Definisi - · PDF fileTujuan Klasifikasi Bentuklahan ... Mass wasting: semua pengangkutan...
Transcript of Definisi - · PDF fileTujuan Klasifikasi Bentuklahan ... Mass wasting: semua pengangkutan...
Definisi GEOMORFOLOGI
• Ilmu yang mendeskripsi (secara genetis) bentuk-lahan dan proses-proses yang mengakibatkan ter-bentuknya bentuklahan ter-sebut serta mencari antar hubungan antara proses-proses 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 proses-proses 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 bekas-
bekasnya 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 Geomorfologi1. 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
LEDOK LAUTANBENUA (DARATAN)
PEGUNUNGAN DATARAN
BENTUK
RESIDUAL
BENTUK
DEPOSISIONAL
BENTUK
EROSIONAL
R. ORDE I
R. ORDE III
R. ORDE II
Bentuklahan inisial
Struktural
Konstruksional
Endogen
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 ENDOGENEKSTRA
TERESTRIAL
DEGRADASI AGRADASI DIASTROFISMEVOLKANISME
Pelapukan ErosiMass Wasting
OrganismKhemisMekanis
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
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:• Slump
• Debris sloipe’
• Debris fall
• Rock slide
• Rock fall
• Subsidence
1. Land slide
2. Debris avalance
3. Earth flow
4. Mud flow
5. Sheet flow
6. Slope wash
7. Stream
LONGSORCROWN
Jalan Putus Oleh Longsoran Akibat Banjir
Bangunan Rumah Rusak Terkena Tanah Longsor
BENTUK LERENG
ASLI
Hubungan timbal
balik masing2 ben-
tuk lereng yang
dapat diukur
PROSES LERENG
MASA LAMPAU
Kemungkinan peng-
ukuran pada perio-
de tertentu
PROSES LERENG
SEKARANG
BENTUK LERENG
SEKARANG
BENTUK LERENG
BARU (YAD)
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 23
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
A. Menurut W. Penk B. Menurut W.M. Davis
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
• People are changing the climate
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
How high is this mountain?
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
Denudation elevation =
Uplift + Denudation
Elevation
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 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 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,
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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