12-Rock CycleCV (1)

Module 3: Geosphere: The Rock Cycle

Introduction

• Focus: Understanding the three major types of rocks and their formation processes.

Major Types of Rocks

• Three Major Types:

  • Igneous Rocks

  • Sedimentary Rocks

  • Metamorphic Rocks

Formation of Rocks

• Igneous Rocks:

  • Formed by the cooling and solidification of molten magma or lava.

  • Extrusive: Cooled above Earth's surface (e.g., basalt).

  • Intrusive: Cooled below Earth's surface (e.g., granite).

• Sedimentary Rocks:

  • Formed through erosion, transportation, deposition, and cementation of sediments or fossils.

  • Example: Shale, Sandstone.

• Metamorphic Rocks:

  • Formed by the re-crystallization of existing rocks due to increased temperature and/or pressure.

  • Exhibit textures such as foliation or distinct mineral crystals.

Driving Forces of the Rock Cycle

• Earth’s Internal Heat:

  • Drives processes like igneous and metamorphic rock formation.

• Water Cycle (Hydrologic Cycle):

  • Primarily drives weathering and sedimentary rock formation.

• Geological Interpretation:

  • Geologists use rock types to interpret past environments and conditions.

Composition and Texture of Rocks

• Understanding a rock's history involves:

  • Composition: The minerals present in a rock, influences nature and appearance.

  • Texture: Size, shape, and arrangement of minerals; affects appearance.

• Comparison of textures in different igneous rocks:

  • Granite vs. Basalt.

Observations of Igneous Rocks

• Extrusive (Volcanic) Rocks: Formed from lava at the surface.

• Intrusive (Plutonic) Rocks: Formed from magma below the surface.

• Texture based on cooling rate:

  • Fast Cooling: Fine-grained or glassy texture.

  • Slow Cooling: Coarse-grained texture.

Sedimentary Rocks Composition

• Clasts: Sediments or rock fragments that form sedimentary rocks by being cemented together.

• Matrix: Finer material filling spaces between clasts.

• Texture Indicators:

  • Small clasts indicate far transport and extensive weathering.

  • Large clasts indicate proximity to source of origin.

Composition of Sedimentary Rocks

• Composition often based on clasts (rocks or fossils).

• Surface processes note that sedimentary rocks frequently contain fossils.

Metamorphic Rocks

• Key Characteristics:

  • Formed under high pressure and temperature without melting.

  • Foliation: Parallel alignment of minerals due to pressure.

• Types of Metamorphic Rocks (based on pressure and temperature):

  • Phyllite: Low pressure/temperature.

  • Schist: Medium pressure/temperature.

  • Gneiss: High pressure/temperature.

Summary: Processes of Rock Formation

• Igneous Rocks:

  • Cooling and crystallization of magma/lava, controlled mainly by Earth's internal heat.

• Sedimentary Rocks:

  • Formed through processes driven by the water cycle. Most likely to contain fossils.

• Metamorphic Rocks:

  • Result from intense heat and pressure; cannot be formed by melting as that would create igneous rocks instead.

Review Questions

• Discuss the two main processes controlling the rock cycle.

• Relate each rock type to its formation process:

  • How do igneous rocks form?

  • Why are sedimentary rocks linked to the water cycle?

  • What happens during the metamorphosis of rocks without melting?

Two Main Processes Controlling the Rock Cycle:

1. Earth’s Internal Heat: Drives processes like igneous and metamorphic rock formation.

   • Igneous rocks are formed by the cooling and crystallization of magma or lava. The process involves cooling that can occur either above (extrusive) or below (intrusive) Earth's surface.

   • Observing texture helps us understand volcanic eruptions: porous volcanic rocks, such as pumice, indicate rapid cooling and gas release during eruptions, which can tell us about the eruption's intensity and style.

2. Water Cycle (Hydrologic Cycle): Primarily controls the weathering, sedimentation, and formation of sedimentary rocks.

   • Sedimentary rocks are formed through weathering, erosion, transportation, deposition, and cementing of clasts from pre-existing rocks. This rock type is most associated with the water cycle due to the processes of erosion and deposition that are driven by water movement in rivers, lakes, and oceans.

   • Sedimentary rocks are most likely to contain fossils as they form in environments where organisms can be preserved in sediments before compaction and cementation occur.

3. Metamorphic Rocks: Formed by the alteration of pre-existing rocks under high temperature and pressure without melting. If a rock is melted, it becomes an igneous rock instead of a metamorphic rock. Metamorphic processes involve re-crystallization rather than the transition from a solid to a liquid state.

Two Main Processes Controlling the Rock Cycle

• Earth’s Internal Heat: Drives processes like igneous and metamorphic rock formation.

  • Igneous Rocks: Formed by the cooling and crystallization of magma or lava; can occur extrusively (above Earth's surface) or intrusively (below Earth's surface). Observing texture helps us understand volcanic eruptions; for example, porous volcanic rocks like pumice indicate rapid cooling and gas release, providing insights into the eruption's intensity and style.

• Water Cycle (Hydrologic Cycle): Primarily controls weathering, sedimentation, and formation of sedimentary rocks.

  • Sedimentary Rocks: Formed through weathering, erosion, transportation, deposition, and cementation of clasts from pre-existing rocks. This rock type is most associated with the water cycle due to processes driven by water movement in rivers, lakes, and oceans, leading to environments where fossils can be preserved before the sediment is compacted and cemented.

• Metamorphic Rocks: Formed by alterations under high temperature and pressure without melting. Heating can change a rock to become a metamorphic rock, but melting would result in the formation of igneous rocks instead.