadium muda 2. Stadium dewasa 3. Stadium tua
Permukaan asli
Muda
Dewasa
Level dasar • •
•
Tua
Stadium muda: lahan masih tinggi, banyak dijumpai permukaan asli, lembah dalam, dinding terjal, erosi aktif Stadium dewasa: lahan mulai rendah, lembah melebar dan terjal, interfluve membulat/ runcing. Disini terjadi “relief maksimum” ketika lembah masih mempunyai puncak (crest) sempit. Stadium tua: permukaan lahan rendah, lereng datar-landai, sungai mengalir memotong dataran banjir, erosi dan deposisi dalam keadaan seimbang.
pengangkatan erosi transportasi mencapai base level erosi terhenti nyaris dataran (peneplain)
Geomorphology in the rock cycle • Every part of the rock cycle that occurs at the Earth’s surface has geomorphic consequences
Relevance of geomorphology • Geomorphology is important because people live on landforms and their lives are affected (sometimes catastrophically) by geomorphic processes: • Slope determines whether soil accumulates and makes arable land • Slope stability controls landslides • Mountains drastically affect the weather: rainshadows, monsoons
• This is also a two-way process: Human action is one of the major processes of geomorphic evolution: • People have been building terraced hillsides for thousands of years • People dam rivers, drain groundwater, engineer coastlines • People plant or burn vegetation on a huge scale • People are paving the world
Geomorphic Concepts • Elevation: height above sea level • Slope: spatial gradients in elevation • Relief: the contrast between minimum and maximum elevation in a region
How high is this mountain?
• Important: a mountain is a feature of relief, not elevation (a high area of low relief is a plateau)
– Slope controls the local stability of hillsides and sediment transport – Relief controls the regional erosion rate and sediment yield – Elevation directly affects erosion and weathering only through temperature, however, high elevation and high relief are generally pretty well-correlated
Geomorphic Concepts • Uplift/subsidence – vertical motions of the crust (i.e., of material points)
• Accumulation/denudation – vertical change in the position of the land surface with respect to material points in the bedrock.
• Important: the net rate of change in elevation of the land surface is the sum of uplift/subsidence rate and accumulation/denudation rate. Denudation
elevation = Uplift + Denudation
Elevation
Uplift
• Isostasy
Geomorphic Concepts
– The result of Archimedes’ principle of buoyancy acting on the height of the land surface in the limit of long timescale (fluid-like mantle below the depth of compensation) and long lengthscale (longer than the flexural wavelength of the lithosphere). – The total mass per unit area above some depth of compensation (in the asthenosphere) should be globally constant. – Areas that satisfy the principle of isostasy are called isostatically compensated.
Geomorphic Concepts • Variation in topography can be compensated through two end-member mechanisms: differences in the thickness of layers or differences in the density of layers. – Isostatic compensation through density differences is Pratt isostasy (in the pure form each layer is of constant thickness). – Isostatic compensation through differences in the thickness of layers (where the layer densities are horizontally constant) is Airy isostasy.
Air ~0
Air ~0
Geomorphic Concepts • In reality, both mechanisms operate together: neither the thickness nor the density of the crust is constant. • However, since the density contrast between crust and mantle is larger than most internal density differences within either crust or mantle, the dominant mechanism of isostatic compensation is variations in crustal thickness, i.e. Airy isostasy.
Geomorphic Concepts • Items for speculation: – Why is the top of the ocean crust lower than the top – – –
– – –
of the continental crust? Why is Iceland above sea level? Are subduction zone trenches isostatically compensated? What controls how long it takes to achieve isostatic compensation? What controls the lengthscale over which isostasy operates? What do gravity anomalies have to do with isostasy? What happens when you put an ice-sheet on a continent? What happens when you take it off?
Drainage networks and Catchment Areas • By mapping local maxima (divides) in topography, natural terrains can always be divided, at all scales (from meters to 1000 km), into catchment areas, each exited by one principal drainage, into which surface runoff is channeled • This is not a necessary property of any surface…it is the result of processes that act to shape the landscape
Geomorphic Concepts • Fractal geometry – the forces that shape landscapes are often scaleindependent and lead to hierarchical regularity across scale, often with fractional scaling relations, hence fractals. The classic examples: • Length of a coastline: coastlines get longer when measured with shorter rulers. • Branching networks: drainage channels come in all sizes, and join together to produce networks whose branching statistics are fractal.
“Process” geomorphology • Quantitative, physically based analysis of morphology in terms of endogenic and exogenic energy sources • Basics of process geomorphology – 1) Assume balance between forms and process (equilibrium and quasi-equilibrium) – 2) Balance created and maintained by the interaction between energy states (kinetic and potential); force and resistance. – 3) Changes in force-resistance balance may push the landscape and processes too far: thresholds of change exist: fundamental change of process and thus form. – 4) Processes are linked with multiple levels of feedback. – 5) Geomorphic analysis occurs at multiple spatial and temporal scales.
Process geomorphology • An example of a quantifiable process: hillslope evolution • What controls stability of a slope? Lithology and water, mostly
Hillslope evolution: qualitative approach
Some rocks are resistant to erosion (they form cliffs), some are weak (they form slopes). Resistant and weak are qualitative terms, but useful for describing landscape evolution.
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