Interior of the Earth and Rocks
NCERT Class 11 - Fundamentals of Physical Geography, Chapters 3 and 5
Interior of the Earth and Rocks
We cannot see inside the earth, so almost everything known about it comes from indirect evidence, above all from the behaviour of earthquake waves. These reveal three main layers - crust, mantle and core - separated by sharp boundaries.
Sources of information
- Direct sources - surface rocks, material thrown out by volcanoes, and deep mining and drilling. Even the deepest holes reach only a small fraction of the way to the centre.
- Indirect sources - the change of temperature, pressure and density with depth, the study of meteorites, and the earth's gravity and magnetic field.
- The most important indirect source is seismic waves, since their speed and path change with the material they pass through.
The layers
| Layer | Depth and nature | Boundary below it |
|---|---|---|
| Crust | Thin outer shell, thicker under continents than under oceans | Mohorovicic discontinuity |
| Mantle | Extends to about 2900 kilometres; includes the soft asthenosphere | Gutenberg discontinuity |
| Outer core | In a liquid state, made largely of iron and nickel | Lehmann discontinuity |
| Inner core | Solid because of enormous pressure, despite very high temperature | Centre at about 6400 kilometres |
- The continental crust is largely silica and aluminium, called SIAL; the oceanic crust is silica and magnesium, called SIMA; the core is mainly nickel and iron, called NIFE.
- The asthenosphere, in the upper mantle, is partly molten and is the source of magma; the rigid lithosphere of crust and uppermost mantle floats on it.
- The earth's magnetic field is generated by motion in the liquid outer core.
Seismic waves and shadow zones
- P waves or primary waves are longitudinal, travel fastest, arrive first, and pass through solids, liquids and gases.
- S waves or secondary waves are transverse, slower, and pass only through solids - the key evidence that the outer core is liquid.
- L waves or surface waves travel along the surface, arrive last and cause the most destruction.
- The S wave shadow zone covers everything beyond about 105 degrees from the epicentre, because S waves cannot cross the liquid outer core.
- The P wave shadow zone lies roughly between 105 and 145 degrees, because P waves are refracted by the core.
The three types of rocks
- Igneous rocks form from cooling magma or lava. Intrusive rocks such as granite cool slowly underground and have large crystals; extrusive rocks such as basalt cool quickly at the surface and have fine grains. All other rocks derive originally from these, so they are called primary rocks.
- Sedimentary rocks form from accumulated and compacted sediment; they occur in layers or strata and are the only rocks that commonly contain fossils. Examples are sandstone, shale, limestone and coal.
- Metamorphic rocks form when existing rocks are changed by heat and pressure without melting.
| Original rock | Metamorphic form |
|---|---|
| Limestone | Marble |
| Sandstone | Quartzite |
| Shale or clay | Slate |
| Granite | Gneiss |
| Coal | Graphite and, under extreme conditions, diamond |
- The rock cycle is the continuous conversion of one type into another - igneous rock weathers into sediment that hardens into sedimentary rock, which heat and pressure turn into metamorphic rock, which on melting becomes magma and then igneous rock again.
हिंदी संकेत: S तरंगें केवल ठोस से गुजरती हैं, और इसी से सिद्ध हुआ कि बाह्य क्रोड तरल है। यह पृथ्वी के आंतरिक भाग का सबसे महत्वपूर्ण प्रमाण है।
Exam me kaise aata hai
Match-the-following on discontinuity and the layers it separates, and on original rock and its metamorphic form, are both very common. Factual questions ask which wave passes through liquids and the extent of the shadow zones. Statement-based questions test which rocks contain fossils.
UPSC / State PSC ke liye
The reasoning chain worth stating is that S waves stop at the outer core, therefore the outer core is liquid, therefore convection there generates the magnetic field. On rocks, note that only sedimentary rocks preserve fossils, which is why they carry the record of past life and also host coal and petroleum.
Yahan confuse hote hain
✗ S waves travel through liquids | ✓ Only P waves do; S waves travel only through solids
✗ The Gutenberg discontinuity separates the crust from the mantle | ✓ That is the Mohorovicic discontinuity; Gutenberg separates mantle from core
✗ Fossils are commonly found in igneous rocks | ✓ They are found in sedimentary rocks
Ek nazar me
- Knowledge of the interior comes mainly from seismic waves and other indirect evidence.
- Layers - crust, mantle, outer core and inner core.
- Mohorovicic separates crust from mantle; Gutenberg separates mantle from core.
- Crust is SIAL and SIMA; the core is NIFE of nickel and iron.
- P waves pass through all media; S waves only through solids; L waves are most destructive.
- S wave shadow zone beyond 105 degrees; P wave shadow zone between 105 and 145 degrees.
- Igneous, sedimentary and metamorphic rocks form a continuous rock cycle; fossils occur in sedimentary rocks.
Ab isi chapter ke questions ExamAtlas mock test me practice karo.