Rock Cycle and Igneous Rocks

Study Notes

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.

Description

This quiz explores the rock cycle's processes, including the formation of igneous, sedimentary, and metamorphic rocks. It also delves into the characteristics and chemical composition of igneous rocks, which make up a significant portion of the Earth's crust. Test your knowledge on how these rocks are formed and transformed in nature.