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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

•
•
•
•
•

• Subsidence

Mud flow
Debris avalance
Land slides:
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.

Terima kasih