rocks 1

Rocks

• Integral part of life; found in buildings, cars, food, and even water (considered a rock).

• Categories of Rock:

  • Magmatic Rocks: Formed by cooling of magma.

  • Metamorphic Rocks: Result from physical and chemical transformation of older rocks.

  • Sedimentary Rocks: Formed by deposits on the Earth's surface.

• Elements in Rocks: Rocks can trap elements such as fossils and minerals which provide evidence of past conditions.

The Crust

• Definition: The outer layer of the Earth.

• Types of Crust:

  • Continental Crust: Contains ancient terrain (up to 3.8 billion years old); primarily metamorphosed rocks and granites; thicker (30-40 km, up to 70 km in mountains).

  • Oceanic Crust: Younger (around 200 million years old); primarily composed of silica-poor basalts; thinner (7-12 km).

• Density: Oceanic crust has a density of 2.7-2.9; continental crust has a density of about 2.7.

• Conrad Discontinuity: Shows a difference in composition in continental crust where upper and lower crust differ.

The Mantle

• Separated from the crust by the Moho.

• Upper Mantle:

  • Composed of a rigid layer attached to the crust (lithosphere) and lower layers with increasing density (up to 700 km).

• Lower Mantle: Extends to 2900 km; density ranges from 4.5 to 6.

• Lithosphere vs. Asthenosphere: Lithosphere is rigid and cold while asthenosphere is warm and plastic; contains molten rock.

Magma Formation

• Forms from melting of mantle/crust and crystallization of minerals.

• Can arise from:

  • Action of an internal heat source.

  • Highly evolved metamorphism (anatexic magma).

• Rock Fusion:

  • Melting mechanism varies by composition and conditions; normal conditions prevent rock melting.

  • Altered conditions may facilitate localized melting due to tectonic activity.

• Magmas are less dense than surrounding rock, allowing them to rise to the surface due to buoyancy.

Characteristics of Magma

• Viscosity: Influences the behavior and ascent of magma; molten rock's fluidity aids in ascent.

• Classification of Rocks:

  • By Minerals:

    • Acidity:

      • Acid rocks: >65% silica.

      • Intermediate rocks: 52-65% silica.

      • Basic rocks: 45-52% silica.

      • Ultrabasic rocks: <45% silica.

    • Alkalinity:

      • Alkaline rocks: Rich in potassium and sodium feldspars.

      • Calc-alkaline: Contains plagioclases.

      • Calc-sodic: Contains only plagioclases.

    • Saturation:

      • Supersaturated: No feldspathoids, rich in quartz.

      • Saturated: No feldspathoids.

      • Undersaturated: Rich in feldspathoids.

  • By Texture:

    • Based on mineral proportions and types. E.g., syenites, granites, gabbros.

Origin of Magmas

• Basaltic Magmas: Thought to originate from mantle's initial peridotite.

• Types of Basalts:

  • Tholeitic Basalts: Ejected from hot spots or oceanic islands, stemming from deep mantle.

  • MORBs: Originating from relatively impoverished mantle due to prior partial melting.