Rock Cycle and Igneous Rocks

Rock Cycle

• The rock cycle demonstrates the continuous transformation of igneous, sedimentary, and metamorphic rocks.
• Erosion creates sediment that is transported and deposited, forming sedimentary rocks.
• Sedimentary rocks are transformed into metamorphic rocks under high temperature and pressure.
• Metamorphic rocks melt at great depths, creating magma.
• Magma rising to the Earth's surface cools and solidifies, forming igneous rocks.
• Weathering and erosion of igneous rocks restarts the cycle.

Igneous Rocks

• Igneous rocks constitute approximately 90% of the Earth's crust.
• They form from the cooling and solidification of magma, a molten silicate melt.
• Magma originates from deep within the Earth and primarily consists of oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K).
• When magma reaches the Earth's surface, it is called lava and loses its gases.

Chemical Composition

• Igneous rocks have a limited range of chemical compositions.
• Silicon dioxide (SiO₂) is the primary constituent, ranging from 40% to 75% by weight.
• Other significant oxides include aluminum oxide (Al₂O₃), calcium oxide (CaO), magnesium oxide (MgO), and iron oxides (Fe), typically comprising 10-20% each.

Acid and Basic Rocks

• The chemical composition of igneous rocks reflects the composition of the magma from which they formed.
• Magma can be broadly classified as acidic or basic.
• Acidic magma is rich in silicon (Si), sodium (Na), and potassium (K), but poor in calcium (Ca), magnesium (Mg), and iron (Fe).
• Basic magma is rich in calcium (Ca), magnesium (Mg), and iron (Fe), but poor in silicon (Si), sodium (Na), and potassium (K).

Classification of Igneous Rocks based on Silica Percentage

• Igneous rocks are categorized based on their silica content:
  • Ultrabasic Rocks: Less than 45% silica (e.g., peridotite)
  • Basic Rocks: 45% to 55% silica (e.g., gabbro and basalt)
  • Intermediate Rocks: 55% to 65% silica (e.g., diorite)
  • Acid Rocks: More than 65% silica (e.g., granite)

General Characteristics

• Acidic igneous rocks are typically light-colored, have a low specific gravity (around 2.7), and contain minerals like quartz and alkali feldspar (felsic rocks).
• Basic igneous rocks are often dark-colored (often black), have a relatively high specific gravity (around 3.2), and mainly contain silica-poor minerals such as olivine, pyroxene, hornblende, or biotite (mafic rocks), with little or no quartz.
• Basalt is an example of a basic rock.