X: The origin of rocks

Formation of rocks

The Earth was born around 4.7 billion years ago. It is a terrestrial planet, also called a rocky planet. The planet’s solid outer layer, the basement, consists of rocks. These rocks consist of minerals, of which we know around 4,000 different ones.

Rocks form in various ways. Igneous rocks are born when magma cools and solidifies into rock. Sedimentary rocks form when erosion breaks down rock into fine particles that accumulate and then lithify (solidify) deep underground. Metamorphic rocks begin as one type of rock, such as igneous or sedimentary rocks, and go through significant changes at high pressure and heat.

The Earth has a metallic core, a mantle formed from mostly solid rock, and a solid crust. The Earth’s crust and the topmost portion of the mantle form the lithosphere, the rocky shell of the planet. It is approximately 100 kilometres thick. The lithosphere is composed of around twenty tectonic plates, which are in constant motion. The movement of these tectonic plates causes earthquakes and volcanic eruptions, as well as mountain formation and volcanic islands. The Earth’s surface has undergone transformations also due to meteorite impacts and several glacial periods, also called ice ages.

Tectonic plates

The tectonic plates move relative to each other due to the convection currents in the underlying mantle layer, causing earthquakes and volcanic eruptions along the plate boundaries, where they collide or slide beneath one another. The tectonic plates in the lithosphere float on the heavier rocks of the mantle. The Earth has one large long-term natural satellite, the Moon.

The bedrock of Finland and glacial periods

The glacial periods may have lasted for up to tens of millions of years, and the thickness of the ice sheets may have been several kilometres. During the Weichselian glaciation, the glacier reached its maximum extent around 25,000 years ago.

The glacial periods cost Finland many of its interesting deposits, but the ice also revealed layers residing deep in the bedrock, generally difficult to access. Now these intrusive rocks can be extracted for durable, high-quality building stone and beautiful migmatites. The most recent glacial period largely eroded away the uppermost layers of the Finnish bedrock. Durable intrusive rocks, such as granite, were revealed underneath.

Finland’s bedrock used to be located near the equator. From there, the Fennoscandian Shield has drifted along the Earth’s crust to its current location. At the same time, it has transformed into its current shape and size due to the collisions and ruptures in the surrounding tectonic plates. As the plates collided, fold mountains were also created. Two of these ancient mountain ranges can be seen in Finland: the Karelides are a mountain range running from Eastern Finland to Lapland that formed around 2 billion years ago. The Svecofennides ran through Southern and Central Finland. They were folded somewhat later, around 1.9 billion years ago.

The Finnish bedrock is primarily old and worn away by erosive forces. The effects of the most recent glacial period can be seen in the bedrock: the uppermost deposits have largely been eroded away and the layers that normally reside deep in the Earth’s crust are visible. These layers consist of intrusive rocks, which have

been created deep in the crust, in slowly cooling magma chambers. Intrusive rocks are hard and durable, making them excellent building stone material. Granite is one type of intrusive rock.