عصر یخ یا دوره یخبندان

معنی کلمه عصر یخ یا دوره یخبندان

معنی واژه عصر یخ یا دوره یخبندان

معنیعصر یخبندان یا عصر یخ (به انگلیسی: An ice age, or more precisely, a glacial age)، دورهٔ دراز مدت کاهش دمای آب و هوای زمین است که در گسترش یخسارهای قاره‌ای، یخسارهای قطبی و یخسارهای آلپی تاثیرگذار است. از دیدگاه یخبندان‌شناسی، عصر یخبندان بیش‌تر به دوره‌ای از یخسارها در نیمکرهٔ شمالی و جنوبی، گفته می‌شود؛ با این تعریف ما هنوز در عصر یخبندان هستیم(چرا که یخسارهای گرینلند و قطب جنوب هنوز وجود دارند). به زبان گفتاری و محاوره‌ای، هنگامی که درباره‌ٔ چند میلیون سال آینده سخن گفته می‌شود، عصر یخبندان برای اشاره به دوره‌های سردتر با یخسارهای پهناور در خُشکاد(قاره)های شمالی آمریکا و اوراسیا گفته می‌شود: با این دید، آخرین عصر یخبندان نزدیک به ۱۱۰۰۰ سال پیش پایان یافت.
بیشتر مردم«عصریخبندان» را چنان می پندارند که گویی هیچ نشانی در زمان ما از آن باقی نمانده است . ولی آیا می دانید که زمینشناسان گفته اند ما هم اکنون تازه به پایان عصر یخبندان رسیده ایم؟
در گرین لند مردمی که در حال حاضر زندگی می کنند هنوز از هر جهت در عصر یخبندان بسر می برند.
تقریباً ۲۵۰۰۰ سال پیش مردمی که در مرکز امریکای شمالی زیست داشتند در سراسر سال با یخ و برف سرو کارشان می بود .
در آنجادیوار کلفتی از یخ وجود داشت که میان دو ساحل شرقی و غربی آمریکا کشیده شده بود . در قسمت شمال نیز یخ بی انتها یی در همه جا گسترده بود.
این آخرین نشانه ی عصر یخبندان در آن سر زمین بود . در عصر یخبندان سرزمین کانادا وهمچنین بیساری از نقاط ایالات متحده و اروپا شمالغربی را ورقه ی کلفتی از یخ به ضخامت هزاران متر می پوشاند.
البته وجود چنین یخ عظیمی سبب آن نبود که در مناطق مزبور سرمای طاقت فرسایی حکمفرما باشد. چه درجه ی هوا در آنجا فقط حدود ده درجه پایین تر از وضع کنونی شمال ایالات متحده ی آمریکا می بود .
آنچه عصر یخبندان را بوجود می آورد تابستان های بسیار سردی بود که نمی گذاشت حرارت کافی یخ و برفهای زمستان را آب کند. از اینرو یخ ها پیوسته روی هم انباشته می شدند تا آنکه سر انجام تمام مناطق شمالی را فرا می گرفتند.
« عصر یخبندان» دارای چهار دوره بود. در طول هر دوره یخهایی تشکیل شده آنگاه گسترش می یافتند. سپس یخها به تدریج آب شده به سوی قطب شمالی سرازیر می گردیدند.
عمل یخبندان و آب شدن یخ ها در چهار دوره انجام گرفته :
نخستین دوره ی یخبندان در آمریکا ی شمالی حدود ۲۰۰۰۰۰۰ سال پیش بود.
دومین دوره ی یخبندان حدود ۱۲۵۰۰۰۰ سال پیش شروع شد.
سومین دوره ی آن نیز حدود پانصد هزار سال پیش بود.
و آخرین دوره ی یخبندان در آمریکای شمالی حدود یکصد هزار سال پیش آغاز شد.
یخها در پایان عصر یخبندان در همه جا به یکسان آب نمی شدند. چون مثلا در جایی که امروزه ویسکانسین نامیده می شود یخچال ها حدود چهل هزار سال پیش رو به ذوب شدن نهادند . ولی در نیوانگلند آب شدن یخ ها حدود بیست و هشت هزار سال پیش آغاز گردید.
یخهایی که بر سطح سرزمین موسوم به «مینزوتا» گسترده بودند تقریباً تا پانزده هزار سال پیش هنوز بر جای خود باقی بودند.
اما چون به اروپا می رسیم می بینیم که آلمان مثلا حدود هفتده هزار سال پیش بود که تازه از زیر یخ بیرون آمد. سوئد نیز تا سیزده هزار سال پیش تقریباً هنوز در زیر یخ پنهان بود.





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منابع

Wikipedia contributors, "Ice age," Wikipedia, The Free Encyclopedia,
رده‌ها: زمین‌شناسی یخچال‌شناسی

قس عربی

العصر الجلیدی فترة فی تاریخ الأرض غطت فیها طبقات الثلج أقالیم کبیرة من الأرض. ویعتقد بوجود العدید من العصور الجلیدیة الرئیسة التی دام کل منها عدة ملایین من السنین. إنقرضت فی العصر الجلیدى الأخیر الثدییات العظمیة (الفقاریة) عندما غطی الجلید معظم المعمورة. وبهذا العصر ظهر الإنسان العاقل الصانع لأدواته وعاشت فیه فیلة الماستدون والماموث وحیوان الدینوثیرم الذی کان یشبه الفیل لکن أنیابه لأسفل وحیوان الخرتیت وکانوا صوفی الشعر الذی کان یصل للأرض.وهذه الفیلة کانت أذناها صغیرتین حتی لاتتأثرا بالصقیع. کما ظهر القط (سابر) ذات الأنیاب الکبیرة والنمور ذات الأسنان التی تشبه السیف وکانت تغمدها فی أجربة بذقونها للحفاظ علی حدتها. وفیه کثرت الأمطار بمصر رغم عدم وجود الجلید بها. وصخور هذا العصر علیها آثار الجلید.
وقد ترک الإنسان الأول آثاره بعد انحسار الجلید. وقد حدث به انقراض کبیر للثدییات الضخمة وکثیر من أنواع الطیور منذ 10 آلاف سنة بسبب الجلید حیث کانت الأرض مغطاة بالأشجار القصیرة کأشجار الصنوبر والبتولا.
محتویات
فترات

حدث أول العصور الجلیدیة المعروفة أثناء زمن ماقبل الکمبری منذ حوالی 2,3 ملیار سنة. وکذلک وجد عصر جلیدی مهم منذ 600 ملیون سنة مضت عند نهایة زمن ماقبل الکمبری. وبدأ العصران الجلیدیان التالیان منذ حوالی 450 ملیون سنة أثناء العصر الأوردوفیشی ومنذ حوالی 300ملیون سنة أثناء العصر الکربونی. واستمر کل عصر جلیدی من 20 إلى 50 ملیون سنة. وجمع العلماء الکثیر من الدلائل لیثبتوا حدوث هذه العصور الجلیدیة. فقد درسوا الصخور التی تشکلت أثناء العصور الجلیدیة القدیمة. وفی هذه الصخور، وجدوا رکامًا جلیدیًا (تربة وأحجارًا نحتت بالمثالج) وأسطحًا صخریة صقلها تحرک الطبقات الجلیدیة فوقها. وعلى سبیل المثال، تحتوی الصخور التی تکونت فی العصرین الکربونی المتأخر والبرمی المبکر فی أمریکا الجنوبیة وإفریقیا والهند وأسترالیا على دلائل مثلجیة. ویعتقد معظم العلماء الآن أنه منذ 300 ملیون سنة مضت کانت هذه الأراضی متجمعة مع أنتارکتیکا حول القطب الجنوبی، مشکِّلة قارة واحدة اسمها أرض الجندوانا التی تفککت فی أو من بعد ذلک العصر ، ثم تحرکت أجزاء الأراضی المنفصلة ببطء إلى مواقعها الحالیة.
العصر البلیستوسینی
أکثر العصور الجلیدیة حداثة کان أثناء العصر البلیستوسینی الذی بدأ منذ ملیونی سنة وانتهى منذ حوالی 10000 سنة. یشیر مصطلح العصر الجلیدی عادة إلى العصر الجلیدی البلیستوسینی.
لم تتأثر الأحافیر ودلائل أخرى من العصر الجلیدی البلیستوسینی بتغیرات فی الأرض مثلما حدث لتلک التی وجدت فی العصور الجلیدیة المبکرة.
التراجع الجلیدی الأخیر

بدأ التراجع الجلیدی الأخیر منذ أقل من 20,000 سنة. ویتوقع معظم العلماء أن فترات مثلجیة سوف تحدث مرة أخرى، حیث یعتقدون بوجود تغیرات منتظمة فی مدار الأرض حول الشمس وفی زاویة میلها، وقد یتسبب هذا فی برودة تزید بدورها من تشکیل کتل جلیدیة.
أثناء المثلجیة تتکون کتل جلیدیة قاریة تنمو سمیکة وتنساب للخارج من مرکزها. وفی أمریکا الشمالیة کان المرکز الرئیسی حول خلیج هدسون، حیث واصل تراکم الثلج بین 2,400 و3,000م. وتسبب الضغط الناتج عن وزنه فی أن ینساب الثلج للخارج فی کل الاتجاهات. وقد غطى معظم أمریکا الشمالیة حتى ودیان نهری میسوری وأوهایو حالیا.
حیوانات

یعتقد بعض العلماء أن الجمال والخیول والفیلة الموجودة الآن ظهرت أولاً فی العصر الجلیدی. نشأ الحصان والجمل فی أمریکا الشمالیة، ثم عبرا مضیق بیرنغ إلى آسیا. وتطور الفیل والثور الأمریکی والغزال والدب فی أوروبا وآسیا، ثم أتت إلى أمریکا الشمالیة. وذهبت الخیول واللاما وکسلان الأرض العملاق والمدرعات إلى أمریکا الجنوبیة.
وحینما دفعت الغطاءات الجلیدیة من الشمال نقلت معها الحیوانات جنوبًا. لکن فی أثناء الفترات بین المثلجیة تتبعت الحیوانات الجلید المذاب عائدة فی اتجاه الشمال.
ویظن بعض العلماء أن التغیر فی المناخ تسبب فی موت ثدییات العصر البلیستوسینی، بینما یعتقد آخرون أن الإنسان أبادها جمیعها. وعلى سبیل المثال عاش کسلان الأرض العملاق والماموث وحیوان الماستودون وهو حیوان بائد شبیه بالفیل ودب الکهوف والمغاور وحیوان الکنغورو العملاق وثدییات أخرى کبیرة فی أمریکا الشمالیة حتى وصل الإنسان إلى القارة منذ 20,000 سنة مضت ولکن سرعان مابدأت هذه الحیوانات فی الاختفاء. وعاشت هذه الحیوانات جنبًا إلى جنب مع البشر خلال معظم العصر البلیستوسینی.
یعتقد العلماء بانقراض حوالی 55 نوعا من الحیوانات الکبیرة الحجم فی أمریکا الشمالیة وقد تمکن العلماء بفضل الأبحاث التی أجروها على حیوانات الکسالى العملاقة فی جنوب شرق الولایات المتحدة الأمریکیة من تحدید انقراض هذا النوع من الحیوانات منذ 11 ألف سنة قبل عصرنا الحالی الذی یتوافق مع تاریخ اندثار صیادی ما قبل التاریخ.
مراجع

↑ أ ب العصر الجلیدی الموسوعة المعرفیة الشاملة
تصنیفان: زمن جیولوجی جیولوجیا


قس عبری

עידני קרח הם תקופות בהיסטוריית כדור הארץ שבהן האטמוספירה התקררה באופן משמעותי ואזורים נרחבים התכסו בים קפוא או בקרחוני ענק.
כדור הארץ קיים כ-4.5 מיליארד שנים. במהלך תקופה זו חווה כדור הארץ מספר עידני קרח, כל אחד מהם ארך עשרות או מאות מיליוני שנים. סך כל השנים מסתכמות בכ-15 עד 20 אחוזים מהיסטוריית הכדור. שכבת הקרח כיסתה ככל הנראה כ-10 עד 30 אחוזים מכלל פני כדור הארץ.
לעתים מכוון המונח עידן הקרח לעידן-הקרח האחרון, שהסתיים, לפי הערכה, לפני כ-10,000 שנה.
גורמים לתקופות הקרח (תופעות גלובליות)

ישנם מספר גורמים אפשריים היכולים ליצור ירידת טמפרטורה, אשר תוביל לתקופת קרח:
שינוי במסלול כדור הארץ סביב השמש - מסלולו של כדור הארץ סביב השמש הוא אליפטי אך אינו קבוע. ישנה מחזוריות של התרחקות המסלול מן השמש המתרחשת כל 100,000 שנים לערך. משמעותה של ההתרחקות מהשמש היא שעוצמת הקרינה והחום של השמש, הנקלטים בכדור הארץ, נחלשים, והטמפרטורה בכדור הארץ יורדת.
שינוי בכיוון ציר כדור הארץ - צירו של כדור הארץ נטוי. נטייתו היא ביחס למישור תנועת הכדור סביב השמש, מישור זה נקרא מישור המילקה. לציר זווית נטייה קבועה וזווית פגיעת קרני השמש בקווי רוחב שונים בכדור הארץ מושפעת מזווית הציר. כאשר זווית הציר משתנה, משתנה גם עוצמת קרני השמש ושינוי זה יכול לגרום להתקררות הכדור. זווית הציר הנוכחית היא בערך 23.4°. ככל שזווית הנטייה קרובה יותר ומאונכת למישור המילקה, פחות קרני שמש פוגעות בקווי הגובה הגבוהים. ככל הנראה שינוי זה עשוי לגרום להיווצרות תקופת קרח.
זרם הגולף - זרם הגולף מביא עמו זרמי מים חמים המשפיעים על האקלים בחופי נורבגיה. בתקופות מסוימות התקרר הזרם ולכן לא היה גורם משמעותי שחימם את הטמפרטורות באזור. בנוסף, בתקופות הקרח עצמן האוקיינוס בו עובר הזרם היה קפוא ומנע מן הזרם להגיע לאזור החופים.
כתמים בשמש - ישנן עדויות שבתקופות מסוימות היו כתמים על השמש. כתמים אלו גרמו לעוצמת הקרינה לרדת ובכך לקרר את האקלים בכדור הארץ.
האפקט ההפוך - בתקופות הבין–קרחוניות האקלים היה חם יחסית וקרחונים החלו להתמוסס. המסת הקרחונים משמעותה שהמים, שהיו בעבר קפואים, נשפכים אל תוך האוקיינוסים בכמויות גדולות, מים אלה קרים מאוד ומתוקים בניגוד למי האוקיינוס. תופעה זו גורמת לאפקט הפוך, המים הקרים משפיעים על טמפרטורת האוקיינוס ויכולים להורידה בכמה מעלות. השינוי הדרמטי בטמפרטורת האוקיינוסים משפיע על האקלים בכדור הארץ כולו ויכול לגרום לעלייה גדולה בשיעור הסערות, בהן יורדים משקעים רבים וכך גדלים הקרחונים. אפקט הפוך נוסף אשר נגרם מההתחממות המהירה הוא עלייה בכמות האדים באטמוספירה. ההתחממות גורמת לאידוי יתר היוצר מסך של עננים באטמוספירה החוסם את קרינת השמש וגורם להתקררות האקלים.
שינוי ברצועות האקלים - בכדור הארץ מספר רצועות אקלים, רצועות אלה מוכתבות על ידי עליות וירידות של לחצים ברומטרים. באזור קו המשווה הלחצים הברומטרים עולים ולכן רצועת אקלים זו היא לחה ומרובת משקעים. באזור קו הרוחב 30 לחצים אלה יורדים, ורצועה זו נותרת צחיחה ומדברית (זהו האזור של מדבר סהרה). באזור קו הרוחב 60 ישנה רצועה נוספת בה עולים הלחצים ואף היא רצועה לחה ומרובת משקעים. בתקופה הנוכחית, שהיא תקופה חמה, רצועת האקלים של צפון אירופה (במיוחד נורבגיה) נהפכת ללחה יותר ולכן יורדים בה משקעים רבים יותר. הדבר מוביל לכך שחלק מן הקרחונים גדלים.
פעילות אנושית - בעקבות פעילות אנושית הכוללת זיהום תעשייתי ופליטת גזים ועשן ממכונות חלה התחממות עולמית. התחממות זו מתבטאת בפגיעה באטמוספירה וחדירה מסוכנת של קרני שמש בעלות קרינה חזקה מדי. אולם יש חוקרים (בהם Andersen, 2000) הטוענים כי התחממות האטמוספירה בשנים האחרונות הינה טבעית (שינוי בגזים באטמוספירה וכדומה).
חורף געשי – דומה במהותו לחורף גרעיני, ונגרם כאשר ענן נרחב של אפר געשי הנפלט בהתפרצות געשית בעוצמה גבוהה מחזיר את קרינת השמש בחזרה לחלל ומוריד את הטמפרטורה על פני השטח.
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קטגוריות: מדעי כדור הארץשינויי אקליםקרח

قس اردو
وہ مدت میں جس میں کوئی تختہ یخ (Ice Sheet) پرورش پاتا اور پھیلتا ہے یخ زدگی (Glaciation)کہلاتی ہے۔ اس دوران خاصی برف باری کا ہونا اور برف کے انبوہ عظیم کا جمع ہو جانا بھی اس اصطلاح میں شامل ہے۔ اس کے برخلاف جو عمل ہوتا ہے جس میں تختہ یخ کی دبازت اور جسامت سکڑنے لگتی ہے۔ برف حاشیوں کے مرکز کی بلندیوں کی جانب پس روی اختیار کرتی ہے۔ یہاں تک کہ تختہ یخ غائب ہو جاتا ہے۔ یہ مدت نایخ زدگی (Deglaciation) کہلاتی ہے۔ نایخ زدگی کے بعد اور اگلے یخ زدگی سے پہلے معتدل آب و ہوا کا زمانہ آتا ہے جسے بین یخ زدگی (Inter Glaciation) کہتے ہیں۔ یکے بعد دیگرے یخ زدگی اور بین یخ زدگی کے تواتر کی مجموعی مدت ایک تا 10 ملین سال یا اس سے زیادہ ہوتی ہے جسے برفانی دور یا برفانی عہد (Ice Age) کہتے ہیں۔
گذشتہ ڈھائی سے تین ملین سالوں میں زمین پر صرف ایک برفانی دور آیا۔ برفانی عہد کا عنوان سب سے پہلے ایک ماہر فطرت لوئس اگاسیز (Louis Agassez) نے انیسویں صدی کی تیسری دہائی میں استعمال کیا۔ فی زمانہ ہم بین یخ زدگی کے دور میں ہیں جو نایخ زدگی کے دور کے بعد آیا جو اب سے تقریباً 15 ہزار سال پہلے شروع ہوا۔
اس سے قبل آنے والا یخ زدگی کے دور کو جدید ترین برفانی عہد (The Pleistocene Ice Age)کہا جا سکتا ہے۔ اس دور کو وسکنسنین یخ زدگی (Wisconsinan Glaciation) کے نام سے موسم کیا جاتا ہے۔ یخ زدگی کے اس دور نے شمالی نصف کرہ کے تقریبا‎ 80 لاکھ مربع میل علاقے کو اپنی لپیٹ میں لیا۔ اس میں سے نصف کے قریب شمالی امریکہ میں تھا۔ شمالی امریکہ کے تختہ کو لورنٹائڈ تختہ یخ (Laurentide Ice Sheet) کہتے ہیں۔ تقریبا‎ 30 لاکھ مربع میل علاقہ جس کا بڑا حصہ یورپ اور ایک چھوٹا حصہ کوہ اورال کے پار شمالی سائبیریا تک چلا گیا تھا۔ یورپ کا تختہ یخ اسکینڈے نیوین تختہ یخ (Scandinavian Ice Sheet) کہلاتا ہے۔ ایک ثانوی کلاہ یخ کوہ الپس سے شروع ہو کر ہمالیہ اور دوسرے سلسلہ ہائے کوہ تک پھیلا ہوا تھا۔
جنوبی نصف کرہ میں آسٹریلیا میں نیو ساؤتھ ویلز، تسمانیہ اور نیوزی لینڈ برف پوش تھے۔ جنوبی امریکہ میں جنوبی چلی اور پیٹاگونیا تک برف موجود تھی۔ انٹارکٹیکا کا تختہ یخ زمانہ حال کے مقابلے میں زیادہ وسیع اور دبیز تھا۔ وسطی افریقہ میں بھی اس کے آثار ملے ہیں۔ اس دور میں زیادہ سے زیادہ برف کا پھیلاؤ اب سے 18 ہزار پہلے تک ہوا۔
زمرہ جات: تاریخ موسمیاتیخ زدگیاتبرفانی عہد

قس انگلیسی
An ice age, or more precisely, a glacial age, is a period of long-term reduction in the temperature of the Earths surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, we are still in the ice age that began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland and Antarctic ice sheets still exist.
Contents
Origin of ice age theory

In 1742 Pierre Martel (1706–1767), an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years later he published an account of his journey. He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the fact that the glaciers had once extended much farther. Later similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin (1767–1858) explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers previously extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier (1786–1855) in 1834. Comparable explanations are also known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethes Scientific Work. Such explanations could also be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra (1806–1878) visited the Chilean Andes in 1849–1850 the natives attributed fossil moraines to the former action of glaciers.
Meanwhile, European scholars had begun to wonder what had caused the dispersal of erratic material. From the middle of the 18th century some discussed ice as a means of transport. The Swedish mining expert Daniel Tilas (1712–1772) was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton (1726–1797), explained erratic boulders in the Alps with the action of glaciers. Two decades later, in 1818, the Swedish botanist Göran Wahlenberg (1780–1851) published his theory of a glaciation of the Scandinavian peninsula. He regarded glaciation as a regional phenomenon. Only a few years later, the Danish-Norwegian Geologist Jens Esmark (1763–1839) argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations. He attempted to show that they originated from changes in the Earths orbit. During the following years, Esmark’s ideas were discussed and taken over in parts by Swedish, Scottish and German scientists. At the University of Edinburgh Robert Jameson (1774–1854) seemed to be relatively open towards Esmarks ideas. Jamesons remarks about ancient glaciers in Scotland were most probably prompted by Esmark. In Germany, Albrecht Reinhard Bernhardi (1797–1849), professor of forestry at Dreissigacker, adopted Esmarks theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe.
Independently of these debates, the Swiss civil engineer Ignaz Venetz (1788–1859) in 1829, explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains and the North German Plain as being due to huge glaciers. When he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical. Finally, Venetz managed to convince his friend Jean de Charpentier. De Charpentier transformed Venetzs idea into a theory with a glaciation limited to the Alps. His thoughts resembled Wahlenbergs theory. In fact, both men shared the same volcanistic, or in de Charpentier’s case rather plutonistic assumptions, about earth history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft. In the meantime, the German botanist Karl Friedrich Schimper (1803–1867) was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder where such masses of stone had come from. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held some lectures in Munich. Schimper then assumed that there must have been global times of obliteration ("Verödungszeiten") with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz (1801–1873) and Jean de Charpentier. Schimper, de Charpentier and possibly Venetz convinced Agassiz that there had been a time of glaciation. During Winter 1836/7 Agassiz and Schimper developed the theory of a sequence of glaciations. They mainly drew upon the preceding works of Goethe, of Venetz, of de Charpentier and on their own fieldwork. There are indications that Agassiz was already familiar with Bernhardis paper at that time. At the beginning of 1837 Schimper coined the term ice age ("Eiszeit"). In July 1837 Agassiz presented their synthesis before the annual meeting of the Schweizerische Naturforschende Gesellschaft at Neuchâtel. The audience was very critical or even opposed the new theory because it contradicted the established opinions on climatic history. Most contemporary scientists thought that the earth had been gradually cooling down since its birth as a molten globe.
In order to overcome this rejection, Agassiz embarked on geological fieldwork. He published his book Study on glaciers ("Études sur les glaciers") in 1840. De Charpentier was put out by this as he had also been preparing a book about the glaciation of the Alps. De Charpentier felt that Agassiz should have given him precedence as it was he who had introduced Agassiz to in depth glacial research. Besides that, Agassiz had, as a result of personal quarrels, omitted any mention of Schimper in his book.
Altogether, it took several decades until the ice age theory was fully accepted. This happened on an international scale in the second half of the 1870s following the work of James Croll including the publication of Climate and Time, in Their Geological Relations in 1875 which provided a credible explanation for the causes of ice ages.
Evidence for ice ages

There are three main types of evidence for ice ages: geological, chemical, and paleontological.
Geological evidence for ice ages comes in various forms, including rock scouring and scratching, glacial moraines, drumlins, valley cutting, and the deposition of till or tillites and glacial erratics. Successive glaciations tend to distort and erase the geological evidence, making it difficult to interpret. Furthermore, this evidence was difficult to date exactly; early theories assumed that the glacials were short compared to the long interglacials. The advent of sediment and ice cores revealed the true situation: glacials are long, interglacials short. It took some time for the current theory to be worked out.
The chemical evidence mainly consists of variations in the ratios of isotopes in fossils present in sediments and sedimentary rocks and ocean sediment cores. For the most recent glacial periods ice cores provide climate proxies from their ice, and atmospheric samples from included bubbles of air. Because water containing heavier isotopes has a higher heat of evaporation, its proportion decreases with colder conditions. This allows a temperature record to be constructed. However, this evidence can be confounded by other factors recorded by isotope ratios.
The paleontological evidence consists of changes in the geographical distribution of fossils. During a glacial period cold-adapted organisms spread into lower latitudes, and organisms that prefer warmer conditions become extinct or are squeezed into lower latitudes. This evidence is also difficult to interpret because it requires (1) sequences of sediments covering a long period of time, over a wide range of latitudes and which are easily correlated; (2) ancient organisms which survive for several million years without change and whose temperature preferences are easily diagnosed; and (3) the finding of the relevant fossils.
Despite the difficulties, analysis of ice core and ocean sediment coresneeded has shown periods of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.
Major ice ages



Ice age map of northern Germany and its northern neighbours. Red: maximum limit of Weichselian glacial; yellow: Saale glacial at maximum (Drenthe stage); blue: Elster glacial maximum glaciation.


Timeline of glaciations, shown in blue.
There have been at least five major ice ages in the Earths past (the Huronian, Cryogenian, Andean-Saharan, Karoo Ice Age and the Quaternary glaciation). Outside these ages, the Earth seems to have been ice-free even in high latitudes.
Rocks from the earliest well established ice age, called the Huronian, formed around 2.4 to 2.1 Ga (billion years) ago during the early Proterozoic Eon. Several hundreds of km of the Huronian Supergroup are exposed 10–100 km north the North Shore of Lake Huron extending from near Sault Ste. Marie to Sudbury NE of Lake Huron, with giant layers of now-lithified till beds, dropstones, varves, outwash, and scoured basement rocks. Correlative Huronian deposits have been found near Marquette, Michigan, and correlation has been made with Paleoproterozic glacial deposits from Western Australia.
The next well-documented ice age, and probably the most severe of the last billion years, occurred from 850 to 630 million years ago (the Cryogenian period) and may have produced a Snowball Earth in which glacial ice sheets reached the equator, possibly being ended by the accumulation of greenhouse gases such as CO2 produced by volcanoes. "The presence of ice on the continents and pack ice on the oceans would inhibit both silicate weathering and photosynthesis, which are the two major sinks for CO2 at present." It has been suggested that the end of this ice age was responsible for the subsequent Ediacaran and Cambrian Explosion, though this model is recent and controversial.
The Andean-Saharan, occurred from 460 to 420 million years ago, during the Late Ordovician and the Silurian period.
The evolution of land plants at the onset of the Devonian period caused a long term increase in planetary oxygen levels and reduction of CO2 levels, which resulted in the Karoo Ice Age. It is named after the glacial tills found in the Karoo region of South Africa, where evidence for this ice age was first clearly identified. There were extensive polar ice caps at intervals from 360 to 260 million years ago in South Africa during the Carboniferous and early Permian Periods. Correlatives are known from Argentina, also in the center of the ancient supercontinent Gondwanaland.


Sediment records showing the fluctuating sequences of glacials and interglacials during the last several million years.
The current ice age, the Pliocene-Quaternary glaciation, started about 2.58 million years ago during the late Pliocene, when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacial periods, glacials or glacial advances, and interglacial periods, interglacials or glacial retreats. The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago. All that remains of the continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island.
Ice ages can be further divided by location and time; for example, the names Riss (180,000–130,000 years bp) and Würm (70,000–10,000 years bp) refer specifically to glaciation in the Alpine region. Note that the maximum extent of the ice is not maintained for the full interval. Unfortunately, the scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage.
Glacials and interglacials

See also: Glacial period and Interglacial


Shows the pattern of temperature and ice volume changes associated with recent glacials and interglacials


Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the northern hemisphere


Minimum (interglacial, black) and maximum (glacial, grey) glaciation of the southern hemisphere
Within the ice ages (or at least within the current one), more temperate and more severe periods occur. The colder periods are called glacial periods, the warmer periods interglacials, such as the Eemian Stage.
Glacials are characterized by cooler and drier climates over most of the Earth and large land and sea ice masses extending outward from the poles. Mountain glaciers in otherwise unglaciated areas extend to lower elevations due to a lower snow line. Sea levels drop due to the removal of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations. Since the Earth has significant continental glaciation in the Arctic and Antarctic, we are currently in a glacial minimum of a glaciation. Such a period between glacial maxima is known as an interglacial.
The Earth has been in an interglacial period known as the Holocene for more than 11,000 years. It was conventional wisdom that the typical interglacial period lasts about 12,000 years, but this has been called into question recently. For example, an article in Nature argues that the current interglacial might be most analogous to a previous interglacial that lasted 28,000 years. Predicted changes in orbital forcing suggest that the next glacial period would begin at least 50,000 years from now, even in absence of human-made global warming (see Milankovitch cycles). Moreover, anthropogenic forcing from increased greenhouse gases might outweigh orbital forcing for as long as intensive use of fossil fuels continues.
Positive and negative feedbacks in glacial periods

Each glacial period is subject to positive feedback which makes it more severe and negative feedback which mitigates and (in all cases so far) eventually ends it.
Positive feedback processes
Ice and snow increase the Earths albedo, i.e. they make it reflect more of the suns energy and absorb less. Hence, when the air temperature decreases, ice and snow fields grow, and this continues until competition with a negative feedback mechanism forces the system to an equilibrium. Also, the reduction in forests caused by the ices expansion increases albedo.
Another theory proposed by Ewing and Donn in 1956 hypothesized that an ice-free Arctic Ocean leads to increased snowfall at high latitudes. When low-temperature ice covers the Arctic Ocean there is little evaporation or sublimation and the polar regions are quite dry in terms of precipitation, comparable to the amount found in mid-latitude deserts. This low precipitation allows high-latitude snowfalls to melt during the summer. An ice-free Arctic Ocean absorbs solar radiation during the long summer days, and evaporates more water into the Arctic atmosphere. With higher precipitation, portions of this snow may not melt during the summer and so glacial ice can form at lower altitudes and more southerly latitudes, reducing the temperatures over land by increased albedo as noted above. Furthermore, under this hypothesis the lack of oceanic pack ice allows increased exchange of waters between the Arctic and the North Atlantic Oceans, warming the Arctic and cooling the North Atlantic. (Current projected consequences of global warming include a largely ice-free Arctic Ocean within 5–20 years, see Arctic shrinkage.) Additional fresh water flowing into the North Atlantic during a warming cycle may also reduce the global ocean water circulation (see Shutdown of thermohaline circulation). Such a reduction (by reducing the effects of the Gulf Stream) would have a cooling effect on northern Europe, which in turn would lead to increased low-latitude snow retention during the summer. It has also been suggested that during an extensive glacial, glaciers may move through the Gulf of Saint Lawrence, extending into the North Atlantic ocean far enough to block the Gulf Stream.
Negative feedback processes
Ice sheets that form during glaciations cause erosion of the land beneath them. After some time, this will reduce land above sea level and thus diminish the amount of space on which ice sheets can form. This mitigates the albedo feedback, as does the lowering in sea level that accompanies the formation of ice sheetsneeded.
Another factor is the increased aridity occurring with glacial maxima, which reduces the precipitation available to maintain glaciation. The glacial retreat induced by this or any other process can be amplified by similar inverse positive feedbacks as for glacial advancesneeded.
According to research published in Nature Geoscience, human emissions of carbon dioxide will defer the next ice age. Researchers used data on the Earths orbit to find the historical warm interglacial period that looks most like the current one and from this have predicted that the next ice age would usually begin within 1,500 years. They go on to say that emissions have been so high that it will not.
Causes of ice ages

The causes of ice ages are not fully understood for both the large-scale ice age periods and the smaller ebb and flow of glacial–interglacial periods within an ice age. The consensus is that several factors are important: atmospheric composition, such as the concentrations of carbon dioxide and methane (the specific levels of the previously mentioned gases are now able to be seen with the new ice core samples from EPICA Dome C in Antarctica over the past 800,000 years ); changes in the Earths orbit around the Sun known as Milankovitch cycles (and possibly the Suns orbit around the galaxy); the motion of tectonic plates resulting in changes in the relative location and amount of continental and oceanic crust on the Earths surface, which affect wind and ocean currents; variations in solar output; the orbital dynamics of the Earth-Moon system; and the impact of relatively large meteorites, and volcanism including eruptions of supervolcanoes.needed
Some of these factors influence each other. For example, changes in Earths atmospheric composition (especially the concentrations of greenhouse gases) may alter the climate, while climate change itself can change the atmospheric composition (for example by changing the rate at which weathering removes CO2).
Maureen Raymo, William Ruddiman and others propose that the Tibetan and Colorado Plateaus are immense CO2 "scrubbers" with a capacity to remove enough CO2 from the global atmosphere to be a significant causal factor of the 40 million year Cenozoic Cooling trend. They further claim that approximately half of their uplift (and CO2 "scrubbing" capacity) occurred in the past 10 million years.
Changes in Earths atmosphere
There is considerable evidence that over the very recent period of the last 100–1000 years, the sharp increases in human activity, especially the burning of fossil fuels, has caused the parallel sharp and accelerating increase in atmospheric greenhouse gases which trap the suns heat. The consensus theory of the scientific community is that the resulting greenhouse effect is a principal cause of the increase in global warming which has occurred over the same period, and a chief contributor to the accelerated melting of the remaining glaciers and polar ice. A 2012 investigation finds that dinosaurs released methane through digestion in a similar amount to humanitys current methane release, which "could have been a key factor" to the very warm climate 150 million years ago.
There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.
The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO2 levels in the atmosphere, and some supporters of Snowball Earth argue that it was caused by a reduction in atmospheric CO2. The hypothesis also warns of future Snowball Earths.
In 2009, further evidence was provided that changes in solar insolation provide the initial trigger for the Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change.
William Ruddiman has proposed the early anthropocene hypothesis, according to which the anthropocene era, as some people call the most recent period in the Earths history when the activities of the human species first began to have a significant global impact on the Earths climate and ecosystems, did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the Milankovitch cycles. In his overdue-glaciation hypothesis Ruddiman states that an incipient glacial would probably have begun several thousand years ago, but the arrival of that scheduled glacial was forestalled by the activities of early farmers.
At a meeting of the American Geophysical Union (December 17, 2008), scientists detailed evidence in support of the controversial idea that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases into the atmosphere over the last 1,000 years. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming, possibly forestalling the onset of a new glacial age.
Position of the continents
The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase the Earths reflectivity and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity in a positive feedback loop. The ice age continues until the reduction in weathering causes an increase in the greenhouse effect.
There are three known configurations of the continents which block or reduce the flow of warm water from the equator to the poles:needed
A continent sits on top of a pole, as Antarctica does today.
A polar sea is almost land-locked, as the Arctic Ocean is today.
A supercontinent covers most of the equator, as Rodinia did during the Cryogenian period.
Since todays Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth will continue to experience glacial periods in the geologically near future.
Some scientists believe that the Himalayas are a major factor in the current ice age, because these mountains have increased Earths total rainfall and therefore the rate at which CO2 is washed out of the atmosphere, decreasing the greenhouse effect. The Himalayas formation started about 70 million years ago when the Indo-Australian Plate collided with the Eurasian Plate, and the Himalayas are still rising by about 5 mm per year because the Indo-Australian plate is still moving at 67 mm/year. The history of the Himalayas broadly fits the long-term decrease in Earths average temperature since the mid-Eocene, 40 million years ago.
Fluctuations in ocean currents
Another important contribution to ancient climate regimes is the variation of ocean currents, which are modified by continent position, sea levels and salinity, as well as other factors. They have the ability to cool (e.g. aiding the creation of Antarctic ice) and the ability to warm (e.g. giving the British Isles a temperate as opposed to a boreal climate). The closing of the Isthmus of Panama about 3 million years ago may have ushered in the present period of strong glaciation over North America by ending the exchange of water between the tropical Atlantic and Pacific Oceans.
Analyses suggest that ocean current fluctuations can adequately account for recent glacial oscillations. During the last glacial period the sea-level has fluctuated 20–30 m as water was sequestered, primarily in the northern hemisphere ice sheets. When ice collected and the sea level dropped sufficiently, flow through the Bering Strait (the narrow strait between Siberia and Alaska is ~50 m deep today) was reduced, resulting in increased flow from the North Atlantic. This realigned the thermohaline circulation in the Atlantic, increasing heat transport into the Arctic, which melted the polar ice accumulation and reduced other continental ice sheets. The release of water raised sea levels again, restoring the ingress of colder water from the Pacific with an accompanying shift to northern hemisphere ice accumulation.
Uplift of the Tibetan plateau and surrounding mountain areas above the snowline
Matthias Kuhles geological theory of Ice Age development was suggested by the existence of an ice sheet covering the Tibetan plateau during the Ice Ages (Last Glacial Maximum?). According to Kuhle, the plate-tectonic uplift of Tibet past the snow-line has led to a c. 2.4 million km² ice surface with a 70% greater albedo than the bare land surface. The reflection of energy into space resulted in a global cooling, triggering the Pleistocene Ice Age. Because this highland is at a subtropical latitude, with 4 to 5 times the insolation of high-latitude areas, what would be Earths strongest heating surface has turned into a cooling surface.
Kuhle explains the interglacial periods by the 100,000-year cycle of radiation changes due to variations of the Earths orbit. This comparatively insignificant warming, when combined with the lowering of the Nordic inland ice areas and Tibet due to the weight of the superimposed ice-load, has led to the repeated complete thawing of the inland ice areas.
Variations in Earths orbit (Milankovitch cycles)
The Milankovitch cycles are a set of cyclic variations in characteristics of the Earths orbit around the Sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.


Past and future of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65 N latitude.
There is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial periods within an ice age. The present ice age is the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milanković orbital forcing periods is so close that orbital forcing is generally accepted. The combined effects of the changing distance to the Sun, the precession of the Earths axis, and the changing tilt of the Earths axis redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earths axis, which affect the intensity of seasons. For example, the amount of solar influx in July at 65 degrees north latitude varies by as much as 25% (from 450 W/m² to 550 W/m²). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter. Some workers believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like CO2 may explain this mismatch.
While Milankovitch forcing predicts that cyclic changes in the Earths orbital elements can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial–interglacial periods. In particular, during the last 800,000 years, the dominant period of glacial–interglacial oscillation has been 100,000 years, which corresponds to changes in Earths orbital eccentricity and orbital inclination. Yet this is by far the weakest of the three frequencies predicted by Milankovitch. During the period 3.0–0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000-year period of changes in Earths obliquity (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earths climate system.
The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000-year cycle over the last 8 cycles. Richard A. Muller, Gordon J. F. MacDonald, and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100,000-year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in insolation, as the Earth moves in and out of known dust bands in the solar system. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Jose Antonio Rial.
Another worker, William Ruddiman, has suggested a model that explains the 100,000-year cycle by the modulating effect of eccentricity (weak 100,000-year cycle) on precession (26,000-year cycle) combined with greenhouse gas feedbacks in the 41,000- and 26,000-year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41,000-year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the second or third cycle triggers an ice age. This would imply that the 100,000-year periodicity is really an illusion created by averaging together cycles lasting 80,000 and 120,000 years. This theory is consistent with a simple empirical multi-state model proposed by Didier Paillard. Paillard suggests that the late Pleistocene glacial cycles can be seen as jumps between three quasi-stable climate states. The jumps are induced by the orbital forcing, while in the early Pleistocene the 41,000-year glacial cycles resulted from jumps between only two climate states. A dynamical model explaining this behavior was proposed by Peter Ditlevsen. This is in support of the suggestion that the late Pleistocene glacial cycles are not due to the weak 100,000-year eccentricity cycle, but a non-linear response to mainly the 41,000-year obliquity cycle.
Variations in the Suns energy output
This section does not cite any references or sources. (January 2012)
There are at least two types of variation in the Suns energy output
In the very long term, astrophysicists believe that the Suns output increases by about 7% every one billion (109) years.
Shorter-term variations such as sunspot cycles, and longer episodes such as the Maunder minimum, which occurred during the coldest part of the Little Ice Age.
The long-term increase in the Suns output cannot be a cause of ice ages.
Volcanism
Volcanic eruptions may have contributed to the inception and/or the end of ice age periods. One suggested explanation of the Paleocene-Eocene Thermal Maximum is that undersea volcanoes released methane from clathrates and thus caused a large and rapid increase in the greenhouse effect.needed There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.
Recent glacial and interglacial phases



Northern hemisphere glaciation during the last ice ages. The set up of 3 to 4 km thick ice sheets caused a sea level lowering of about 120 m.
Main article: Timeline of glaciation
This section requires expansion with: Recent glacial and interglacial phases in other areas outside North America. (March 2008)
Glacial stages in North America
The major glacial stages of the current ice age in North America are the Illinoian, Sangamonian and Wisconsin stages. The use of the Nebraskan, Afton, Kansan, and Yarmouthian (Yarmouth) stages to subdivide the ice age in North America have been discontinued by Quaternary geologists and geomorphologists. These stages have all been merged into the Pre-Illinoian Stage in the 1980s.
During the most recent North American glaciation, during the latter part of the Wisconsin Stage (26,000 to 13,300 years ago), ice sheets extended to about 45 degrees north latitude. These sheets were 3 to 4 km thick.
This Wisconsin glaciation left widespread impacts on the North American landscape. The Great Lakes and the Finger Lakes were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old Teays River drainage system was radically altered and largely reshaped into the Ohio River drainage system. Other rivers were dammed and diverted to new channels, such as the Niagara, which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall, at the present Clark Reservation State Park near Syracuse, New York, is now dry.
The area from Long Island to Nantucket was formed from glacial till, and the plethora of lakes on the Canadian Shield in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of loess many dozens of feet thick in the Missouri Valley. Isostatic rebound continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets.
The Driftless Zone, a portion of western and southwestern Wisconsin along with parts of adjacent Minnesota, Iowa, and Illinois, was not covered by glaciers.
See also: Glacial history of Minnesota
Last Glacial Period in the semiarid Andes around Aconcagua and Tupungato
A specially interesting climatic change during glacial times has taken place in the semi-arid Andes. Beside the expected cooling down in comparison with the current climate, a significant precipitation is concerned here. So, researches in the presently semiarid subtropic Aconcagua-massif (6962 m) have shown an unexpectedly extensive glacial glaciation of the type "ice stream network". The connected valley glaciers exceeding 100 km in length, flowed down on the East-side of this section of the Andes at 32–34°S and 69–71°W as far as a height of 2060 m and on the western luff-side still clearly deeper. Where current glaciers scarcely reach 10 km in length, the snowline (ELA) runs at a height of 4600 m and at that time was lowered to 3200 m asl, i.e. about 1400 m. From this follows that—beside of an annual depression of temperature about c. 8.4°C— there was an increase in precipitation. Accordingly, at glacial times the humid climatic belt that today is situated several latitude degrees further to the S, was shifted much further to the N.
Effects of glaciation



Scandinavia exhibits some of the typical effects of ice age glaciation such as fjords and lakes.
See also: Glacial landforms
Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out the landscape in Canada (See Canadian Arctic Archipelago), Greenland, northern Eurasia and Antarctica. The erratic boulders, till, drumlins, eskers, fjords, kettle lakes, moraines, cirques, horns, etc., are typical features left behind by the glaciers.
The weight of the ice sheets was so great that they deformed the Earths crust and mantle. After the ice sheets melted, the ice-covered land rebounded. Due to the high viscosity of the Earths mantle, the flow of mantle rocks which controls the rebound process is very slow—at a rate of about 1 cm/year near the center of rebound area today.
During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level dropped by about 110 meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During deglaciation, the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed ice dams resulting in salination of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale. It can even cause temporary reglaciation. This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the Baltic and Scandinavian regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe.
The redistribution of ice-water on the surface of the Earth and the flow of mantle rocks causes changes in the gravitational field as well as changes to t
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