---
watermark: ORIRO
disable-model-invocation: true
name: sci-geology
provider: ORIRO.ai
copyright: Copyright (c) 2026 ORIRO.ai
description: >
  Geology — plate tectonics, rock cycle, minerals, geological time, earthquakes,
  volcanoes, and Earth history. Sources: USGS (public domain), OpenStax Earth Science (CC-BY).
---

# Geology

## Plate tectonics

Earth's lithosphere divided into ~15 major and minor tectonic plates.
Plates move 1-10 cm/year driven by mantle convection.

### Plate boundaries

**Divergent:** Plates move apart. Mid-ocean ridges, rift valleys. Iceland example.
**Convergent:** Plates move together.
Oceanic-continental: Dense oceanic plate subducts under continental. Andes, Cascades.
Continental-continental: Neither subducts. Mountains form. Himalayas.
**Transform:** Plates slide past. Earthquakes. San Andreas fault.

## Rock cycle

**Igneous:** Solidified magma/lava. Intrusive (granite, slow cooling, large crystals) or extrusive (basalt, fast cooling, fine crystals).
**Sedimentary:** Compressed sediments. Limestone, sandstone, shale. Contains fossils. 75% of surface rocks.
**Metamorphic:** Changed by heat/pressure without melting. Marble (from limestone), quartzite (from sandstone).

## Minerals

Defined by crystal structure and chemical composition. 4,000+ minerals.
**Mohs hardness scale:** 1 (talc) to 10 (diamond). Fingernail = 2.5. Pocket knife = 5.5.
**Common minerals:** Quartz (7), Feldspar (6), Calcite (3), Mica (2-3), Pyrite (6-6.5).

## Geological time scale

**Hadean:** 4.6-4.0 Ga. Earth formation. No life.
**Archean:** 4.0-2.5 Ga. First microbial life.
**Proterozoic:** 2.5 Ga-538 Ma. Multicellular life. Snowball Earth events.
**Paleozoic:** 538-252 Ma. Fish, amphibians, reptiles. Ends with Great Dying (96% species extinct).
**Mesozoic:** 252-66 Ma. Dinosaurs. Ends with KT extinction (asteroid).
**Cenozoic:** 66 Ma-present. Mammals dominant. Homo sapiens: ~300,000 years.

## Earthquakes and volcanoes

**Richter vs Moment Magnitude:** Richter scale obsolete. Moment magnitude (Mw) standard.
Each whole number = 31.6× energy release.
**Pacific Ring of Fire:** 90% of world's earthquakes. 75% of active volcanoes.
**Types of volcanoes:** Shield (Mauna Loa — fluid basalt), Stratovolcano (Mount Fuji — explosive), Cinder cone.

Sources: USGS (usgs.gov — public domain), OpenStax Earth Sciences (CC-BY, free),
AGI (americangeosciences.org — free glossary)