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Obsidian

Obsidian is a type of volcanic glass that forms when lava cools rapidly at Earth's surface. It has a smooth, shiny, glassy texture and is typically black or dark-colored.

Fine-grained rock

Rocks that cool quickly from lava at Earth's surface, like obsidian, appear fine-grained or glassy, with no visible crystals due to rapid cooling preventing crystal growth.

Igneous rock

When granite melts and solidifies, it becomes igneous rock again, although the texture and mineral composition may change based on cooling conditions.

Pegmatite

The large crystal size of pegmatite is the best evidence that it solidified deep underground, as slower cooling allows larger crystals to form.

Sedimentary rocks

Fossils are most likely to be found in sedimentary rocks. Two common examples are limestone and shale.

Conglomerate rock

This describes a conglomerate rock, which is a type of sedimentary rock composed of rounded pebbles and sand grains cemented together.

Absolute dating

Absolute dating gives the actual age of a rock or fossil, usually using methods like radiometric dating.

Relative dating

Relative dating determines the age of a rock or fossil in relation to others (e.g., older or younger), using principles like stratigraphy.

Fossil

A fossil is the preserved remains or traces of organisms from the past. Examples include imprints, molds, casts, amber-preserved fossils, and trace fossils (like footprints).

Metamorphic rock

Metamorphic rocks typically show foliation (layering) or a non-foliated texture due to pressure and temperature changes. They may also appear shiny, reflective, or banded.

Rock Cycle

Look over the Rock Cycle Gizmo—be sure to know each of the different rock classes and how each is formed.

Igneous

Formed by cooling and solidification of molten rock.

Sedimentary

Formed from the compaction and cementation of sediments.

Metamorphic

Formed by the alteration of existing rocks under heat and pressure.

Rock Cycle

Called the 'Rock Cycle' because rocks are constantly transformed from one type to another through processes like melting, cooling, erosion, and compression, in a continuous cycle.

Half-life

Refers to the time it takes for half of a substance to decay.

Half-life of Carbon-14

About 5,730 years.

Carbon dating

Cannot be used on rocks because rocks are typically much older than the range of Carbon dating, and any original carbon would have decayed by the time the rock formed.

Eon

The longest divisions of geological time.

Era

Divisions within eons.

Period

Divisions within eras.

Epoch

Divisions within periods.

Cenozoic Era

The era we live in now, which began about 66 million years ago.

Geosphere

Formed through accretion and differentiation.

Atmosphere

Formed from volcanic gases and biological processes.

Biosphere

Evolved as life began.

Hydrosphere

Formed from water released by volcanic activity and extraterrestrial sources like comets.

Earth's water origin

Likely came from volcanic outgassing and comets or asteroids that bombarded Earth during its early formation.

Geological processes

Processes that show the transitions between the three types of rocks (igneous, sedimentary, metamorphic).

Making of North America

Review how the geological history of North America unfolded, including tectonic movements, mountain formation, and the evolution of the continent over time.

Life's Rock Start Video

Describes rock formation, types of rocks, and how they relate to Earth's geologic history.