Earth surface processes and landform

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1 Earth surface processes and landform Ch. 1 Introduction Ch. 2 geomorphic system Earth surface processes now is a team of USGS (United States Geological Survey), it is not limited in landform or geomorphology. but in this course we emphasize landform only. Earth surface processes and landform is also a journal name.

2 Earth surface processes and landform ---Introduction Introduction what is landform study or geomorphology? Form Process History

3 What is landform study or Geomorphology? Geomorphology is the study of landforms, i.e. the shape of the Earth s surface. It attempts to explain why landscapes look as they do in terms of the structures, materials, processes, and history affecting regions. Geomorphology relates to all the other disciplines of geology in two directions: 討論 Tectonics, petrology, geochemistry, stratigraphy, and climate determine the geomorphology of the earth and its regions by controlling the principal influences on landscape. Therefore evidence from observations of the landscape in turn constrain the tectonic, petrologic, geochemical, stratigraphic, and climatic history of the earth and its regions.

4 Uluru, central Australia Green-grey arkosic sandstone formed during Cambrian Period, with thin coating of iron oxide

5 Coast in Chile, South Ameica

6 Digital elevation model (40m)

7 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

8 Slope evolution Penck view Davis view

9 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. Elevation Denudation elevation = Uplift + Denudation Uplift

10 Geomorphic Concepts Isostasy (First order factor) 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). That is: 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.

Isostatic compensation through density differences is called Pratt isostasy (in the pure form each layer is of constant thickness).

11 Geomorphic Concepts Variation in topography can be made consistent with isostasy through two end-member mechanisms: differences in the thickness of layers or differences in the density of layers. Isostatic compensation through density differences is called Pratt isostasy (in the pure form each layer is of constant thickness). Isostatic compensation through differences in the thickness of layers (where each layer has a different density than the layer above or below, but the layer densities are horizontally constant) is called Airy isostasy. Air ~0 Air ~0

12 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.

13 Process in Geomorphology Processes and the resulting landforms will be analyzed as balances between driving force (e.g., climate and gravity) and the resistance offered by the geologic framework that makes up Earth s surface. Surface features of Mars How to form? processes?

14 Process geomorphology Quantitative, physically based analysis of morphology in terms of endogenic and exogenic energy sources (thus, climate, tectonic factors) 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: processes are linked with multiple levels of feedback. 5) Geomorphic analysis occurs at different temporal scales.

15 Build up a database of process rates in various parts of the globe Build increasing refined models for predicting the short term (in some cases long term) changes of landform. Generated some enormously powerful ideas about stability and instability in geomorphic systems

16 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.

18 Work on mitigation of natural hazards of geomorphic origin : landslide; debri flow; soil erosion; sediment load of rivers; erosion of beaches; cliffs; submerged of coastal lowlands) Contribute to environmental management when Global warming effect on natural resources - water, vegetation crops etc. Predictions of critical boundary changes: such as shift of tree line, and then guide decisions.

19 Form Expressed by Contour map 等高線 Morphological map 地貌圖 DEM (Digital elevation model) 數值地形

20 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 and groundwater flow are channeled. This is not a necessary property of any surface it is the result of processes that act to shape the landscape. Contour map

21 Morphological map

23 Digital elevation model (40m)

24 Process Geomorphic systems Geomorphic system dynamics equilibrium and steady state Magnitude and frequency Modelling in geomorphology

25 Process Geomorphic system Geomorphic system as form and processes structures positive and negative feedbacks Geomorphic system as simple or complex structures Geomorphic system dynamics Equilibrium and steady state thresholds magnitude and frequency

26 A hillslope as a system : storages (waste mantle), inputs (wind dep. debris produc.), outputs (wind erosion), throughputs (debries transport) and units (channel, valley-side slope, interfluve)

28 feedbacks Negative feedback loop Positive feedback loop Slope erosion + Stream Bed load Infiltration capacity + Surface runoff + + Valley-side Slope angle + Channel erosion Slope erosion

29 Types of equilibrium in geomorphology

discharge) If a flood of a particular magnitude has a recurrence interval of 10

30 n+1 T = m T: recurrence interval n: no. of years recorded m: magnitude of flood (with m=1 at the highest recorded discharge) If a flood of a particular magnitude has a recurrence interval of 10 yrs, means that there is a 1 in 10 (10%) chance a flood of this magnitude will occur in any year Magnitude and frequency of flood Gumbel graph

31 Uniformity and non uniformity : a note on methodology Uniformity of process? some processes acted in the past very different from those experienced today Such as when no plants on earth

32 Homework 1. What does earth surface processes study? 2. What can be computed from DEMs? 3.If you have a 100 year Gumbel graph (recurrence interval versus discharge) of Keelung River, Taipei, what information you may obtain when you need to build a bridge?

33 TECTONICS AND GEOMORPHOLOGY for area with active tectonics, like TAIWAN, geomorphology is clearly reflected by the tectonics. Digital terrain model, 40 m grid by Inst. Applied Geology, Nat. Central Univ.

35 Rational and Empirical Approaches to Geomorphology Multiple Working Hypotheses

36 Rational and Empirical Approaches to Geomorphology Multiple Working Hypotheses

The Next Generation Science Standards (NGSS) Correlation to. EarthComm, Second Edition. Project-Based Space and Earth System Science

The Next Generation Science Standards (NGSS) Achieve, Inc. on behalf of the twenty-six states and partners that collaborated on the NGSS Copyright 2013 Achieve, Inc. All rights reserved. Correlation to,