Junior Cycle Geography Rocks

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The Rock Cycle

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The Rock Cycle

The rock cycle is a continuous process where rocks are formed, broken down, and transformed into new types of rocks over time. It explains the relationship between the three main rock types: igneous, sedimentary, and metamorphic.

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Stages of the Rock Cycle

1. Formation of Igneous Rocks:

   • Process: Magma (molten rock) cools and solidifies.

   • Examples:

     • Granite (intrusive, cools slowly underground).

     • Basalt (extrusive, cools quickly on the surface).

2. Weathering and Erosion:

   • Rocks on the Earth's surface are broken into small particles by wind, water, ice, and temperature changes.

   • These particles are called sediments.

3. Formation of Sedimentary Rocks:

   • Process: Sediments are transported by water, wind, or ice and settle in layers.

   • Over time, layers are compacted and cemented together to form sedimentary rocks.

   • Examples:

     • Limestone (made from marine fossils).

     • Sandstone (made from sand grains).

4. Formation of Metamorphic Rocks:

   • Process: Rocks are changed by heat and pressure deep within the Earth during tectonic activity.

   • Examples:

     • Limestone becomes marble.

     • Shale becomes slate.

5. Melting:

   • Rocks are buried deep in the Earth and melt due to intense heat, forming magma.

   • The cycle then begins again when magma cools into igneous rocks.

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Key Points

• The rock cycle is continuous and does not have a starting or ending point.

• Rocks can change into any type depending on the conditions (e.g., heat, pressure, or erosion).

• Examples in Ireland:

  • Igneous Rock: Granite in the Wicklow Mountains.

  • Sedimentary Rock: Limestone in The Burren.

  • Metamorphic Rock: Marble in Connemara.

This cycle shows how rocks are constantly recycled and transformed over millions of years!

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Igneous Rocks

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Igneous rocks are rocks formed from the cooling and solidification of molten rock material. There are two main types based on where this process occurs:

1. Intrusive Igneous Rocks (also called Plutonic Rocks):

   • Formed when magma cools slowly beneath the Earth's surface.

   • This slow cooling allows large crystals to form.

   • Example: Granite.

2. Extrusive Igneous Rocks (also called Volcanic Rocks):

   • Formed when lava cools quickly on the Earth's surface.

   • The rapid cooling leads to the formation of small crystals.

   • Example: Basalt.

Igneous rocks are the foundation of the Earth's crust and can vary in texture, colour, and mineral composition depending on the conditions under which they form.

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Intrusive & Extrusive

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Granite

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Granite

• Type: Intrusive igneous rock.

• Formation:

  • Forms when magma cools slowly beneath the Earth's surface, allowing large crystals to develop.

  • Made of quartz, feldspar, and mica.

• Features:

  • Hard, durable, and resistant to weathering.

  • Typically light-coloured with visible crystals (e.g., grey, pink, or white).

• Example in Ireland: Found in the Wicklow Mountains.

• Uses:

  • Used in construction, buildings, monuments, and kitchen countertops.

  • Polished for decorative purposes in tiles and sculptures.

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Basalt

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Basalt

• Type: Extrusive igneous rock.

• Formation:

  • Forms when lava cools quickly on the Earth's surface, resulting in small crystals.

  • Composed mainly of silica, iron, and magnesium minerals.

• Features:

  • Dark-coloured (black or dark grey).

  • Fine-grained due to rapid cooling.

  • Hard and resistant to weathering.

• Example in Ireland: Found at the Giant's Causeway, Co. Antrim, with unique hexagonal columns formed by cooling lava.

• Uses:

  • Used in road building, foundations, and as crushed stone in construction.

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The Giant's Causeway

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The Giant's Causeway

• Type: Natural basalt rock formation.

• Location: County Antrim, Northern Ireland, along the northeast coast.

• Formation:

  1. Volcanic Activity: Around 60 million years ago, lava from volcanic eruptions cooled and solidified rapidly.

  2. Cooling and Cracking: As the lava cooled, it contracted and formed hexagonal columns.

• Features:

  • Composed of around 40,000 basalt columns, mostly hexagonal, though some have 4-8 sides.

  • Columns vary in height, with some reaching 12 metres.

• Significance:

  • Declared a UNESCO World Heritage Site in 1986.

  • Linked to Irish mythology, said to be built by the giant Finn McCool as a pathway to Scotland.

• Tourism:

  • One of Ireland's most famous landmarks, attracting visitors for its unique geology and folklore.

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Sedimentary Rocks

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Sedimentary Rocks:

• Formation: Made when sediments (small particles) are broken down by weathering, moved by wind, water, or ice, and settle in layers. These layers are then compacted and cemented together.

• Types:

  • Clastic: Made of broken rock pieces (e.g., sandstone).

  • Organic: Made of plant or animal remains (e.g., limestone).

  • Chemical: Made from dissolved minerals (e.g., rock salt).

• Examples in Ireland:

  • Limestone: Found in The Burren, Co. Clare.

  • Sandstone: Found in Macgillycuddy's Reeks, Co. Kerry.

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Sediments

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• Definition: Particles of rock, soil, and organic material that are transported and deposited by natural forces such as water, wind, or ice.

• Formation:

  • These particles often settle in layers over time in environments like rivers, lakes, oceans, or glacial areas.

• Result: Over time, these deposited layers may undergo compaction and cementation, eventually forming sedimentary rocks.

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Limestone

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Limestone

• Type: Organic sedimentary rock formed in warm, shallow, underwater environments, such as ancient seas.

• Formation:

  1. Organic Material: The remains of marine organisms (e.g., corals, shells) settle on the sea floor in warm, shallow waters.

  2. Strata: These remains accumulate in layers (strata) over millions of years.

  3. Lithification: Layers are compacted under pressure and cemented by calcite (CaCO₃) to form solid rock.

• Features:

  • Permeable and Porous: Water can pass through it due to pores between particles.

  • Bedding Planes: Horizontal layers where water flows.

  • Vertical Joints: Cracks formed during rock compression, increasing permeability.

  • Contains fossils, reacts with acid, and forms karst landscapes.

• Example in Ireland: Found in The Burren, Co. Clare, with features like grikes (deep cracks) and clints (flat rock slabs).

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Sandstone

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Sandstone

• Type: Clastic sedimentary rock made from sand grains.

• Formation:

  1. Weathering and Erosion: Rocks are broken into sand-sized particles.

  2. Transportation: Sand is carried by rivers, wind, or ice and deposited in layers in deserts, riverbeds, or shallow seas.

  3. Lithification: Over millions of years, layers of sand are compacted and cemented by minerals like silica or calcite, forming solid rock.

• Features:

  • Strata: Formed in horizontal layers.

  • Porous: Water can pass through its small spaces.

  • Durable: Resistant to erosion due to high quartz content.

• Example in Ireland: Found in Macgillycuddy's Reeks, Co. Kerry.