Definisi
GEOMORFOLOGI
• Ilmu yang mendeskripsi (secara genetis) bentuk-lahan dan proses-proses yang mengakibatkan
bentuknya bentuklahan ter-sebut serta mencari antar hubungan antara proses-proses dalam susunan
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
proses-proses yang bekerja
selama pembentuklahan
bentuklahan
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
bekas-bekasnya yang nyata pada bentuklahan dan
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
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
8. Apresiasi iklim dunia adalah perlu untuk
mengetahui bebagai kepentingan suatu
proses geomorfik yang berbeda’
9. Walaupun geomorfologi menekankan
pada bentanglahan sekarang, namun
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
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)
PERMUKAAN BUMI
LEDOK LAUTAN BENUA (DARATAN)
PEGUNUNGAN DATARAN
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
Application
•
Resource Distribution
– Soils and Agriculture– Water resources
– Forest and Biological
•
Natural Hazards
– Monitoring– Prediction
•
Planning
– Transportation
– Development
PROSES GEOMORFOLOGI
EKSOGEN ENDOGEN EKSTRA
TERESTRIAL
Pelapukan Mass Wasting Erosi
Organism
Khemis
1. Pelapukan: pecahnya batuan akibat disintegrasi dan dekomposisi; belum ada gerakan massa (tidak
termasuk pelepasan dan pengangkutan)
2. Mass wasting: semua pengangkutan massa puing-puing batuan menuruni lereng akibat pengaruh
langsung tenaga gravitasi
1.
Permulaan penyebab terjadinya gerak massa
batuan dan erosi’
2.
Faktor perendahan permukaan lahan secara
umum
3.
Pengaruh terbentuknya berbagai bentuklahan
• Aliran lambat: creep : soil creep. talus creep
rock creep rock glacier creeps.
solifluction
• Aliran cepat: Earth flow Mud flow
Debris avalance Land slides:
• Slump
• Debris sloipe’
• Debris fall
• Rock slide
• Rock fall
1.
Land slide
2.
Debris avalance
3.
Earth flow
4.
Mud flow
5.
Sheet flow
6.
Slope wash
BENTUK LERENG ASLI
Hubungan timbal balik masing2
ben-tuk lereng yang dapat diukur PROSES LERENG
MASA LAMPAU
Kemungkinan peng-ukuran pada
1. Main slope retreat 2. Main slope decline 3. Main slope shortening
• Hal ini tergantung pada:
1. Bentuk lereng asli
2. Karakteristik internal lereng
3. Seluruh karakteristik lereng utama (internaldan eksternal)
1 2
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.
Geomorphology in the rock cycle
•
Every part of
the rock cycle
that occurs at
the Earth’s
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
• 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
• Elevation: height above sea level
• Slope: spatial gradients in elevation
• Relief: the contrast between minimum and maximum
elevation in a region
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
.
Uplift
Geomorphic Concepts
•
Isostasy
– 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
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.
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
Geomorphic Concepts
•
Items for speculation:
–
Why is the top of the ocean crust lower than the topof 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
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
Geomorphic Concepts
•
Fractal geometry
– the forces that shape landscapes are often
scale-independent 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,
“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
Process
geomorphology
• An example of
a quantifiable process:
hillslope evolution
• What controls
stability of a slope?
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
