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.

Terima kasih