Acasta Gneiss

At 4 billion years old, this gneiss is the oldest known rock that formed on Earth. The Earth is about 500 million years older still, but little record of that early time has survived our planet's geologic activity.

Actinolite-talc Schist

The container (right) shows how much water this rock contains. It formed deep in a subduction zone when the altered oceanic lithosphere began to release water.

Almandine-bearing schist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth:T = 500°C, P = 7,000-8,000 atm.

Andalusite-bearing phyllite

Var. chiastolite. Andalusite is stable at low pressures and temperatures.

Andalusite-bearing slate

Var. chiastolite. Andalusite is stable at low pressures and temperatures.

Augen Gneiss

Contains 0.7% water by weight. Water volume: 70 milliliters.

Biotite Schist

This schist forms the ancient metamorphic basement of the North American craton in southern Nevada. Over time it was covered by near-shore sandstone, then deeper-water limestone, as a shallow sea encroached.

Blue Quartz Veins in Gneiss

The iridescent blue quartz in this folded rock formed under high temperatures produced by a continental collision 1.1 billion years ago. During a later collision, the rock was incorporated into the Appalachian Mountains.

Blueschist

Changed by subduction, this rock was once sea-floor basalt. It was metamorphosed at high pressures and low temperatures, transforming into a blueschist.

Corundum-bearing gneiss

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 750-800°C, P = 6,000-8,000 atm.

Deformed Spinel Peridotite

Under mantle temperatures and pressures, solid rocks can flow like a glacier. The spinel peridotite contains bands of strongly sheared and deformed crystals.

Earth's Oldest-known Rock

The white bands in this 3.96-billion-year-old gneiss consist of quartz and feldspar, two minerals common in granite. Their presence tells us the gneiss was metamorphosed from granitic rock contained in Earth's earliest continental crust.

Eclogite

The eclogite was originally a basalt erupted at Earth's surface. It was subducted into the mantle, changed to eclogite, caught up in a kimberlite magma, and blasted back to the surface.

Eclogite

The eclogite was originally a basalt erupted at Earth's surface. It was subducted into the mantle, changed to eclogite, caught up in a kimberlite magma, and blasted back to the surface.

Eclogite

Changed by subduction, this rock was once sea-floor basalt. It was metamorphosed at high pressures and high temperatures, transforming into an eclogite.

Eclogite

If greenstone is buried to 18,000 atmospheres and heated to 700°C (1,290°F), its minerals convert to other ones, including garnet and clinopyroxene. The result is a new metamorphic rock: eclogite. The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 700°C, P = 18,000 atm.

Epidote Blueschist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T= 350 - 400°C, P= 7,000 - 10,000 atm.

Garnet amphibolite

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 640-750°C, P = 10,000 atm.

Garnet amphibolite

About a billion years ago, a collision between North American and another continent formed this metamorphic rock. The dark red areas are huge garnet crystals, mined to make abrasives like sandpaper or emery boards. The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 750°C, P = 8,000 atm.

Garnet blueschist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 400-450°C, P = 7,000-10,000 atm.

Garnet Peridotite

Peridotite is the most abundant rock of the upper mantle. The tiny dark red garnet crystals are evidence that it formed deeper than 60 km (37 mi) below Earth's surface. Nearer the surface, garnet is transformed to spinel.

Garnet Peridotite

Peridotite is the most abundant rock of the upper mantle. The tiny dark red garnet crystals are evidence that it formed deeper than 60 km (37 mi) below Earth's surface.

Garnet Peridotite

Peridotite, the upper mantle's most abundant rock, is named for peridot -- the gem variety of olivine. Peridotites contain mineral clues to how deep the rocks were formed. The mineral garnet, present in this sample, is stable at depths greater than 60 km (37 mi).

Garnet-muscovite schist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 550-650°C, P = 5,000-7,000 atm.

Gedrite-garnet schist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 650°C, P = 5,000-7,000 atm.

Gneiss

This gneiss from near Denver, Colorado, formed deep in Earth's crust 1.7 billion years ago. Stress and high temperatures reorganized its minerals into alternating pink and dark gray layers. Notice the very thin pink-and-white vein that cuts across the layers.

Gneiss

This 1.7 billion-year-old gneiss is part of a 500-km (300-mi) wide area that accreted to North America’s southern margin. Once sedimentary, the rock was metamorphosed and folded. It contains layers and veins of granite.

Gneiss

Gneiss is a Metamorphic rock--one that was transformed from older rocks by heat and pressure within the Earth. Two continents collided 1.7 billion years ago, burying and heating a sandstone. Its minerals recrystallized and separated into alternating layers. Molten granite invaded this gneiss, forming a vein that was later deformed into a zigzag shape.

Gneiss with pencil structure

This gneiss was caught in a deep, hot part of a fault zone between rocks moving in opposite directions. The movement stretched the mineral grains into long ribbons. You can see the grains that form the ribbons by viewing the end of the rock.

Granitic Gneiss

About 2.6 billion years ago, a granite crystallized deep in the crust. Later, it was metamorphosed into this gneiss. The collision of the Wyoming Province and North American craton 1.8 billion years ago folded the layers of the gneiss.

Greenstone

If basalt is buried to 2,000-4,000 atmospheres and heated to 350°C (660°F), its minerals and glass react to form new minerals--epidote and albite. The result is a metamorphic rock: greenstone.

Kyanite-bearing schist

Blue kyanite crystals in a matrix of fine-grained mica. Kyanite is stable at high pressures and moderate to high temperatures.

Kyanite-bearing schist

Kyanite is stable at high pressures and moderate to high temperatures.

Kyanite-bearing schist

Kyanite is stable at high pressures and moderate to high temperatures.

Marble

Marble, Precambrian Era. This marble was once limestone, it was transformed by heat and pressure within the Earth.

Metaconglomerate

Coarse-grained sediment first became the sedimentary rock conglomerate, which was later metamorphosed during subduction. It contains the high-pressure and low-temperature minerals that are diagnostic of this tectonic setting: lawsonite, glaucophane, and jadeite.

Metagraywacke

This rock, originally sand deposited in a deep-sea trench, was metamorphosed during subduction. It contains lawsonite, glaucophane, and jadeite (the same mineral in jade).

Metamorphosed Basalt

This rock is part of a 1,000-cu-km (240-cu-mi) lava flow -- one of Earth's largest. It solidified from basaltic magma that erupted during rifting 1.1 billion years ago.

Mineralized Basalt

This mineralized basalt was metamorphosed by hot water near a black smoker. Hole 504B: 83-504B-80-1. 106-120 cm, #7B

Peridotite

Earth's mantle contains more water than all the world's oceans. Where is it? Locked up within the structures of minerals. The golden brown crystals in this peridotite are the mineral phlogopite, a Mg-rich mica that has four percent water (by weight).

Peridotite Xenoliths in Basalt

This lava is chock-full of green and dark reddish-brown peridotite xenoliths.

Peridotite Xenoliths in Basalt

This lava is chock-full of green peridotite xenoliths.

Pyrite bearing Phyllite

Contains 6.9% water by weight. Water volume: 638 milliliters.

Schist

Bedrock under this museum, 530 million years old. During the continental collisions that built the Appalachian Mountains, heat and pressure transformed a sedimentary rock into this folded metamorphic rock.

Schist with rolled garnets

The red thumbprints in this rock are garnets that grew when the rock was glowing hot, 16 km (10 mi) below Earth's surface. The garnets took about 12 million years to get this big. In the process they rolled slowly like snowballs! The peculiar S-shaped patterns within the garnets are made of minerals trapped as the garnets grew. 400 million years old

Serpentinite

Contains 12.5% water by weight. Water volume: 1,302 milliliters.

Sillimanite-bearing schist

Sillimanite is stable at a wide range of pressures and high temperatures.

Sillimanite-bearing schist

Sillimanite is stable at a wide range of pressures and high temperatures.

Slate

Slate shingles cover the Museum's central dome and roof. Slate makes good shingles because it splits easily into thin, durable sheets of solid rock. Why? It contains platelike mica minerals arranged in narrowly spaced layers.

Slate with deformed trilobite fossil

Deformation related to faulting flattened the black fractured rock and its fossil at an angle to the original layers. You can see the result: a long, asymmetric trilobite.

Slate with Chevron folds

Deformed while cooler and less ductile, this slate buckled into more angular folds.

Slate with undeformed trilobite fossil

Flowing rocks can be stretched into elongated forms. Compare this specimen and the other piece with the same number. Once, both of these trilobite fossils looked alike.

Slickensides in iron ore

The shiny surface on this specimen was polished by fault movements.

Spinel Peridotite

Peridotite, the upper mantle's most abundant rock, is named for peridot -- the gem variety of olivine. Peridotites contain mineral clues to how deep the rocks were formed. The mineral spinel, present in this sample, is stable at depths shallower than 60 km (37 mi).

Staurolite-bearing schist

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 500-550°C, P = 6,000-7,000 atm.

Wollastonite Skarn

The white crystals of wollastonite formed when limestone was metamorphosed during a continental collision 1.1 billion years ago. New York is a major producer of wollastonite, used to reinforce plastics and stabilize ceramics.

Wollastonite-bearing marble

The minerals in this metamorphic rock record the following temperature (T) and pressure (P) conditions within the Earth: T = 675°C, P = 7,000 atm.