Igneous rocks are the correct answer! The word "igneous" comes from the Latin "ignis" (fire). Magma is molten rock underground; lava is molten rock on the surface. When magma cools slowly underground, it forms intrusive igneous rocks with large crystals (granite). When lava cools quickly on the surface, it forms extrusive igneous rocks with small or no crystals (basalt, obsidian). Sedimentary rocks form in layers (strata) and often contain fossils. Examples: sandstone (sand compressed), limestone (shells and calcium carbonate), shale (mud compressed), coal (compressed plant matter). Metamorphic rocks form when existing rocks are subjected to high heat and pressure (without melting). Examples: marble (from limestone), slate (from shale), schist (from mudstone or shale). The rock cycle shows how rocks can change from one type to another over millions of years.

Granite is intrusive! Intrusive rocks cool slowly over thousands to millions of years, allowing large crystals to grow. They are often exposed at the surface after erosion removes overlying rock. Granite is a common continental crust rock (forms mountain ranges like the Sierra Nevada). Extrusive rocks cool quickly (days to years), forming fine-grained or glassy textures. Basalt makes up most of the ocean floor and is the most common rock on Earth's surface. Pumice (vesicular, frothy) is extrusive and so lightweight that it floats on water (used for exfoliating skin). Scoria is similar but darker and denser. Obsidian (volcanic glass) was used by ancient peoples for arrowheads and knives because it can be flaked into very sharp edges. The Giant's Causeway in Ireland is made of hexagonal basalt columns formed by cooling lava. The Hawaiian islands are made of basalt from shield volcanoes. The Sierra Nevada batholith in California is a massive granite intrusion.

Sedimentary rocks contain fossils! Fossils are the remains or traces of ancient organisms preserved in rock. The process of fossilization requires rapid burial in sediment. Sedimentary rocks also provide clues about Earth's history: layers (strata) record changes over time (older at the bottom, younger at the top – law of superposition). Limestone often forms in warm, shallow seas from the shells of marine organisms (coral, clams, foraminifera). Chalk is a soft limestone made of microscopic algae (coccolithophores). Coal forms from compressed plant matter in swamps; it is an organic sedimentary rock. Sandstone is made of sand-sized particles cemented together (quartz is common). Shale is made of clay-sized particles; it is the most common sedimentary rock. Conglomerate has rounded pebbles cemented together; breccia has angular fragments. Rock salt (halite) and gypsum form when seawater evaporates. The Grand Canyon exposes layers of sedimentary rock over 1.5 billion years old. Fossils in sedimentary rocks include dinosaur bones, shells, leaves, footprints, and burrows. The study of fossils is paleontology. The principle of cross-cutting relationships: any feature that cuts across rocks is younger than the rocks it cuts.

Marble is the correct answer! Metamorphism occurs deep within the Earth's crust (regional metamorphism) or near igneous intrusions (contact metamorphism). Foliated rocks have aligned minerals that give a layered appearance. Slate (from shale) splits into thin sheets, used for roofing tiles. Schist (more intense metamorphism) has visible mica flakes. Gneiss (highest grade) has alternating light and dark bands (like granite gneiss). Non-foliated rocks: quartzite (from sandstone) is very hard, used in construction; marble (from limestone) reacts with acid (effervesces). Metamorphic rocks are often harder and denser than their parent rocks. The original rock is called the protolith. Metamorphic grade refers to the intensity of metamorphism (low grade → slate; high grade → gneiss). The oldest rocks on Earth (Acasta Gneiss, 4.03 billion years) are metamorphic. The Himalayan mountains are composed partly of metamorphic rocks formed during the collision of India and Asia. Index minerals (chlorite, garnet, staurolite, sillimanite) indicate the temperature and pressure conditions of metamorphism. Blue schist is a high-pressure, low-temperature metamorphic rock found in subduction zones.

Lithification (compaction and cementation) turns sediments into rock! Weathering breaks down existing rocks into sediments (pieces). Erosion transports sediments. Deposition (settling) accumulates sediments in layers. Over time, the weight of overlying layers compacts the sediments, and minerals (calcite, silica, iron) precipitate from groundwater, cementing them together into sedimentary rock. Melting turns rocks into magma (igneous). Cooling and crystallization turn magma into igneous rock. Heat and pressure (metamorphism) turn rocks into metamorphic rocks. Uplift and exposure bring rocks to the surface, where weathering begins again. The rock cycle has no beginning or end; it is a continuous loop. The concept of the rock cycle was developed by James Hutton (the "father of modern geology") in the 18th century. Hutton recognized that Earth's processes operate over vast timescales (uniformitarianism). The rock cycle is related to plate tectonics: igneous rocks form at divergent boundaries (mid-ocean ridges) and convergent boundaries (subduction zones); metamorphic rocks form in collision zones; sedimentary rocks form in basins. The rock cycle operates on timescales of millions to billions of years. The oldest rocks on Earth are about 4.03 billion years old; the Earth is about 4.54 billion years old. Meteorites (4.56 billion years old) are pieces of the early solar system.

Frost wedging is mechanical weathering! It is common in cold climates with daily freeze-thaw cycles. Talus slopes (rock piles at the base of cliffs) are caused by frost wedging. Root wedging (plant roots growing in cracks) is another type of mechanical weathering. Exfoliation (sheeting) occurs when overlying rock erodes, releasing pressure, causing the rock to crack into sheets (like onion layers). Chemical weathering: oxidation (rusting of iron-bearing minerals); hydrolysis (reaction with water, turning feldspar into clay); dissolution (limestone dissolving in acidic water, forming caves). Acid rain (from pollution) accelerates chemical weathering. The Grand Canyon was carved by erosion (Colorado River). The rate of weathering depends on climate (warm, wet = faster chemical weathering; cold, wet = faster mechanical weathering), rock type (marble dissolves easily; quartz is resistant), and surface area (more cracks = faster weathering). Weathering produces soil, which is essential for plant life. Laterite soils (tropical) are rich in iron and aluminum oxides. Loess is wind-blown silt (fertile soil). The red color of Martian soil is due to iron oxide (rust) from chemical weathering.

Obsidian is volcanic glass! It is usually black but can be brown, green, or red (depending on impurities). Snowflake obsidian has white crystals (cristobalite) that formed after eruption. Obsidian is chemically similar to granite (rhyolite) but cooled too fast for crystals to form. Apache tears are small, rounded obsidian nodules. Basalt is dark gray to black, fine-grained, rich in iron and magnesium (mafic). Basalt is the primary rock of the ocean floor and the dark areas of the Moon (maria). Granite is light-colored (pink, white, gray), rich in silica (felsic), with large crystals of quartz, feldspar, and mica. Pumice is vesicular (full of gas bubbles) from explosive eruptions; it is so light that it floats (used for exfoliation). Scoria is similar but darker, denser, and not as vesicular. Pumice and scoria are both extrusive. Pegmatite is an intrusive igneous rock with extremely large crystals (over 1 cm, sometimes meters).

Slate is the correct answer! Slate is fine-grained and foliated (splits along flat planes). It is used for pool tables (slate beds), floor tiles, and roof tiles. Schist has larger mica flakes that reflect light (sparkly). Gneiss has distinct banding (light layers of quartz/feldspar, dark layers of mica/amphibole). The oldest rocks on Earth (Acasta Gneiss, 4.03 Ga) are gneiss. Marble is soft enough to be carved (used for sculptures, buildings). The Taj Mahal (India) and the Lincoln Memorial (Washington, D.C.) are made of marble. Marble reacts with acid (effervesces). Quartzite is extremely hard and resistant to weathering (used in construction). Quartzite forms from sandstone; it was used by Native Americans for arrowheads. Hornfels is a non-foliated metamorphic rock formed by contact metamorphism (adjacent to igneous intrusions). Metamorphic rocks are important indicators of plate tectonic history; the presence of blue schist indicates subduction zones. The Himalayas are made partly of metamorphic rocks (including gneiss) formed during the collision of India and Asia. The term "metamorphic" means "changed form" (Greek).

Chalk is the correct answer! Chalk forms from the accumulation of coccoliths (calcium carbonate plates) from single-celled algae. The White Cliffs of Dover (England) are made of chalk. Limestone is used for building stone, cement, and aggregate. Many caves form in limestone by dissolution (carbonic acid in groundwater). Shale is the most common sedimentary rock (about 70% of sedimentary rocks). It is often the source rock for oil and natural gas (fracking). Sandstone is used for building stone (sandstone buildings) and for grinding stones (millstones). Coal is a fossil fuel burned for electricity; it is the remains of plants from ancient swamps (Carboniferous period). Anthracite is the highest grade of coal (hard, shiny). Bituminous coal is the most common. Lignite is soft, brown, low-grade coal. Peat is the precursor to coal (not a rock). Oil shale and tar sands are unconventional fossil fuels. Sedimentary rocks contain important resources: groundwater (aquifers in sandstone and limestone), oil, natural gas, coal, salt, gypsum, phosphate (fertilizer), and building materials (limestone, sandstone, dimension stone). The study of sedimentary rocks is critical for understanding Earth's history, climate, and resources.

Melting is the correct answer! The rock cycle is driven by Earth's internal heat (magma, metamorphism) and external processes (weathering, erosion, deposition). Plate tectonics recycles rocks: subduction carries rocks into the mantle, where they melt. Volcanic eruptions bring magma to the surface, forming igneous rocks. Uplift exposes rocks to weathering and erosion. The rock cycle is a closed system (no new rock is created or destroyed; matter is conserved). However, some rocks are subducted into the mantle and may melt completely; new crust forms at mid-ocean ridges. The oldest rocks on Earth are about 4.03 billion years old, but most rocks are much younger because the rock cycle constantly recycles them. The rock cycle is related to the carbon cycle: weathering of silicate rocks consumes CO₂, which is released by metamorphism (decarbonation) and volcanism. The rock cycle is also linked to the formation of mineral resources (ores, gems, industrial minerals). Understanding the rock cycle helps geologists locate resources (e.g., diamonds in kimberlite pipes). You have learned about igneous, sedimentary, and metamorphic rocks, their formation processes, and the rock cycle. You are now a rock cycle expert!