How Rocks Form: Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks

Why Study Rocks?

-Many, many uses! – writing tools (chalk, pencils), building materials (marble, concrete), fuels (coal), scouring powder (pumice, sand), weapons (flint, obsidian)

Rock: a group of minerals bound together

–  Found in the Earth’s crust and mantle

The Rock Cycle: the continuous process of rock formation and change

– Presents a “map” of the basic processes, the transformation of rock from one form to another.  Many variations!!

– These rocks are classified according to their origins, or processes by which they formed:

1. igneous rocks: formed by the cooling and hardening of magma (molten rock) from inside Earth

2. sedimentary rocks: formed by the compaction and cementing of layers of sediment (rock fragments, plant and animal remains, minerals from water)

3. metamorphic rocks: formed by the effect of heat and pressure on other rocks

Igneous Rocks

*Earth’s crust is made-up primarily of igneous rocks.

*Classified according to composition and texture.

–Composition determined by the type of magma from which it formed.

–Texture determined by rate at which the magma cooled.

2 main types of magma (plus intermediate):

1. felsic– thick; slow-moving; large amounts of silica (SiO₂); light-colored silicate rocks (contain mainly quartz and feldspar)

2. mafic– hotter; thinner; faster-moving; low amounts of silica (SiO₂); dark-colored rocks (contain mainly hornblende, augite, biotite)

3 main rates at which magma cools and hardens:

1.  Magma trapped inside the Earth’s crust hardens very slowly to form intrusive igneous rocks.

–Pluton: a rock mass underground that forms from the magma cooling there.

–Batholith: largest of all plutons that forms the cores of many mountains

-These rocks only appear on Earth after they are pushed up and the overlying rock is worn away.

-granular, coarse-grained texture; larger crystals (because magma is cooling slowly, giving crystals time to form and “grow”)

-examples: gabbro, granite

2. Magma that pours onto Earth’s surface (lava) hardens very quickly to form extrusive igneous rocks.

-smoother, fine-grained, to glassy texture; microscopic crystals or no crystals at all (because magma is cooling quickly giving crystals very little time to form)

-examples: obsidian, basalt, pumice, rhyolite

3. Sometimes, magma that’s trapped inside the Earth and begins cooling slowly, can then be suddenly forced to the surface and finish cooling quickly.  This forms porphyritic igneous rocks.

-large crystals surrounded by a fine-grained mass of rock. (2 different textures)

Sedimentary Rocks

*Most of the Earth’s surface is covered by sedimentary rocks.

*Classified by formation process and particle size.

3 basic formation processes:

1. clastic rocks– formed from fragments of other rocks. How does this happen?

a. A river moves sediments into a body of water.

b. Particles are sorted by size.  Largest, heaviest stones are deposited first, followed by sands, and finally silts and clays.

c. Over time, the sediments are buried and compacted and may become cemented together.

–cementation: process during which loose sediments become bound together by natural cements

-Influences a rock’s color.

d. -Larger particles form conglomerates; coarsest texture of clastic rock

-Sands form sandstone; rough, gritty texture

-Silt and clay form shale; smooth, soft, easily broken (p.128)

2. chemical rocks– formed from chemical sediments that precipitate out of water

–Geodes form in this way! (sphere of silica rock filled with crystals)

-other examples: rock salt (halite), some limestones

3. organic rocks– formed from sediments made-up of the remains of plants and animals (clam shells, coral, bone)

-examples: coal, limestone, coquina

Sedimentary rocks contain 3 distinct features:

• stratification: change in the type of sediment being laid down produces stripes or visible layers of rock
• The single most characteristic feature, common in most sedimentary rocks!
• fossils: the remains, or impressions, of a plant or animal preserved in rock
• ripple marks and mud cracks: sand patterns formed by wind or water, and/or cracks caused by the drying and contracting of sediments

Metamorphic Rocks

*Formed from preexisting rocks called parent rocks.

*Metamorphism: the process by which a rock’s structure is changed by pressure and heat

–Pressure comes from the great weight of overlying rocks; Also from plate movement.

–Heat comes from inside the Earth.

Effects of metamorphism on rocks:

1. affects chemical composition– new minerals may appear

2. affects texture– rock could become smoother or coarser

3. affects internal structure– minerals may be enlarged or re-formed; pressure may force grains to flatten, stretch, and/or become more compact

-Often, metamorphosized rocks will become foliated: form bands of minerals or split along parallel lines

-Examples: sandstone (sed.) > quartzite

limestone (sed.) > marble

granite (ign.) > gneiss

shale (sed.) > slate > phyllite > schist

*Classified according to parent rock, mineral content, and texture.

William Anderson (Schoolworkhelper Editorial Team)

William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

See Full Bio