Igneous Rocks

What are igneous rocks?

Rocks that form from the cooling and solidification of magma or lava.

What are sedimentary rocks?

Rocks that form from the accumulation, compaction, and cementation of sediments derived from weathered preexisting rocks.

What are metamorphic rocks

Rocks that form when existing rocks are changed by heat, pressure, or chemically active fluids without melting

What is decompression melting

Melting that occurs when pressure decreases on hot mantle rock, allowing it to melt. Common at divergent boundaries and hot spots.

What is partial melting?

When only some minerals in a rock melt while others remain solid. This produces magma with a different composition than the parent rock.

What is lava?

Molten rock that has reached the surface of Earth and lost most of its gas content.

What does felsic mean?

Magma or rock that is high in silica and rich in light-colored minerals like quartz and feldspar (e.g., granite, rhyolite).

What does intermediate mean in igneous rocks?

Rock or magma with a moderate silica content and a mix of light and dark minerals (e.g., andesite, diorite).

What does mafic mean?

Rock or magma that is low in silica and rich in iron and magnesium minerals (e.g., basalt, gabbro).

What is assimilation in magma formation?

The process by which magma melts and incorporates surrounding country rock, changing its chemical composition.

What is mixing in magma evolution?

When two different magmas combine, forming a hybrid magma of intermediate composition.

What does phaneritic texture mean?

A coarse-grained texture where mineral crystals are large enough to be seen with the naked eye. Indicates slow cooling underground.

What does aphanitic texture mean?

A fine-grained texture where crystals are too small to see. Indicates rapid cooling at or near the surface.

What does porphyritic texture mean?

A texture with large crystals (phenocrysts) embedded in a fine-grained groundmass, showing two stages of cooling.

What are phenocrysts?

The large, well-formed crystals in a porphyritic rock that formed during slow early cooling

What is the matrix or groundmass?

The fine-grained material surrounding phenocrysts in a porphyritic rock, formed during later, rapid cooling.

What is a glassy texture?

Texture formed when lava cools too rapidly for crystals to form, producing a smooth, glass-like surface (e.g., obsidian).

What is a vesicular texture?

Texture with gas bubbles (vesicles) trapped in lava as it cools, creating a porous structure (e.g., pumice, scoria).

What are plutonic rocks?

Intrusive igneous rocks that form when magma cools slowly beneath Earth’s surface, producing large crystals (e.g., granite, gabbro).

What are volcanic rocks?

Extrusive igneous rocks that form when lava cools quickly at or near the surface, producing fine-grained textures (e.g., basalt, rhyolite).

What is the rock cycle?

The continuous process in which rocks transform between igneous, sedimentary, and metamorphic types through melting, cooling, weathering, and pressure/heat.

How do igneous rocks form?

By the cooling and crystallization of magma or lava—either below (intrusive) or above (extrusive) the surface.

What is the difference between magma and lava?

Magma is molten rock beneath Earth’s surface (gas-rich), while lava is molten rock at the surface (gas-poor).

How and why do rocks melt?

Rocks melt due to increased temperature, decreased pressure (decompression), or addition of water (flux melting), which lower their melting points.

What are the main types of melting at different plate boundaries?

Divergent boundaries: Decompression melting.

Convergent boundaries: Flux melting (water added).

Hot spots: Heat and decompression melting.

How does water lower the melting temperature of rocks?

Water weakens atomic bonds in minerals, allowing them to melt at lower temperatures — especially in subduction zones.

How can magma chemistry change to form multiple rock types?

Through magma differentiation (minerals crystallize and settle), assimilation (melting surrounding rock), and mixing (combining magmas).

What is Bowen’s Reaction Series?

A model showing how different minerals crystallize from magma at different temperatures, explaining why one magma can produce several rock types.

What are xenoliths and what do they indicate?

Foreign rock fragments trapped in magma that reveal the composition of the crust or mantle where they originated.

What do igneous textures indicate?

The cooling history and environment of formation

What is the difference between plutonic and volcanic rocks?

Plutonic (intrusive): Cooled slowly underground, large crystals.

Volcanic (extrusive): Cooled quickly at surface, small crystals.

What are batholiths, sills, and dikes?

Batholith: Massive, deep-seated igneous body (e.g., Sierra Nevada).

Sill: Magma sheet parallel to rock layers (horizontal).

Dike: Magma sheet that cuts across rock layers (vertical)

What are related igneous rock pairs?

Intrusive and extrusive rocks of the same composition, such as:

• Granite ↔ Rhyolite

• Diorite ↔ Andesite

• Gabbro ↔ Basalt