(a) What are the fundamental concepts used in Geomorphological studies? Explain the concept "little of the earth's topography is…

(a) Fundamental concepts of geomorphology; "little of the Earth's topography is older than Tertiary"

Geomorphology rests on a set of principles set out by Thornbury:

• The same physical processes and laws operating today operated through geologic time (uniformitarianism), though not always at the same rates.
• Geologic structure is a dominant control of landforms and is reflected in them.
• Relief exists because geomorphic processes act at different rates on different rocks.
• Each geomorphic process leaves its own imprint and produces its own assemblage of landforms.
• As agents act, landforms evolve through a sequence of stages.
• Geomorphic evolution is usually complex rather than simple.
• Little of the Earth's topography is older than Tertiary, and most is no older than Pleistocene.
• Present landscapes cannot be understood without knowing the climatic and geologic changes of the Tertiary and Pleistocene.
• Knowledge of world climates is needed to judge the significance of different processes.

The concept of age: denudation removes relief rapidly on a geological scale. Average denudation of 0.1 to 1 mm a year can lower a landscape by hundreds of metres in a few million years, so old land surfaces are destroyed or buried. Most present relief is therefore young: Tertiary uplift built the Himalaya, Alps and Andes, and the Pleistocene glaciations and sea-level changes shaped glacial valleys, lakes, river terraces and coastlines. Exceptions occur on stable cratons and arid areas, such as parts of the Australian shield or ancient plateau surfaces, which preserve older surfaces.

(b) Isostasy and its theories

Isostasy is the state of gravitational equilibrium between the Earth's lithosphere and the denser, plastic asthenosphere, so that blocks of crust of different height float at the same level of compensation, as icebergs float in water. The term was used by Dutton (1889).

Theories:

• Airy (1855): the crust has uniform density but varies in thickness; mountains have deep roots into the denser substratum. Root thickness r = h × ρc / (ρm − ρc), where h is elevation, ρc crust density and ρm mantle density.
• Pratt (1855): the crust varies in density, with lighter columns under high land, up to a uniform depth of compensation, so that the mass of each column is equal.
• Heiskanen (1931): a combination of Airy and Pratt, using a crust with roots and density variation.
• Vening Meinesz (1931): regional compensation, where the lithosphere bends elastically over a wide area, so that a load is supported by flexure.

Evidence is the gravity anomaly pattern, which shows mountains compensated by low-density roots, and isostatic rebound, as in Scandinavia and Canada after the ice melted. Isostasy explains uplift after erosion and subsidence under sediment.

(c) Fold domain and types of folds by closure

A fold domain is a part of a deformed region within which the folds have the same style and orientation (hinge lines and axial surfaces), separate from neighbouring domains. It is used to analyse the fold structure of an area in units.

Folds can be classed by closure, which is the tightness given by the interlimb angle between the limbs:

• Gentle fold: interlimb angle 180–120°, slightly bent beds.
• Open fold: 120–70°.
• Close fold: 70–30°.
• Tight fold: 30–0°, limbs nearly parallel.
• Isoclinal fold: limbs parallel, interlimb angle about 0°.
• Mushroom (fan or negative-angle) fold: limbs converge and then diverge, with interlimb angle below zero.
• Chevron fold: straight limbs and sharp hinge.
• Box fold: flat crest with two hinges and steep limbs.

Closure shows the amount of shortening and strain; tight and isoclinal folds mark high strain.