Unit 5 - Rock Cycle and Igneous Rocks

Igneous rocks

Rocks that form when melted rock (magma or lava) cools and hardens.

Sedimentary rocks

Rocks formed from small particles of other rocks, minerals, or organic materials that get pressed and cemented together over time.

Metamorphic rocks

Rocks that have changed from one type to another due to heat, pressure, or chemical processes deep inside the Earth.

Intrusive (Plutonic) Igneous Rocks

Form inside the Earth's crust when magma cools slowly, resulting in large crystals and a coarse-grained texture. Example: Granite.

Extrusive (Volcanic) Igneous Rocks

Form on the surface when lava cools quickly, resulting in small or no crystals and a fine-grained or glassy texture. Examples: Basalt, Obsidian, Pumice.

Felsic Rocks

Igneous rocks with high silica content (over 65%), light-colored, rich in minerals like quartz and feldspar, and less dense. Examples: Granite, Rhyolite.

Mafic Rocks

Igneous rocks with low silica content (45-55%), dark-colored, rich in minerals like pyroxene and olivine, and denser. Examples: Basalt, Gabbro.

Coarse-Grained (Phaneritic)

Igneous rocks with large, visible crystals that form when magma cools slowly inside the Earth. Example: Granite.

Fine-Grained (Aphanitic)

Igneous rocks with small, microscopic crystals that form when lava cools quickly on the surface. Example: Basalt.

Vesicular/Frothy

Igneous rocks that have holes or bubbles (vesicles) formed by trapped gas, resulting from rapid cooling of lava. Example: Pumice.

Glassy

Igneous rocks with no crystals and a smooth, glass-like appearance formed when lava cools extremely fast. Example: Obsidian.

Porphyritic

Igneous rocks that have two different crystal sizes, with large crystals embedded in a fine-grained matrix, formed from partial slow cooling followed by rapid cooling. Example: Porphyritic Andesite.

Felsic or Mafic Composition

Felsic Rocks are light-colored, high silica, rich in quartz & feldspar; Mafic Rocks are dark-colored, low silica, rich in pyroxene & olivine.

Intrusive or Extrusive

Intrusive (Plutonic) rocks are coarse-grained (large crystals, slow cooling); Extrusive (Volcanic) rocks are fine-grained, glassy, or vesicular (small/no crystals, fast cooling).

Batholith

The largest intrusive feature (over 100 km²) that forms deep underground from slow-cooling magma, mostly composed of granite. Example: Sierra Nevada Batholith (USA).

Stock

Similar to a batholith but smaller (less than 100 km²), also made of coarse-grained igneous rock.

Laccolith

An intrusive feature where magma pushes upward, creating a dome-shaped intrusion that can cause the overlying rock to bulge.

Lopolith

A bowl-shaped intrusion formed by magma.

Lopolith

A bowl-shaped intrusion formed by dense, mafic magma that sinks and spreads in layers.

Sill

A horizontal intrusion of magma between rock layers, parallel to surrounding rock layers. Example: Palisades Sill (New York, USA).

Dike

A vertical or diagonal intrusion that cuts across rock layers, forming when magma moves through cracks in the crust.

Silica (SiO₂)

A major component that determines the viscosity of molten rock (magma/lava).

High Silica

Results in high viscosity (thick and slow-moving) magma, leading to explosive volcanic eruptions.

Felsic magma

Magma rich in silica (>65%) that has high viscosity, making it thick and sticky.

Low Silica

Results in low viscosity (runny and fast-flowing) magma, allowing gas to escape smoothly.

Mafic magma

Magma low in silica (45-55%) that has low viscosity, flowing easily and leading to gentle eruptions.

Composite Volcanoes (Stratovolcanoes)

Tall, steep-sided, cone-shaped volcanoes built from alternating layers of lava flows, ash, and volcanic debris.

Shield Volcanoes

Broad, dome-shaped volcanoes with gentle slopes, built from thin, overlapping lava flows.

Cinder Cone Volcanoes

Small, steep-sided cones built from loose volcanic fragments (cinders, ash, and bombs) with short, explosive eruptions.

Difference between Magma and Lava

Magma is molten rock beneath the Earth's surface, while lava is molten rock that has reached the surface.

Magma

Molten rock that is still beneath the Earth's surface, found deep inside the Earth, in the mantle or in chambers beneath the crust.

Lava

Molten rock that has reached the Earth's surface during a volcanic eruption, flowing out of a volcano or fissure.

High-silica magma

Leads to explosive eruptions due to trapped gas and builds pressure.

Low-silica magma

Leads to effusive eruptions where lava flows easily with minimal explosions.

Examples of Composite Volcanoes

Mount St. Helens (USA), Mount Fuji (Japan), Mount Vesuvius (Italy).

Examples of Shield Volcanoes

Mauna Loa & Kilauea (Hawaii), Galápagos Islands volcanoes.

Examples of Cinder Cone Volcanoes

Paricutin (Mexico), Sunset Crater (USA), Cerro Negro (Nicaragua).

Magma

Underground molten rock.

Lava

Molten rock that has erupted onto the Earth's surface.

Subduction Boundaries

Where one tectonic plate is forced beneath another, leading to volcanic eruptions.

Oceanic Plate Subduction

Occurs when an oceanic plate sinks into the mantle, melting and creating magma.

Pacific Plate

An oceanic plate that is subducting beneath the North American Plate along the Cascadia Subduction Zone.

Nazca Plate

An oceanic plate subducting beneath the South American Plate, causing volcanic activity in the Andes.

Hot Spot Volcanoes

Volcanoes that occur far from plate boundaries, typically due to mantle plumes.

Mantle Plume

An upwelling of abnormally hot rock from deep within the Earth's mantle that melts and creates magma.

Hawaii

A location where the Pacific Plate moves over a hot spot, creating volcanoes like Mauna Loa and Kilauea.

Yellowstone

A hot spot under a continental plate in the U.S.

Dike

A vertical or diagonal sheet-like intrusion of magma that cuts across existing rock layers.

Sill

A horizontal sheet-like intrusion of magma that lies parallel to the layers of surrounding rock.

Laccolith

A dome-shaped intrusion that pushes up the layers of rock above it, creating a bulging structure.

Batholith

A large, massive body of intrusive igneous rock that forms deep within the Earth's crust.

Giant's Causeway

An example of a dike located in Northern Ireland.

Palisades Sill

An example of a sill located along the Hudson River in the USA.

Henry Mountains

An example of a laccolith located in Utah, USA.

Sierra Nevada Batholith

An example of a batholith in the USA, which includes Yosemite National Park.

Contour Lines

Contour lines are lines drawn on a map that connect points of equal elevation above a specific reference point, such as sea level. Purpose: These lines represent the shape of the terrain (topography) and help depict hills, valleys, slopes, and other landforms on a two-dimensional map. Characteristics: The closer the contour lines are to each other, the steeper the slope. The further apart they are, the gentler the slope.

Contour Interval

The vertical distance or difference in elevation between two adjacent contour lines on a map. Purpose: It shows how much elevation changes between one contour line and the next. Example: If the contour interval is 10 meters, each contour line represents an elevation that is 10 meters higher than the previous one. Variation: The contour interval may vary depending on the scale of the map and the level of detail needed.

Index Contour Lines

Thicker or heavier contour lines that are marked with elevation values at regular intervals to help read the map more easily. Purpose: Index contour lines are used to show the main elevations on a map and make it easier to identify the elevation of nearby contour lines. They are typically drawn every fifth contour line, but the interval can vary based on the map's scale. Example: On a map with a 20-meter contour interval, index contour lines might be marked every 100 meters (5 x 20 meters) with the elevation written on them.

How to Determine the Contour Interval for a Map

Contour Interval: Difference in elevation between two adjacent index contour lines, divided by the number of lines between them.

Elevation of a Point

Look at the nearest contour lines and estimate the elevation based on their proximity.

Matching Profile Drawings with Topographic Map Sections

To match a profile drawing with a topographic map section, follow these steps: Identify the Line of Section: Choose a straight line on the map that you want to create the profile for. This line represents the path along which you will examine elevation changes. Mark Elevations: Find the contour lines that intersect the line of section on the map. Note the elevation values of these lines. If the line is between contour lines, estimate the elevation based on the contour interval. Plot the Profile: On graph paper, draw a horizontal axis for distance and a vertical axis for elevation. Plot the elevation points along the distance of the line of section, then connect these points with a smooth curve to form the profile. Match the Profile to the Map: Compare the plotted profile with the terrain on the map. Ensure the profile's shape (hills, valleys, ridges, etc.) reflects the changes in elevation seen in the map's contour lines.

Rule of V's

The Rule of V's is a method used to determine the direction of flow of a stream or river based on its interaction with contour lines on a topographic map.

How the Rule of V's Works

When contour lines cross a stream or river, they form a 'V' shape. The point or apex of the V always points upstream, meaning it points toward the source of the river or stream. The open end of the V points downstream, indicating the direction the river or stream is flowing.

Using the Rule of V's

To determine the direction of flow: Look at the V-shape formed by the contour lines around the stream. If the V is pointing upward, the stream is flowing downhill in the opposite direction (downstream). If the V is pointing downward, the stream is flowing uphill toward the source (upstream).

Example of Rule of V's

If you see a V-shape where the contour lines meet the stream, and the point of the V is facing up the hill, you know that the stream flows downhill in the direction of the open end of the V. If the V is upside down, the water flows toward the point of the V.