IIT JAM Geology: Earth Materials, Structures and Time
IIT JAM GG tests mineralogy, petrology, structural geology, stratigraphy, paleontology, geophysics, geochemistry and hydrogeology. Strong answers link processes to observable rock textures, structures and measurable properties rather than listing terms.
Core rules and concepts
- Mineralogy: silicate structures depend on SiO₄ tetrahedra; Bowen’s reaction series predicts crystallization order; Mohs hardness and cleavage reflect bonding.
- Petrology: igneous rocks are classified by texture and composition; sedimentary rocks record transport and deposition; metamorphic facies indicate pressure–temperature conditions.
- Structural geology: strike is the compass direction of a horizontal line on a plane; dip is the angle of steepest descent. Folds, faults and joints record stress and strain.
- Stratigraphy: superposition, original horizontality, lateral continuity and cross-cutting relationships establish relative age; unconformities represent missing time.
- Geophysics: P-waves are faster than S-waves; S-waves do not travel through liquids; seismic shadow zones constrain Earth’s interior.
- Hydrogeology: Darcy’s law Q = KAi relates discharge, hydraulic conductivity, area and hydraulic gradient.
Worked example
A mineral sample contains 25% parent isotope and 75% daughter isotope. If the half-life is 1.3 Ga, how old is the sample? 25% parent means one-quarter remains, and (1/2)² = 1/4, so two half-lives have passed. Age = 2 × 1.3 = 2.6 Ga. This assumes no loss or gain of parent or daughter.
Traps that cost marks
Do not confuse parent remaining with daughter accumulated. Do not interchange strike and dip. P-waves arrive before S-waves, and S-waves are not transmitted through fluids. An unconformity is a time gap, not necessarily a fault. Porosity is not the same as permeability.
Key points
- Parent fraction (1/2)ⁿ gives the number of half-lives n.
- Strike is horizontal; dip is steepest descent.
- P-waves are faster; S-waves require rigidity.
- Superposition and cross-cutting relations give relative age.
- Darcy’s law links discharge to gradient and conductivity.