Keurusselkä

Key takeaways

Keurusselkä belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Keurusselkä to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Keurusselkä from memory before moving on to harder problems.

Reference excerpt

Keurusselkä is a lake in Central Finland between the towns of Keuruu to the north and Mänttä to the south. It covers an area of 117.3 km2 (45.3 sq mi). Its average depth is 6.4 m (21 ft) with a maximum depth of 40 m (130 ft). The surface lies at 105.4 m (346 ft) above sea level. The lake is 27 km (17 mi) long and is a part of the Kokemäenjoki River basin. Keurusselkä gained international publicity in 2004 when a pair of amateur geologists discovered an ancient impact structure on the western shore of the lake.

Etymology Keurusselkä was originally only the name of the largest fjard of the lake, while the lake was known as Väärinkeuru or Keuruvesi. The element keuru is a dialectal word meaning "crooked", in this context referring to the shape of the lake. The older names were displaced by the name Keurusselkä in the 19th century. The name of Keuruu is derived from the lake's name.

Environmental issues In 1986, the Keurusselkä region was heavily contaminated (70 kBq/m2) by radioactive caesium, 137Cs, from the Chernobyl disaster fallout. In 2003, some fish near Mänttä still had caesium concentrations several times higher than in Olkiluoto and Loviisa, which host the only two nuclear plants of Finland. This is due to the difference between the uptake of caesium in fresh water and brackish or saline water. However, the concentration levels are so small that eating the fish is not considered a health risk. Apart from radioactivity, the water quality is also weakened by humic substances and local sewage. Despite that, the water quality is generally good, and the central part of the lake is in almost pristine condition. The lake is considered good for fishing, and the pike (Esox lucius) and perch (Perca fluviatilis) populations are especially large.

Keurusselkä impact structure Keurusselkä covers an ancient impact crater remnant, which was discovered in 2003 by amateur geologists. Shatter cones, horsetail-shaped formations in rocks specifically formed in meteor impacts, have been found in an 11.5 km (7.1 mi) wide area, but it is possible that the area containing shatter cones may be only the central uplift of the crater. Weak traces based on digital elevation data suggest possible ring structures from 10 km (6.2 mi) to as wide as about 30 km (19 mi) in diameter. This would make Keurusselkä the largest impact structure in Finland surpassing the Lappajärvi crater. In addition to the shatter cones, microscopic studies of samples from a breccia boulder have revealed shock metamorphic features, (planar fractures and planar deformation features) in quartz grains, which formed when the rock underwent extreme shock pressures of between 7–35 GPa (1,000,000–5,100,000 psi). Argon-argon dating of a pseudotachylitic breccia from the central uplift of the Keurusselkä impact structure yielded a late Mesoproterozoic age of 1.14-1.15 Ga (thousand million years) for the impact., which makes Keurusselkä one of the oldest impact structures known in Europe. The age of the local granite bedrock of the Central Finland Granite Complex is 1.88 Ga (Paleoproterozoic). Since the impact, 7–8 km of rock has eroded away, leaving no visible depressions (compare, in contrast, Iso-Naakkima, Lumparn). Sparse gravity data shows a negative anomaly (an area of lower gravity) in the area of the impact structure. However, there are other negative gravity anomalies nearby which are not believed to be related to the impact event. In addition, there are less dense felsic rocks in the western shore of the lake, which can contribute to the gravity low. The anomaly will be mapped in a much greater detail to determine if it is related to the impact event. Airborne magnetic data show a strikingly circular magnetic anomaly, but for a reason not yet known it is slightly east from the gravity low area as well as the region where the shatter cones are found. It is possible that the ring was created for example by a granitic intrusion instead of the impact event. No specific anomalies have been found from radiometric or electromagnetic data. Paleomagnetic data will be collected to define the age of the structure. Only some 30 km (19 mi) east from the centre of the Keurusselkä impact structure is Karikkoselkä, a much smaller and younger impact crater.

Illustrations

Worked examples

Example 1 — a first encounter with Keurusselkä

Start with the simplest possible case. Write down what Keurusselkä claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Keurusselkä before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Keurusselkä ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Keurusselkä

In research: Keurusselkä appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Keurusselkä in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Keurusselkä is common in secondary-school and first-year university syllabi. It links to neighbouring topics Earth Impact Database, Impact crater lakes, Impact craters of Finland, so understanding it makes those chapters shorter.
In everyday life: Look for Keurusselkä outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Keurusselkä in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Keurusselkä means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Keurusselkä out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Keurusselkä in simple terms?

Keurusselkä is a lake in Central Finland between the towns of Keuruu to the north and Mänttä to the south. It covers an area of 117.3 km2 (45.3 sq mi).

Why does Keurusselkä matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Keurusselkä?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Keurusselkä.